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Thread: Wood - Plywood - Epoxy bond, re-building my ketch Tonga

  1. #1
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    Default Re-building my ketch Tonga (1960)

    Hi together,

    I've found some amzing projects and very interesting information in your Forum, and am now one of your newbies !

    Maybe I better introduce us first: we, that's Tonga my composite ketch launched 1960, Mr. Max the cat (arrived here in winter 2015 and couldn't be convinced that life on a boat will be difficult for him) and me (Dody, 1962, ex office-worker but didn't do much practial stuff with my hands, female). Tonga and me have been sailing together since 1996 when in 2010 I discovered her plywood-deck needs replacing. I started, but sailed away with a mate for 3 months, but it turned out in the end that we've actually been away for 23 months. When I came back in 2013 I had to take my whole interior out as rainwater had got in (only kept my bunk). All the interior was covered in fungus and spak, but the hull was not affected. So, I really started with my deck. From bow to cockpit the new deck is installed now. When I ripped off the aft-deck I found - as kind of expected - that the previous owner and builder of Tonga had altered the form of the stern. Originally she was positive stern, he added a negative stern to it. Water was sitting in between the two not doing any good. So, together with the local shipwright for wooden fishing-boats we re-built her stern. During this operation we found out that after 57 years the nails connecting the planks to the frames are giving up, meaning I've got to bang new nails in each connection. On top of the planks was one layer of plywood in 4 mm, 14 cm wide and nailed on in a 45-degree angle, on top of that fibreglass and filler. Fortunately for me now, this was done with Polyester. Fortunately in so far as it never managed to get a good bond with the plywood and is fairly easy to take off.

    I've already ordered the plywood for the hull, had it cut in 14 cm strips (am using 6 mm ply as I tested and found out it bends easy enough in the closest bends), and I will go for 3 layers, one crossing the previous one. The layers of ply will be bonded together with Epoxy, and on top of that 3 or so layers of fibreglass, probably the exterior one in Carbon or Kevlar for more protection against little icy bits etc. So far so good.

    I would like to know your opinion about what you think would be the best way to do the connection with the planks (they have been heavily treated against all kind of ugly stuff in the building-process of Tonga, they are not painted or varnished, but they received from me several layers of linseed-oil mixed with wood-preserver), and the first layer of plywood.

    The guys where I get the epoxy from try to convince me to epoxy and filler-glue this first layer on, with staples till the epoxy has cured.

    For some reason I don't trust this bond to hold forever, although it might be pretty stiff. I've made some tests with one of the old planks and the new ply. With the hammer I managed to break the plywood, but not the bond. Still, I've ordered a pneumatic nailgun and 20.000 nails, in inox because, strange enough the inox-screws that were used during the building-process in several places under and above water are still perfectly alright.

    What scares me is the question how the planks will love it long-term. They will have no possibility to breathe on the side where they are glued to the plywood, which they can on all other 3 sides.

    Please let me know your ideas and thoughts about it!

    Fair winds and sunny greetings from Portugal
    Dody

    Sorry, I resized the photos 6 times now, but he still wouldn't take it, must be doing something wrong!
    https://www.facebook.com/photo.php?f...type=3&theater
    Last edited by Dody; 06-27-2017 at 06:48 AM.

  2. #2
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    Default Re: Wood - Plywood - Epoxy bond, re-building my ketch Tonga

    Hi, and welcome to the forum.
    The FAQ page has got instructions for posting your pictures on here.









    I think that gluing on with epoxy is the right way to go, otherwise you will have a suspect carvel boat sitting inside a weak cold moulded boat. You could ask the epocy manufacturer about removing some of the oil from the surface, and if it is necessary.
    The owners of the Falmouth Quay Punt Curlew laid a new diagonal skin over her, using techniques similar to your proposal. http://forum.woodenboat.com/showthre...ay-over-carvel
    It really is quite difficult to build an ugly wooden boat.

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  3. #3
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    Default Re: Wood - Plywood - Epoxy bond, re-building my ketch Tonga

    I would not put "plywood" over the goofy planking,but rather mill actual lumber into thin strips, and continue to cold mold her.In other words, make your own plywood as you proceed.
    It is a screwy lookin construction, but as she is almost 60 years old....
    I recon you are looking at an easy 50 gallons of epoxy.

    welcome to the forum
    bruce

  4. #4
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    Default Re: Wood - Plywood - Epoxy bond, re-building my ketch Tonga

    It's a big job you have there. Cold molding over carvel or sheating is often done. Search the forum for it.

    From the look of the planking I suspect the boat was intended from the start to be sheated and constructed that way. Stop asking yourself how the planks will do. Up until now they also could not "breathe" on one side and the boat was fine until the deck let go. Just keep the deck leck free and it will be ok. Neglect the deck and you will have the same problems you have now. Glass the deck and please glass the hull deck joint aditionally.
    Plywood sheating is not ideal, wood veneer would be better, but ply also does the job.

    Do you know what wood was used initially for planking and structure? And what are you using now for the repairs?

    In order to assure a good bonding you have to have sound, clean and dry planking. So first thing to do is completley remove the existing sheating and plywood. The big rub/toerail also needs to go. Then you have to replace every single rotten piece of wood in the boat, and refasten where needed. Then you sand the whole hull agressively until you have clean wood. Tell the workers to stop using red lead between the planks and stop soaking the planking with anything. You will need to clean all the seams. Depending on how open they are you then either use epoxy putty to fill them, or spline with thin battens. Another sanding to get everything fair and after degreasing with acetone you are ready to glue the ply on. Since this is a big job done outside and all the ply has to be spiled I would first spile all the ply strips (hold them in place with some temporary screws or staples) number them (permanent marker please not pencil) and glue them on all at once. This would also give the hull more time to dry out in the sun. So you first clean the seams, then sand the hull, then spile all the strips, then remove strips, fill seams, glue the ply on.

    I would only use two layers of ply at 45°, the third is not necessary. The hull has carveel scantlings already and was fine with only one layer of 4mm ply so two layers of 6mm is more then it needs. After all this the glass on top is only for abrasion. How much glass you want to put on is up to you. One layer of 200g/sqm with some reinforcements along the stem and keelline would be enough but you can go as high as you like. For "icy bits" you also need reinforcement along the waterline. Just keep in mind that putting a lot of glass on the topsides is just dead weight and aded expense. Carbon is not necesarry it will do nothing for you. Kevlar has good abrasion resistance but is difficult to use. If you feel you need more protection just use more glass.

    Download the Gougeon Brothers on Boat Construction and read it.

    Ask around if there is someone who can sandblast with soda (sodium bicabonate) that would be the fastest way to clean the hull inside. But please only soda not sand or corund you only want to clean the wood of paint not destroy it. And don't do it if the operator has no experiece with blasting wood.

  5. #5
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    Default Re: Wood - Plywood - Epoxy bond, re-building my ketch Tonga

    Thank you alll, this is super-helpful just a quick reply as have to get on before it gets too hot: the wood originally used was oak for the frames and larch for the planking. The frames are ok, I can see all the planks in the hull from the inside and one or two need replacing in the process. I can't get larch, for the stern we used Gamballa, and for the parts where the Gamballa was too rigid the only wood we could get was scotch fir. As scotch fir is not very rot-resistant I treated them with red-led (there are no workers apart from Albertino the local shipwright with whom I am doing the planking, so the bang with the hammer for using this is for me). Is it really necessary to epoxy-putty or spline with thin battens? I was hoping to get away without doing this as she didn't have that before?
    I've read the Gougeon Brothers and also Buehler's Backyard Boatbuilding. Have ordered 2 more Boatbuilding-Books which should arrive today or on Monday, one by Howard Irwing Chapelle, the other by Robert M Steward and Carl Cramer as there is heaps of other questions I've got and havent found the right way yet.

  6. #6
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    Default Re: Wood - Plywood - Epoxy bond, re-building my ketch Tonga

    If you don't close the planking seams the moment you begin stapling the ply on, all the thickend epoxy will go into the seams and you will be left with a starved joint at best. It will be like pressing dough trough a sieve.
    I don't know how the original ply layer was fastend, but I suspect nails or screws not adhesive. Epoxy is stronger than nails. Plastic staples can be left in place and sanded.

    Gamballa would be Iroko (Kambala)?

  7. #7
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    Default Re: Wood - Plywood - Epoxy bond, re-building my ketch Tonga

    I would like to know your opinion about what you think would be the best way to do the connection with the planks (they have been heavily treated against all kind of ugly stuff in the building-process of Tonga, they are not painted or varnished, but they received from me several layers of linseed-oil mixed with wood-preserver), and the first layer of plywood.

    The guys where I get the epoxy from try to convince me to epoxy and filler-glue this first layer on, with staples till the epoxy has cured.

    For some reason I don't trust this bond to hold forever, although it might be pretty stiff.For some reason I don't trust this bond to hold forever, although it might be pretty stiff. I've made some tests with one of the old planks and the new ply. With the hammer I managed to break the plywood, but not the bond.
    Have you tried this test on a board treated with the linseed oil and preservative the same way you treated the new planks?

    There was a thread here in the last year about a boat builder who deliberately planks his boats like carvel but with gaps a few mm wide and then caulks them with thickened epoxy. Even if the bond between the treated wood and plywood sheathing is weak, the thickened epoxy forced into the gaps and the staples will provide considerable shear strength. I don't see a problem with starved glue joints, but swelling of the planks with a rigid material between them can generate a lot of pressure. I think that you will need to coat the inside of the planks to keep them as dry as possible. I don't know the solution, but I see a potential problem. An alternative would be to add a layer of cloth between the planks and the sheathing to prevent the epoxy from filling the gaps. Hopefully someone with more insight in this area will correct me or provide better information.

    Red lead comment: The red lead is not a good preservative. Red lead does keep the paint more flexible and reduces cracking of the paint, so it is still a good protective coating. Personally, I don't like creating so much hazardous waste, but until the boat is scrapped, the paint works well enough.

    Edit : Related thread http://forum.woodenboat.com/showthre...ay-over-carvel
    Last edited by MN Dave; 06-23-2017 at 11:52 AM.
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  8. #8
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    Default Re: Wood - Plywood - Epoxy bond, re-building my ketch Tonga

    That's a big project, good luck with it. Seems to me that doing it the same way it was done before is a pretty reasonable approach. I'd worry that linseed oil on the planks might compromise the bond with epoxy though. As someone commented above, you will want to get the planking as clean and dry as possible, and rough them up plenty too.

  9. #9
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    Default Re: Wood - Plywood - Epoxy bond, re-building my ketch Tonga

    Thanks Nick for posting the photo, and thanks to you all of course and thanks for the links!

    Where do I start? Best with the wood. In the shop it was marked "Kambala Escura", on the bill it says "Zazange" which is 2 different names for the same thing. I tried to find out something on internet. The english name is "West African Albizia" which hardly gave me any information, the French name is "Iatandza" and there some info popped up. It comes from Africa, seems pretty similar to Iroko from the looks, smell, and qualities but is a bit less difficult to work with - although tools do get blunt faster than with other woods.

    Linseed oil. I only know about it since I'm in Portugal (although it is used on ships for hundreds of years). Every 3 - 4 years most of the local fishermen strip their paint completely off the boats, apply one coat of linseed-oil, let it soak in for a few days and then start with a fresh paint job. So far I found out it's anticorrosive, strenghtens the wood and makes it more water-resistant. It doesn't affect the breathability and is part of the better range of oil-based paints. I applied it together with wood-preserver on the inside of the hull the moment I had my interior out. Mainly because I was too scared fungus or anything else might have had ideas to find a new home in my planks, but also to give them a bit of a treat after 50 something years and increase their resistance against water.
    My whole boat got 3 coats of linseed-oil 50:50 with wood-preserver from the inside in 2014, the aftcabin an additional coat in January 2017.

    The plank I used for the test was one we had to shorten to install the new planks while re-building the stern. Which means it was old, came from the aftcabin, and her last linseed-oil was applied in January 2017. To make the test more interesting and find out how it affects the bonding I used the treated side (inside of the plank) for glueing the plywood on. Thickness of the plank 30 mm, thickness of the ply 6 mm.

    Test. I tried out 7 scenarios
    1 - no sanding, no cleaning, held in place by a screw till the epoxy cured
    2 - no cleaning but sanding, held in place by a screw till the epoxy cured
    3 - sanding, cleaning, held with 2 staples
    4 - sanding, cleaning, held with 2 nails
    5 - sanding, cleaning, held with 2 screws
    6 - sanding, cleaning, held in place by a screw till the epoxy cured
    7 - sanding & cleaning as before, Epoxy on both faces but NO filler, held in place by a screw till the epoxy cured

    After waiting for 7 days I tightened the plank in the bench, got my biggest hammer out (maybe 4 KG?) and started with all my available force on No. 7, the weakest of them all. A huge surprise for me happened: this weak bond without filler held, the ply broke! The next weakest should be the one where I didn't sand and didn't clean, No. 1. The same thing happened: the bond was perfectly alright but the ply broke. This makes me believe that big impacts don't affect the bond. Today I tried with flexing the ply to see what happens, but, as the ply only sticks out about 8 cm on each side I wasn't successful in breaking anything. I'll do some tests with a chisel tomorrow (just sharpened them all today) to see if and how I can destroy the bond. Will let you know what happens :-D!

    Acetone. When reading the book of the Gougeon Brothers a huge surprise happened: nowhere in this book they mentioned Acetone. Whenever they were talking about cleaning, they were using Alcohol!!! Buying heaps of Vodka for the purpose was probably not what they had in mind I guess :-D. I asked around and what I found here is something called "blue alcohol", which is 90 % strong industrial alcohol with something else in it, and darkish blue so nobody drinks it by accident. I tried this stuff cleaning my epoxy-mess from the tools and it actually worked much better than Acetone. I also used this stuff for tests 2 to 7. Friends of mine then gave it a try with Epoxy-paint and 2-pack Polyurethane paint and were quite happy how much better it works. I've got no idea if it's less harmful than Acetone, but at least it doesn't dry the skin as bad as Acetone if you forgot the gloves for a moment.

    Epoxy. I've tried a lot of stuff over the years. Although I'm always very particlular about getting the mixture right, the only one I never had a problem with was WEST SYSTEM. I absolutely hate doing things twice or clean up another nice mess. Imagine on a job like this being unlucky with a wrong batch of epoxy ... well, I'm not gonna try something else. I will end up with more than 200 KG of Epoxy-resin. Fortunately my supplier is giving me his yard-discount. I could save quite some more if I would next buy a 200 KG drum instead of sticking with the 30 KG containers, but I haven't figured out a cheap way of filling the resin into the 30 KG containers which I can easily carry around my workplace, the drum would stay in my workshop about 100 m away from my boat (was lucky to be able to rent one of the fishermen's workshops here in the port).

    Planks and distance between planks. The planks have a thickness of 30 mm, the original 4 mm ply was nailed onto the planks without any glue or bedding-compound in between. Somewhere in the forecabin there is 2 planks where I've got a distance of 3 cm, with the rest of the hull it's between 0,5 and 2 cm. Albertino (the local shipwright) did a neat job with the planks at the stern, but the original planking is not that neat.

    You are absolutely right, I should take the whole fibreglass off now to profit from the summerwinds drying it all out nicely. And yes, the toerail is coming off and the fibreglass of the hull and the deck will be joined together so no water can come in (there is another nice little mess waiting for me concerning keel and keelbolts ...). Trouble is, I've only got my 2 hands and last winter turned out pretty nasty sleeping in a boat where the complete stern was missing, the temperatures dropping below zero and the wind blowing fresh.
    Right now I've still got plenty of work to do with the deck. The tiny little elevation on the aftdeck didn't have a carlin which I've started to make yesterday. When both of them and the knees are in I would like to close the transom first, so I can completely lay the rest of the deck from the cockpit to the transom (1st layer is 12 mm, screwed to the deckbeams with bedding compound, consecutive 3 layers 6 mm ply glued on with epoxy and epoxy-filler). I would then like to fibreglass the whole deck with filler or primer on top to protect everything from UV and water (right now the finished part has one coat of epoxy with peel-off cloth). When this is done I can remove my tent and get the toe-rail off (the hoops of the tent are fixed to the stanchions, which are screwed into the toe-rail). With the growing experience I'm getting much faster now, but still, somehow, everything seems to take ages and I can't have stress any more. I recon by the time I'm done with all this there will be just enough time to close the open stern-area with the first layer of ply so the wind and cold stay out and winter will be there (the rains start something around November normally). Winterjobs will be my new rudder, the whole construction aft where the rudderstock sits in etc.
    Last edited by Dody; 06-24-2017 at 03:10 PM.

  10. #10
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    Default Re: Wood - Plywood - Epoxy bond, re-building my ketch Tonga

    Quote Originally Posted by MN Dave View Post
    There was a thread here in the last year about a boat builder who deliberately planks his boats like carvel but with gaps a few mm wide and then caulks them with thickened epoxy. Even if the bond between the treated wood and plywood sheathing is weak, the thickened epoxy forced into the gaps and the staples will provide considerable shear strength. I don't see a problem with starved glue joints, but swelling of the planks with a rigid material between them can generate a lot of pressure. I think that you will need to coat the inside of the planks to keep them as dry as possible. I don't know the solution, but I see a potential problem. An alternative would be to add a layer of cloth between the planks and the sheathing to prevent the epoxy from filling the gaps. Hopefully someone with more insight in this area will correct me or provide better information.

    Red lead comment: The red lead is not a good preservative. Red lead does keep the paint more flexible and reduces cracking of the paint, so it is still a good protective coating. Personally, I don't like creating so much hazardous waste, but until the boat is scrapped, the paint works well enough.

    Edit : Related thread http://forum.woodenboat.com/showthre...ay-over-carvel
    In this Gougeon Brother's book I found one construction that had many stringers with a distance of maybe 30 cm between them going all along the hull (and no planking). The plywood would be glued on the stringers and when explaining how and what they said to just scrape off the excess filler from the inside. I haven't got a clue but was wondering if I maybe could apply this technique on my boat?

    I agree with you, something needs to be done from the inside to prevent water soaking into the wood. Trouble is I also keep reading again and again that the wood needs to be able to breathe so water that does get in has a chance to get out again. Tonga used to be fairly dry, but there is no way to completely keep water out and be it only that you come inside with wet rainies, a hose or a fitting breaks, a hatch accidentally left open when a shower goes down and and and.

    Red lead, I wish there was something less poisonous on the market but it hasn't happened yet.
    Last edited by Dody; 06-24-2017 at 03:17 PM.

  11. #11
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    Default Re: Wood - Plywood - Epoxy bond, re-building my ketch Tonga

    Never heard of that african wood, but if similar to Iroko it's ok. Pine for the planking is a classic, no problems there.

    Using epoxy from a drum requires either the use of a pump or you put the barrel on the side and use a tap. There are manual and electric transfer pumps and also taps that screw on standard 200L steel drums. Available at any better industrial supply shop. Just ask the diesel guy in the harbour where he got his, he uses it to transfer motor oil from the drums to the canisters. You need two of them one for the resin and one for the hardener, and never mix them up.

    Why are you using first a layer of 12mm ply and then 3 layers of 6mm for the deck? Just use two layers of 12mm and one of 6mm. I suppose 18mm plywood will not conform to the deck chamber? Doing more layers then strictly necessary is just a waste of epoxy and time. After glassing paint it well and use nonslip.

    How big is the boat?

    Looking at the pictures I'm guessing that if the planking is 30mm thick then most planks are 50-60mm wide. And reading about the gaps I am pretty certain your boat was designed and constructed this way from the begining. The builder saved time (and money) by combining several building methods known to him. Or someone got creative and fitted out a plug intended to create a fiberglass mold and knowing that used good materials for building it from the begining. However it was he used narrow planking without spiling like in strip planking, nailed plywood over it to create a seamless surface and glassed it to be watertight, You are now converting the boat to a more orthodox building style, namely strip planking followed by cold molding. You need to close or open the gaps to about 1-2mm then fill them with thickend epoxy. You can make your own mixture or use Resoltech 2040G it's your call. It's not hard to do, I'll link some videos. If the planks are indeed only 50mm wide you could maybe just glass over instead of cold molding with plywood.
    The inside is a philosophical question. Some are of the opinion that once you use epoxy you should encapsulate, meaning you sand all of the paint away to bare wood and paint three coats of epoxy on followed by paint or varnish. Others say it's okay without encapsulation just paint it well inside. It's for you to study and decide.

    The construction method from the Gougeons book is ply or coldmolding over stringers. It results in a stiff and light hull. Converting your boat to it would require recalculating the structure.


  12. #12
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    Default Re: Wood - Plywood - Epoxy bond, re-building my ketch Tonga

    Interesting construction. I wonder if epoxy is a mistake then? Those planks are going to move with changes in moisture, and their movement is much more powerful than your hammer. If you do go epoxy, I agree with others, you need to spline those gaps between the planks down to nothing. Of course I've never done anything like this, I'm just an arm chair quarterback here.

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    Default Re: Wood - Plywood - Epoxy bond, re-building my ketch Tonga

    I found the threads I was thinking of.
    http://forum.woodenboat.com/showthre...strip-planking
    http://forum.woodenboat.com/showthre...strip-planking
    It looks like he glasses the interior to seal the water out of the wood, which could be very difficult in this case. You can PM Mark Bowdidge about sealing the interior since he obviously knows a lot more than I do about this.

    I used Google to translate "Kambala Escura" from Portuguese to English and got dark kambala, which comes up more readily on google. It is same as Iroko.
    https://www.balticwood.pl/eng/news/n...1470/type:blog
    Kambala – a wooden chamaeleon
    "This exotic wood, which is known in the countries of its origin under such names as iroko, abang, odum, intule, rokko and moreira, comes from the hot African continent."

    See also http://www.wood-database.com/iroko/
    Last edited by MN Dave; 06-24-2017 at 07:07 PM. Reason: corrected link
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    Default Re: Wood - Plywood - Epoxy bond, re-building my ketch Tonga

    In theory one could repair as initially done and save a boatload of epoxy. In practice when this boat was constructed ocoume ply was regarded as barely good enough for packing crates, and marine ply was made out of rot resistant species. So now it's either epoxy or a complete replank in a traditional manner. Galvanized fasteners into oak frames have a defined lifespan so this would have been a expected outcome and the original boatbuilder probably didn't expect the boat to last 57 years without a complete overhaul, so we can say this is one lucky boat.
    Do we know were the boat was constructed?

    The Kambala escura is Albizia ferruginea not related to Iroko wich is Milicia excelsa. Seems to be a good wood for boatbuilding.

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    Default Re: Wood - Plywood - Epoxy bond, re-building my ketch Tonga

    After rereading the OP
    On top of the planks was one layer of plywood in 4 mm, 14 cm wide and nailed on in a 45-degree angle, on top of that fibreglass and filler.
    One layer of plywood, held only with nails and it lasted 57 years? It may be best to allow the planking under the plywood sheathing to work as it shrinks and swells by not attempting to bond rigidly. Perhaps a softer material like Sikaflex or 3M 4000 or 3M 4200 is better than a stiffer material like epoxy.

    More information of no practical value to the OP:
    Common names are always problematic. Kambala can be either iroko or albizia, but I think that the name is more commonly applied to iroko. The wood of both albizia and iroko is similar. Even the end grain looks similar. Albizzia has a bright green fluorescence under UV light. Iroko tends to dull tools.

    http://www.wood-database.com/albizia/
    http://www.wood-database.com/iroko/

    http://www.prowebcanada.com/taxa/alp...ialspecies.php
    Woody Species Milicia regia
    Common name(s): Most known as Iroko Angola: Moreira Benin: Lokotin Cameroon: Abang Dem. Republic of Congo: Kambala, Lusanga, Mokongo, Moloundou Equatorial Guinea: Abang Gabon: Abang, Mandji Ghana: Odoum Guinea: Simme Liberia: Semli Mozambique: Mufula, Tule Nigeria: Rokko Sierra Leone: Semli Belgium: Kambala

    Plant Resources of Tropical Africa Can't copy from google books.
    Last edited by MN Dave; 06-25-2017 at 01:55 AM.
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    Default Re: Wood - Plywood - Epoxy bond, re-building my ketch Tonga

    Quote Originally Posted by MN Dave View Post
    After rereading the OP
    One layer of plywood, held only with nails and it lasted 57 years? It may be best to allow the planking under the plywood sheathing to work as it shrinks and swells by not attempting to bond rigidly. Perhaps a softer material like Sikaflex or 3M 4000 or 3M 4200 is better than a stiffer material like epoxy.
    Awesome! This is exactly where I started with my supplier for Epoxy! I already had to start wondering about it before I began replacing my deck (I didn't know about the hull by then). The old deck was just nailed on. The next layer nailed on top of this etc. In the end I decided to use bedding compound (Sikaflex) between the deckbeams and beamshelf and the first layer of plywood, screw the first layer of plywood on, and, as the 12 mm ply didn't bend easy enough when epoxy-filler-glueing to use with staples and me thinking using screws will be a hell of a job, changed over to 6 mm ply instead which I could use easily with staples.

    History: The guy who built her had wooden boats all his life. He was dreaming of a new boat, but couldn't afford to have one built or buy one. Being a tax-inspector he still knew lots about wooden boats, but didn't trust his abilities with the complicated joinery-work needed to build in wood. We are talking about the year 1956 now. By this time polyester as a building-material had just come to the market of boatbuilding, but nobody could tell by then how durable, strong or longlasting this material would turn out to be. He decided to build in wood as good as his abilities would permit and use this new material to keep her watertight. He was planning extensive voyages with this boat, together with his family and friends from his sailing-association and felt he has a huge responsibility, so: he wanted this boat to be as strong as she could possibly be.
    He bought plans for a 11 m, but when thinking about the interior he quickly found out that this is too small. So, he altered the plans and changed her length to 14,50 m, keeping the same width of 3,45 m. He started building her in 1957, first upside down and then later in the process turned her right way up. Tonga was launched the 3rd of July 1960. When she was launched she only had one layer of fibreglass with polyester and heaps of filler, but he changed this and many other things at a later time. By this time she was quite a big boat up in the baltic and North Sea and looked wonderful with her wishbone-rigg.

    During the 36 years he owned her, he did many long and happy voyages in her, sailing all over the Atlantic, Caribbean and Med. I bought her from him in 1996, caught up with a lot of maintenance he couldn't do any more as he was quite ill by then, Tonga got brand new masts in 2008 and and and. Unfortunately he sailed for his last voyage, so I can't ask him questions any more.

    I've got a copy of the photo-album from the building-process of Tonga. And 3 days after I was born in 1962 the german magazine "Die Yacht" published 2 consecuetive articles about the building of Tonga. Even though she's "only" a homebuilt by amateur, she's a great ship with wonderful sailing-abilities, strong and safe. In my cruising-life with her I met many people who have seen her before and/or been on board in previous years. All of them were super-happy to see her still going strong. I've got hardly any data-volume on my phone left and wifi is very poorly at the moment. When I find a possibility I will try to upload some pics.

    Tonga is (or was before I started with this project) 22 Tons, 14,52 m long (16 m with bowsprit), waterline 13 m, 3,45 m beam, Longkeeler 2,10 m draft, Centrecockpit, Ketch, Mercedes OM352 and 130 HP. My fastest run under sail so far (not that anyone might think I am a fan of speed, this was accidental and unexpected!!!) was Portimao - Madeira in 3 days and 1 hour, doing 9,5 knots over ground for 14 hours in comfort.
    Last edited by Dody; 06-25-2017 at 09:35 AM.

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    Default Re: Wood - Plywood - Epoxy bond, re-building my ketch Tonga

    Quote Originally Posted by Rumars View Post
    Using epoxy from a drum requires either the use of a pump or you put the barrel on the side and use a tap. There are manual and electric transfer pumps and also taps that screw on standard 200L steel drums. Available at any better industrial supply shop. Just ask the diesel guy in the harbour where he got his, he uses it to transfer motor oil from the drums to the canisters. You need two of them one for the resin and one for the hardener, and never mix them up.

    The construction method from the Gougeons book is ply or coldmolding over stringers. It results in a stiff and light hull. Converting your boat to it would require recalculating the structure.
    Thanks for all the infos Rumars!!
    And, could you tell me please how to get this "recalculating of the structure" done?
    Last edited by Dody; 06-25-2017 at 10:45 AM.

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    Default Re: Wood - Plywood - Epoxy bond, re-building my ketch Tonga

    Quote Originally Posted by Rumars View Post
    In theory one could repair as initially done and save a boatload of epoxy. In practice when this boat was constructed ocoume ply was regarded as barely good enough for packing crates, and marine ply was made out of rot resistant species. So now it's either epoxy or a complete replank in a traditional manner. Galvanized fasteners into oak frames have a defined lifespan so this would have been a expected outcome and the original boatbuilder probably didn't expect the boat to last 57 years without a complete overhaul, so we can say this is one lucky boat.
    Do we know were the boat was constructed?

    The Kambala escura is Albizia ferruginea not related to Iroko wich is Milicia excelsa. Seems to be a good wood for boatbuilding.
    Now this is the next interesting thing I'm facing: the species the plywood is made of. My gut-feeling says that there is no way I could leave okume ply without a protective coating. Which would be no problem at all if I Epoxy-glue the strips on the planks because in this case they will be completely covered in epoxy and thus protected. If I use a different method to attach them to the planks, be it with bedding-compound or whatever, I will have to varnish them before I install them. The varnish will have to dry first before I do this, which means automatically that each strip will be quite a bit more stiff then when applied with wet Epoxy. With the bending-test I did a few days ago the flexibility around the tighter curves of my boat was just the limit. It might be that, when attaching it plank after plank, it still will bend enough without problems which I didn't try in the test. I only attached it with a woodclamp on the top and checked how good or bad it will bend.

    She was built by a small river/channel called "Bille" in Hamburg/Germany.
    Last edited by Dody; 06-25-2017 at 10:38 AM.

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    Default Re: Wood - Plywood - Epoxy bond, re-building my ketch Tonga

    Just a thought: what might also be important thinking about all this and strenght and stuff: distance between frames is 50 cm, the frames are 35 mm thick oak, sawn into segments held together by bolts and the knees (Edit: just found out they are called gussets) by nails, each frame from the floors down to the keel with 2 mm steel plate attached. In theory she's chined, but this chine is rounded, so, from the outside one can't notice and might think she's round bilge. The steel, strange enough, is in very good condition where I can see it, although I will have to remove the concrete that was filled in there for inspection, but also to replace keel-bolts etc.. But this I will do at a later stage. As concrete is anticorrosive and the steel was painted with some kind of sticky black stuff to protect it, I don't expect to find nasty stuff from this end but one never knows before one has seen it !
    Last edited by Dody; 06-25-2017 at 06:05 PM.

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    Default Re: Wood - Plywood - Epoxy bond, re-building my ketch Tonga

    Quote Originally Posted by Phil Y View Post
    Interesting construction. I wonder if epoxy is a mistake then? Those planks are going to move with changes in moisture, and their movement is much more powerful than your hammer. If you do go epoxy, I agree with others, you need to spline those gaps between the planks down to nothing. Of course I've never done anything like this, I'm just an arm chair quarterback here.
    I am starting to ask myself the question, what kind of forces there will be to cope with?

    Let's imagine there were no splines. The plywood strips diagonally glued to the planks with Epoxy and filler. Filler that has squeezed out scraped off before it cures. And the plywood-strips additionally held in place with 35 mm nails, 2 for each plank-plywood-strip-connection.

    As the frames are 50 cm apart, the movement of each plank is limited to these 50 cm lengthwise anyway. The movement of the plank would be expansion with humidity - provided it is allowed to take the humidity of the air etc., and a movement up and down? At the same time it would be held quite rigidly in place by the Epoxy-plywood-connection which will make the up and down movement difficult because the plywood will be too rigid to be buckled in the small gap between the planks. This will be even more difficult, the more layers of plywood are laminated on top of the first one.

    Presumably and theoretically, the moment the plywood would reach the same thickness as the plank itself, the plywood would be a lot more rigid then the plank. Would then the expanding and possibly up or down movement of the plank within these 50 cm it has available in length make her break off the bond? If this would be the case, it would still be held by the 2 nails each and the situation would be exactly the same as before. Only if the plank would break itself I would end up with a problem - or do I overlook something here? Would it be possible that a plank, held rigidly in place on one of the wider sides could break itself over a distance of 50 cm?

    Sure, the boat working in a heavy sea has a big impact on all this movement as well. Much of this will be dealt with the fibreglass and the plywood to start with. There are plenty of boats out there with far less framework behind their skins although similar in displacement and weight.

    Gosh, so glad I've still got some time left to think about it and even more to have you guys around to exchange ideas!!!
    Last edited by Dody; 06-25-2017 at 01:15 PM.

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    Default Re: Wood - Plywood - Epoxy bond, re-building my ketch Tonga

    Do you know the history of the old boat? Has she actually sailed ?

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    Default Re: Wood - Plywood - Epoxy bond, re-building my ketch Tonga

    Quote Originally Posted by wizbang 13 View Post
    Do you know the history of the old boat? Has she actually sailed ?
    Hahhhhaaaaahhaaaa, you are funny! This is what I was just writing about above . Yes, both, hundredthousands of miles and quite a lot of them together with me.
    Last edited by Dody; 06-25-2017 at 02:28 PM.

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    Default Re: Wood - Plywood - Epoxy bond, re-building my ketch Tonga

    I suppose potentially the plank to ply bond shears away, the ply planks move apart and the glass sheathing is torn. Or it all just firms up in a stiff stressed monocoque structure, with no breakage. My boat is 50 feet, built in 1974. Strip planked, edge glued and nailed, and glassed on the outsid from new. Just paint inside. Built with Celery Top Pine, which is quite stable. I occasionally get a fine crack in the sheathing due to movement in the planking, cracking the glass. I understand there is a fine art to traditional caulking in carvel construction. Too soft and it leaks, too hard and it starts breaking frames.

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    Default Re: Wood - Plywood - Epoxy bond, re-building my ketch Tonga

    So I read the original articles from the 1962 Yacht (number 10 and 11, beginning at page 22 both) they are available online at http://www.yachtsportmuseum.de/dokum...tschriften.htm and understand the boat better.
    I'll describe the construction as per the articles since not everyone reads german.

    The backbone:
    Frames are 4x15cm oak, single sawn with butt blocks glued on the joints. The frames are glued to the keel and floors. Keel and floors are also connected by 5mm thick plates bolted to them. The stem is laminated from multiple boards (I suspect like giant plywood). The sternpost is reinforced with an iron U channel. The keel is made out of several segments glued to the frames. There is no deadwood, the bilge goes down to the ballast and was intended to house the diesel and watertanks and some internal ballast. The keelform is somewhere between short cutaway longkeel and a long fin. Under this fin is the ballast consisting of three steel boxes, welded togheter and filled with concrete and boiler punchings for a total of 3,5t. The boat is not chined it has true round bilges with typical wineglass shape.

    The planking:
    On the frames there are stringers, 50x50mm. This stringers are spaced 20mm apart midships and converge on the boat ends. On top of this there is a layer of 4mm plywood. On the bottom, between ballast and frames the ply is in full sheets (cut to size), the rest is in 15cm strips at 45 degrees. The ply is nailed and riveted to frames and stringers with 40 000 nails. On top of the ply are two layers on fiberglass in polyester.

    The deck:
    Deckbeams are laminated out of 7 layers 10x70mm. Deck is one 8mm layer glassed with polyester nailed to the beams and another 12mm layer also glassed on top.

    Rig:
    Staysail ketch with bermudan mizzen (13sqm),boomed staysail (17sqm) and mainsail (29sqm) and selftacking boomed jib (26sqm). Steel mainmast and wooden mizzen, both hinged for folding.

    Interiour was mainly plywood, motor was a Mercedes OM636 with a variable pitch propeller.
    The article also has a detailed materials and cost list (there should be 0,8 cm of teak somewhere). It cost 37 036 DM and was done in 27 months from lofting to sail away launching by a guy working a fulltime job and building in his spare time (riveting the 40 000 nails of the plywood planking consumed a 4 week vacation). At the time a VW Beetle was 3400DM, and the most expensive Mercedes would have cost 76000DM.

    IMHO this boat is the epitone of thin skin construction. The propper keel is actually the boxed ballast not the wood. I am amazed it sailed for 57 years.

    Dody, understanding the boat better I now think your replanking can go as follows:

    You need to sand and clean the stringers. The best would be on all sides, so interior also. Mark a line along the hull were the stringers begin to have less then 5mm between them. Gaps less then 5mm will need to be filled with thickend epoxy prior to glueing the ply on in order not to starve the joint. This will probably differ from stringer to stringer. Gaps bigger then 5mm you can reach in with a tool and form a fillet between ply and stringer from the squeezeout or remove it completley. This will considerably slow your planking time but should save epoxy. If you want to leave the nails or staples in you need to adjust the gun so that it seats them slightly below the surface of the ply. When you are done planking paint the interior of the planking and the stringers with 3 coats of epoxy and then paint or varnish. You need to protect the ocoume ply between the stringers so just paint everything. Even if you don't paint them with epoxy it will be ok if you paint or varnish the interior. 50mm square stringers will not break loose from the skin by moisture.
    When you remove the existing planking you will see the condition of the frames, floors, keel and the bottom ply sheets. After you remove the ballast you can glass the ply there.
    I don't see why you want a 30mm deck, two layers of 12mm should be enough. The deckbeams are laminated so you can use epoxy to glue the ply to them if you want.

    Recalculating the structure would take a naval arhitect. He would then be able to say take out so many stringers, replace that frame with a laminated one, and so on. Basicly he has to reengineer your boat to a different construction system, or more exactly adapt it to the modern materials.
    Last edited by Rumars; 06-25-2017 at 07:28 PM.

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    Default Re: Wood - Plywood - Epoxy bond, re-building my ketch Tonga

    Quote Originally Posted by Dody View Post
    Now this is the next interesting thing I'm facing: the species the plywood is made of. My gut-feeling says that there is no way I could leave okume ply without a protective coating. Which would be no problem at all if I Epoxy-glue the strips on the planks because in this case they will be completely covered in epoxy and thus protected. If I use a different method to attach them to the planks, be it with bedding-compound or whatever, I will have to varnish them before I install them. The varnish will have to dry first before I do this, which means automatically that each strip will be quite a bit more stiff then when applied with wet Epoxy. With the bending-test I did a few days ago the flexibility around the tighter curves of my boat was just the limit. It might be that, when attaching it plank after plank, it still will bend enough without problems which I didn't try in the test. I only attached it with a woodclamp on the top and checked how good or bad it will bend.

    She was built by a small river/channel called "Bille" in Hamburg/Germany.
    The math for paints stiffening the strips isn't what you would expect. Wood is a lot stiffer than varnish or epoxy, and the added thickness, 0.025mm, on each side of 6mm works out at maybe 1% stiffer, which is less than the difference between any two sheets of plywood. We have gone into the math on adding 6oz fiberglass cloth and epoxy to 6mm ply and it still doesn't change much. Treat the surface with something and don't worry about the stiffness.
    Management is the art of counting beans. Leadership is the art of making every being count. --Joe Finch

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    Default Re: Wood - Plywood - Epoxy bond, re-building my ketch Tonga

    Rumars, I am thunderstruck, you made my day! All the time and effort you have spent to help me solve my problem is exceptional and I don't know how to properly thank you. I hope there is something I can do for you to give you the same feeling of happiness I'm feeling right now !!!

    Thank you for the translation! Over the years I read this article several times but, by then not being familiar (and now just a little bit) with boatbuilding, the significance of several things didn't come to me.

    As I said before, lots of things were changed in later years. More plywood was added to the deck, the interior got replaced by a professional carpenter (I was devastated when I had to rip the interior out and hardly being able to rescue some of it) and, I just measured the thickness of pieces of the skin we took out. The Fibreglass under the waterline was between 9 and 11 mm, above the waterline between 6 and 7 mm. I've found heaps of Mahogany, but no Teak yet, it must be somewhere ...

    I think this is a good solution, and I like the idea with the fillets. It's giving additional strength to the whole system. Concerning the painting I would prefer to use oil-based paint on the inside. It's more flexible in it's structure for possible movements to happen and also less likely to get into conflict with the linseed-oil as this is part of the paint already. Also, I've got a compressor and a spraygun. So far I haven't got too much experience with the spraygun yet as I've only used it for the linseed-oil mixture, but I start where nobody ever will look and do some tests before, so it'll be alright I guess. Spray-painting Epoxy sounds difficult to me as you are not meant to use thinners if I got that right.

    Concerning the deck I just copied the thickness I had before, and, as the bigger part of the deck is done by now it would be silly to change the thickness. Would it make a difference if I changed from now on the way the first layer is attached to the deckbeams (I've been using Sikaflex for this)?

    Recalculating the structure would then probably mean to reduce weight but keep the same strength? If it's just about this I don't think it's worth it. On a boat with 22 Tons a few KG more or less don't really matter to me. Or is there more behind it?

    Just for fun I've calculated the weight of 1 m2 of the fibreglass I took off and the result was 5 KG for 1 m2, this is without the 4 mm plywood.
    Last edited by Dody; 06-26-2017 at 09:23 AM.

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    Default Re: Wood - Plywood - Epoxy bond, re-building my ketch Tonga

    Quote Originally Posted by MN Dave View Post
    The math for paints stiffening the strips isn't what you would expect. Wood is a lot stiffer than varnish or epoxy, and the added thickness, 0.025mm, on each side of 6mm works out at maybe 1% stiffer, which is less than the difference between any two sheets of plywood. We have gone into the math on adding 6oz fiberglass cloth and epoxy to 6mm ply and it still doesn't change much. Treat the surface with something and don't worry about the stiffness.
    Very interesting, I didn't expect that!

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    Default Re: Wood - Plywood - Epoxy bond, re-building my ketch Tonga

    No problem Dody, it's nothing. I enjoyed the articles and I like the boat. Maybe someday I'll see it in person. I'm happy I could help.

    I think the best thing out of the articles is that your boat is not carveel. Maybe the boat had a teak deck at some point? 0,8 cubic meters of teak had to be somewhere. The mahagony can be reused as faces and trim for plywood furniture. You get furniture that stays square and has real mahagony looks.
    The fiberglass thickness contains a lot of filler, the article mentioned they used a lot of it. Maybe it's worth to sand the stringers fair with a longboard, then you need less filler (less epoxy, less money).
    The inside is fine just painted with oil paint. If you put linseed oil first the wood will absorb less water. Be careful with linseed oil, soaked rags can self-ignite (not oiled wood, or the canister, but paper and rags are dangerous, put them in a bucket of water). Epoxy can not be sprayed on, it needs to be brushed or rolled.
    As Dave said you could epoxy coat the plywood off the boat and even fiberglass it then nail it to the stringers and there would be no problem. I think that glueing the ply on is stronger so I recomend epoxy glueing.
    Unfortunately slapping more plywood over the existing deck was the quick fix. So now you have stronger deck, not a real problem. Using Sikaflex (btw. wich one are you using there a ton of them?) and screws or using epoxy and screws is more or less the same. There would be a difference if the deck would be glued only, that can only be done with epoxy. Maybe in your case it's even better to use Sika and screws since I suspect the deckbeams of being oak (the article does not elaborate), and oak is difficult to glue with epoxy. West has G-flex and that is reported to work with oak.

    Recalculating the structure to proper use of modern materials would probably loose you half the weight of the boat for the same strenght. For example a similar boat, SV Safari buildt in Hamburg in 1955 out of steel is only 13t displacement. And it's 14,8m long, 3,2m wide, has 3,5t of ballast with 2m depth and also a wishbone rig, so pretty similar, and you would normally expect it to weigh in the same area, not 9t lighter. Only the underwater shape is different and she has more sailarea, but you mentioned a bowsprit so I think Tonga now also carries more sail. You can check it out under www.sy-safari.de they also had a big refit and reverted the interior to original.
    Your vessel is a semi-monocoque construction with extremly thin skin. The only reason that thin plywood skin works is because you have so many stringers. If you put a 12mm skin on her half the stringers and all frames are probably redundant. Only the bulkheads and floors will see some work. Your safety factors are probably also over the top. If the stringers would be 25x50mm and spaced the same they would probably be enough even with the 4mm skin. The article lists 2,5t of galvanized fittings (plates, channels, pipes, etc.) that would also be cut by half. But all this would practically mean almost a new boat.

    Maybe you can rename the thread "rebuilding my 50 ft. ketch" or something like it, that would attract a lot more attention and qualified comments than something that deals with epoxy adhesion. And I hope you will keep posting photos.
    Last edited by Rumars; 06-26-2017 at 08:29 PM.

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    Default Re: Re-building my ketch Tonga (1960)

    Still, thanks again, Rumars!

    I have been thinking about it again. the seats in my cockpit are teak. And there were some other pieces I took out where I was wondering if it was teak. But 0.8 cubic meters?

    For some reason he applied a lot of filler to get a certain thickness, concerning the strength this does nothing of course. I will fair her before I start, we actually did that already a little bit at the stern. Somewhere I read you can use carpenters chalk on a long piece of wood to facilitate spotting the high and low spots and I already got myself a big pot of that.

    Glueing the ply on is much stronger in my opinion too, and this is the way I will go.

    Now, I don't know if I explained this with the deck very well. I didn't slap more plywood over the existing deck but took the whole deck off (well, about 2 square meters still have to go), replaced 2 deckbeams, replaced the steel-structure where the mainmast is sitting on (this is now also bolted through the deckbeam in front and behind), replaced the beamshelf in several places, replaced the knees wherever suitable - because at this stage you can easily work in these spaces. And then I laid the new deck with wooden re-inforcing bolted to the deckbeams where the windlass is going etc.. The first layer in 12 mm got painted/varnished on the underside and the edges, Sikaflex 291i was applied to the deckbeams and the beamshelf, and then the sheet was screwed in place with fillets (? don't remember how this is called) between sheets where they meet as I couldn't get the ply wide enough to go across the whole boat. On top of that 3 layers of 6 mm ply were epoxy-and-epoxy-filler glued on and held with staples till the epoxy has cured. In the whole of Portugal I couldn't find stainless staples so they were applied with roller-shutter-belt and pulled out the next day. A tedious job. Thanks to internet I later found a supplier for stainless staples which I'm using since. They stay in and I just smooth the surface above them with filler. Oh and by the way: the deckbeams, when I cut them, smell a bit like pine, but so do the stringers. They could be pine, but they also could be larch as the rest of the hull.

    I know Ulli and Michl of Safari, we never met but exchange emails for a long time now. We got in contact because of the wishbone-rig. Unfortunately Dieter, the guy who built Tonga changed her rigging after the wishbone came down on him several times. When I replaced her masts in 2008 I didn't think about going back to wishbone. But then, I'm sailing singlehanded most of the time and you want to keep things simple. Maybe in 30 years time when she needs her next set of masts I'll go back to wishbone, who knows?

    The sailarea has changed since the article and also since I stepped my new masts (haven't properly calculated the lot with the sailarea yet). We had done all the measuring and calculating when my mastbuilder called me one day in tears: Dody, I just can't bring it over my heart to cut this beautiful 18 m profile down to 14.50 m. I've checked it 5 times now, she can easily carry 16.50 m. And that's what we did. The new mainmast is 16.50 m and the mizzen 11 (was 7 m before). And what a great decision this was! She only slightly heels a tiny bit more over till she's completely stable and she overreaches her theoretical hull speed with joy, something that was never possible before. On my way from Terceira (Azores) to mainland Europe, about 400 miles out, I had an encounter with Hurricane Bill (2009) which was a bit very rough for 36 hours, Tonga and her new masts were just doing fine!

    You know, I have no intentions of racing, safety is more important to me and I really don't care about the weight. I know of other steel-boats with the same length, even have sailed on some of them, and their weight is similar to the one of Tonga. The main difference is the thickness of the steel used in the construction. One of the ships I've been sailing on for 23 months was a Melquiades 50 (http://www.georgebuehler.com/Melquiades%2050.html). She's got 6 mm steel on the hull and 5 on the deck, the keel is 10 and 25 mm. A good, strong and seaworthy boat. Her weight is 23 Tons. We went on the rocks in Douarnenez (Brittany, France) and she survived. I don't guess Tonga would survive the same bashing she received though ... but I do want Tonga to be able to happily cope with some rough treatment in fishing-ports or wherever.

    Saying this, last night - I'm still in the thinking- and planning-process which keeps me from having proper progress with my work on Tonga at the moment - it came to me that the strength of the hull, the deck etc. need to be in some kind of balance to each other. There is no point in having a strong deck and a weaker hull. I prefer it the other way around, if the deck fails I will still be able to survive in a strong hull. Now, the deck is 30 mm laminated together, the hull will be 18 mm laminated together (I will go for 3 layers). I haven't got a clue how the stringers fall into the equation, but 30 mm and 18 mm are definitely not the same. I checked with my new boatbuilding-books, but they are about the building-process itself and you are supposed to check with your designer if you wish to alter things. Well, I haven't got a designer. George Buehler takes the approach: what looks strong is strong, and I think this is a very good approach. But, is there any possibility I could find out how much the stringers will throw into the equation?

    And thanks, yes, I renamed it and when I have found a better possibility for internet I will do something about pictures.
    Last edited by Dody; 06-27-2017 at 09:59 AM.

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    Default Re: Re-building my ketch Tonga (1960)

    Somehow the name of the thread does not want to change. Does someone know how to do that please? Thanks!

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    Default Re: Wood - Plywood - Epoxy bond, re-building my ketch Tonga

    I didn't think you put new ply over the old deck. But sometime someone did, and when you changed the deck you respected the thickness you found, so now yo have a stronger deck.

    The deck and hullsides do not have to be in some relation to eachother. Your vessel works like this: keel and deck are the flanges of an I-beam and resist up and down flexing. For this the hullsides only need to keep them apart. The hullsides resist torsional forces, hidrostatic pressure, puncture loads and so on. The stringers impart longitudinal stiffness and the frames and bulkheads transversal. They also divide the planking in panels and the smaller this panels are the thinner the outside skin can be, because stifness is provided for it. Case in point your vessel could use a 4mm skin with some semi-atached fiberglass. In a semi-monocoque like yours the skin also carries torsional loads, that's why you align veneers at 45° to the frames. The skin acts as a gusset or knee between stringers and frames. The fewer stringers and frames you have the thicker the skin must be because it carries more and more loads and needs to be stiffer to resist buckling, until you come to a true monocoque structure where there are no frames and stringers and only one or two bulkheads to keep the hullsides apart under the rigging stress because the skin carries all the loads.

    The structure you have is proven to work with a 4mm skin. Increasing skin thickness does little. It provides more reserve in case stringers and frames break. You already have so many stringers that you sailed with a few rotten ones without problems. Thicker skin has better puncture resistance but your stringers are spaced so closely apart that it would matter only if a sharp pointy object like a piece of rebar would hit the hull exacltly between two stringers. For any other scenario your effective hull thickness is 50mm plus whatever skin you have. And if you smash your boat onto a piece of rebar 12 or 18mm plywood will not really make a difference considering the weight of the boat. Abrasion resistance is also increased but plywood is not great at that anyway. For this reasons I would stay at two layers of ply, it is already 3 times what was proven to work. It is a cost/benefit analysis after all, and a third layer does not bring much except increasing time and cost. Weight is not really consideration, the aded displacement will probably ofsett the aded weight. With carveel construction sometimes it even is a net positive because the timbers are not waterlogged anymore, but I suppose it's not your case.
    If you do stay with 3 layers of plywood then you can arrange the first layer at 90° to the stringers and the next two at +/-45° for a complete isotropic sequence. This is just best practice derived from veneer use since plywood per se is regarded as an isotropic material, but the 90° layer will require less spiling so it is worth doing it.
    Reread the ply-on-frame chapter from the Gougeons book, they also have rules that explain stringer and frame spacing.

    Wishbone rigs were favoured when the boat was built because of the available materials. Today a boat like yours would probably be rigged as a sloop or cutter with a big roach square headed main, and be faster, pointing higher, and have improved handling. Just be glad that you don't have the wishbone anymore and have more sailarea.

    I like the Melquiades design, but all Buehler boats are overbuildt. With metal and fiberglass boatbuilding what looks strong is probaly masively overbuildt. But that is often the case with amateur construction anyway. Not much of a problem on big boats who can carry the weight but smaller ones do struggle with it. On the other hand they do have inbuildt corrosion allowance with that plate thickness.

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    Default Re: Wood - Plywood - Epoxy bond, re-building my ketch Tonga

    I looked again at the Melquiades 50 design and Buehler does have a wooden version. For that he specifies a layer of 1.5" of wood and two layers of 1/2" of plywood for a total of 2.5" or 63.5mm. That would be consistent with your 50mm close spaced stringers and 12mm skin for a total of 62mm. Keep in mind his is a chined design with developable surfaces (flat panels) and your boat is round bilge and that is stronger than flat surfaces by nature. 18mm for the skin is just overkill, but it's your boat after all and if that makes you feel better it's ok.

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    Default Re-building my ketch Tonga (1960)

    Thanks for the explanation, it makes the whole thing much more transparent for me!

    I just noticed we've got a bit of a confusion here, the stringers are 50 mm wide but only 30 mm thick. Together with 3 layers of 6 mm ply the result would only be 48 mm. But I'll re-read the section of the book, that's for sure!

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    Default Re: Wood - Plywood - Epoxy bond, re-building my ketch Tonga

    Sorry the article said 50x50mm and I asumed it was right. But it does not really matter, if it was good with 4mm plywood it will be only better with 12 or 18mm.
    Btw. I still think that closing the gaps with tapered pine battens is better, it is simply easier to put the plywood skin on a continuous surface. That would also simplify interior fitout and painting and you loose all the grime catching corners. It's also stronger if that is your concern.

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    Default Re: Wood - Plywood - Epoxy bond, re-building my ketch Tonga

    Quote Originally Posted by Dody View Post
    I just measured the thickness of pieces of the skin we took out. The Fibreglass under the waterline was between 9 and 11 mm, above the waterline between 6 and 7 mm. I've found heaps of Mahogany, but no Teak yet, it must be somewhere ...

    Recalculating the structure would then probably mean to reduce weight but keep the same strength? If it's just about this I don't think it's worth it. On a boat with 22 Tons a few KG more or less don't really matter to me. Or is there more behind it?

    Just for fun I've calculated the weight of 1 m2 of the fibreglass I took off and the result was 5 KG for 1 m2, this is without the 4 mm plywood.
    Fiberglass is probably more than twice as strong and stiff for a given thickness as plywood. Laying up 18 mm of plywood to replace 11 mm of fiberglass will... take me a while to work out? I am back to urging caution based on insufficient information. This should be thought through carefully. My inexpert opinion is that there is so much structure supporting a very substantial fiberglass shell on this boat that you could live without much of
    one or the other. 3 layers (18mm total) of plywood is probably more than adequate even if it is not as strong as the original 10 mm of glass.

    I see a lot of thickness numbers here, some plans, some historical and some measured. There seems to be a discrepancy between the measured fiberglass thickness and the originally reported 2 layers of cloth.
    Management is the art of counting beans. Leadership is the art of making every being count. --Joe Finch

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