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Thread: Exterior ply for trailer sailer?

  1. #1
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    Default Exterior ply for trailer sailer?

    Greetings all,

    I'm building a 14' Hartley Trailer Sailer and have a thread describing my trials and tribulations as a first timer.. http://forum.woodenboat.com/showthre...8-More-Hartley.

    Anyway, I've found that BS1088 is much more expensive than big box store, but has 5 plys in 1/4" sheets. 1/4" exterior ply has 3. So at the recommendation of a reader more experienced than myself, I decided to conduct a testing program to see how the cheap stuff would perform.

    I've looked for other threads on this subject, but didn't find anything that was applicable to my question:

    Can I use 1/4" exterior plywood in lieu of BS1088 certified plywood on my boat?

    I thought that a separate thread was appropriate for the question of using exterior plywood for dinghy trailer sailers. I've read so much about the deterioration of certified materials and the perceived equality of exterior glue plywood.

    Boil test and scarf joint trials. Can't think of a better way to spend a Sunday afternoon.

    OK, I can, but not this weekend.

    DSCF7237.jpg

    So I cut 2 - 9.75" x 9.75" plywood squares, one a 11/32" sanded pine and the other a 1/4" AB Douglas Fir, both with exterior ratings and from different stores. Photo above show the pine closest and the DF in the back.

    DSCF7238.jpg

    Official testing requires 8 hours at a boil. I only had time for 1 hour at boil.

    Sanded Pine after boil.

    DSCF7247.jpg

    DF

    DSCF7248.jpg

    Neither sample showed signs of de-lamination, both warped a bit and the DF had some adhesive spots appear on the B side.

    Cleaning the pot afterward required some serious solvents, hope the next turkey isn't tainted.

    Both materials performed equally with no separation or surface deterioration. DF may have swelled a bit, I need to look at original material measurements. So a first blush evaluation would indicate that both materials can meet the needs of a marine application. As much as a plywood hull could.

    Next up is making a couple of scarf joints out of both of these materials and testing them. Not sure how to test them right now and would certainly appreciate any suggestions.

    Eric
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    Default Re: Exterior ply for trailer sailer?

    Ive been using 1/4 and 3/8 exterior ply, pine or fir - "exterior waterproof" on my boats (row and power dories). they're new so I can't speak for the long term durability, and I build things kinda "workboaty" in finish. but I'm a fan so far. the worst bit is it generally takes some picking for sheets without surface voids/loose knots, and there's also the possibility of internal voids. that seems to be the deal breaker for most people, Moreso than the certification or glue type.

    I also built a punt a few years ago out of 1/4" (or 4mm) birch ply (when it was like 10$ a sheet) and it held up surprisingly well for living outdoors and only being painted. it was really nice ply despite being birch.

    i wouldn't touch that sanded pine stuff you have with that huge central ply - and I'm not sure your 1hr boil test is worth drawing much of a conclusion from. but I think you are of the right mind to do some testing and experimentation if you have any concern. I look forward to the scarph tests.

    ive also pondered the viability of using MDO/HDO or "signboard", I've seen some discussion and reccomendation of it on the forum, but not much else about it actually being used. i almost bought a sheet to play with but there were other things where my money was better spent.

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    Default Re: Exterior ply for trailer sailer?

    Dynamite Payson was an advocate of exterior ply. http://www.mcssl.com/store/hhpaysoncompany

    I used BCX or ABX fir to build a Gypsy, glassed on outside and most, but not all inside surfaces. No problem after 9 years. It spends the summer on the water. A friend took me for a ride in his Gypsy which was also exterior fir and 25 years old at the time. The fir ply did check after a few years because fir always checks if you don't glass it, marine or not.

    Using exterior ply is considered by most here to be penny wise and pound foolish. It depends on the finish you want. Good marine ply is nicer, and IF the stuff is really made properly, somewhat stronger and more durable. It looks like you put a lot of work into the boat, which argues in favor of the better plywood.

    Your build thread mentioned scarph joints, and in the short time that I spent skimming through, I didn't see if the issue was resolved. The Payson fiberglass reinforced butt joint is good for thin plywood. http://www.simplicityboats.com/epoxyknowhowcarnel.htm
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    Default Re: Exterior ply for trailer sailer?

    Testing the scarfs? Do a bend test with the scarfed panel on trestles and load the scarf with weights.
    Be careful to select your exterior grade ply to reject voids and overlaps of the inner plys.
    Last edited by Peerie Maa; 07-16-2018 at 06:09 AM. Reason: Speeling wrong wurd speeled correctly.
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    Default Re: Exterior ply for trailer sailer?

    Make the test joints in some strips, needn't be very big pieces. Place one end in the vice and pull the other by hand or clamp a lever in place to help you pull it back until it breaks. Then take a look at where and how the break formed. I expect it will be self evident if the scarf or butt joint has contributed to early failure.

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    Default Re: Exterior ply for trailer sailer?

    If you must use plywood, MDO ("Medium Density Overlay") is excellent, provided you seal the hell out of the edges. MDO was developed for making highway signs. So far, I haven't seen any highway signs delaminating anywhere. It is sheathed with a waterproof sheet material. Another advantage is that it doesn't check and is "paint ready" without any face sanding. Again, however, be sure to seal the edges well with epoxy. The interior softwood plies (which are more numerous that other plywood sheet material) are hardly decay resistant, although they are apparently free of voids.

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    Default Re: Exterior ply for trailer sailer?

    Greetings all,

    Quote Originally Posted by telenorth View Post
    i wouldn't touch that sanded pine stuff you have with that huge central ply - and I'm not sure your 1hr boil test is worth drawing much of a conclusion from. but I think you are of the right mind to do some testing and experimentation if you have any concern. I look forward to the scarph tests.
    I think I agree about the sanded pine, that piece gained a lot of weight in the boil test. That central ply just seemed like a piece of sponge. Yeah, that 1 hour boil didn't really tell me anything did it.

    Using exterior ply is considered by most here to be penny wise and pound foolish. It depends on the finish you want. Good marine ply is nicer, and IF the stuff is really made properly, somewhat stronger and more durable. It looks like you put a lot of work into the boat, which argues in favor of the better plywood.

    Your build thread mentioned scarph joints, and in the short time that I spent skimming through, I didn't see if the issue was resolved. The Payson fiberglass reinforced butt joint is good for thin plywood. http://www.simplicityboats.com/epoxyknowhowcarnel.htm
    My original thought process was to use BS 1088 plywood without question, but lately I've been questioning that because of the cost differential. The question comes as the result of the intended use of the boat. It is a trailer sailor...meaning that it will spend the majority of it's life out of the water.

    But as I get closer to planking, the question of the scarf becomes bigger. I had some problems with scarfing stringers and chines earlier in the build so part of my questions are motivated by conquering my failures. While I was thinking about going with the butt joint with a backing plate, the need to get by this roadblock keeps me on the scarf trail. The upcoming scarf test should help on a variety of points. I certainly appreciate your experience comments on exterior with glass, right down my path!

    Testing the scarfs? Do a bend test with the scarfed panel on trestles and load the scarf with weights.
    Sounds fair. I've got big weights though, hard to fine tune with 10# discks. But I'm not building a rocketship so my thought is that the first to break looses.
    Thanks Nick!

    Make the test joints in some strips, needn't be very big pieces. Place one end in the vice and pull the other by hand or clamp a lever in place to help you pull it back until it breaks. Then take a look at where and how the break formed.
    Good idea Geoff, I'll cut my test pieces into strips and test for both bending and tension. Failure method is definitely noteworthy.

    If you must use plywood, MDO ("Medium Density Overlay") is excellent, provided you seal the hell out of the edges.
    Hey Bob, I hadn't considered MDO in this application. I'll have to look into it. Would the boil test be an issue with that material?

    Thanks for the input and questions, makes the testing process a bit more interesting with additional information.

    Onward!

    Eric
    Last edited by FishoutaFlorida; 07-17-2018 at 01:07 AM.
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    Default Re: Exterior ply for trailer sailer?

    OK, back at it.

    The boil test was intended to be the deal breaker in material choice, but that was not the case. Other than noticing a weight gain in the sanded pine sample and some minor checking on the DF sample, all was similar.

    So on to the scarf test. I don't think this is a test to determine which material to use but is perhaps more of a test to determine the type of joint to use.

    Scarf or butt with a backing?

    So I built two scarf joint samples, one with the 11/32 sanded pine and the other with 1/4 DF. 12:1 taper.

    DSCF7249.jpg

    Belt sander with 60 grit gave me this.

    DSCF7252.jpg

    Planted wide board and weights to hold it all together.

    DSCF7254.jpg

    Creating the 12:1 tapers was a bit easier that I thought (although the ply limits in the 2nd photo may not be showing the most consistent taper), hopefully the joint construction will work out. It is certainly a skill based on observations and experience.

    Anyway, I've stuck them back in the garage and will probably let them sit there till the weekend. I'll be dreaming up a testing protocol up until then.

    Thanks for all observations, questions and comments, they all help.

    Later,

    Eric
    Last edited by FishoutaFlorida; 07-17-2018 at 12:35 AM.
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    Default Re: Exterior ply for trailer sailer?

    If that "sanded pine" plywood you're talking about is the "Sandeplywood" that Home Depot sells, then you should know that that species of wood used for the outer ply - Sande (Brosimum utile) - is rated as non-durable as far as its rot resistance goes. I don't know what the inner ply is.
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    Default Re: Exterior ply for trailer sailer?

    Generally speaking, materials are the cheapest part of the boat.

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    Default Re: Exterior ply for trailer sailer?

    Greetings all,

    Quote Originally Posted by BrianY View Post
    If that "sanded pine" plywood you're talking about is the "Sandeplywood" that Home Depot sells, then you should know that that species of wood used for the outer ply - Sande (Brosimum utile) - is rated as non-durable as far as its rot resistance goes. I don't know what the inner ply is.
    Yes it is and thanks for the info on the durability capabilities of Sande. Odd story about how I ended up with this sheet.

    Either way, choose the sheets carefully.

    Also, ditch the sander for scarfing fir. Use a blade. It won’t leave the “humps” from the grain, as badly. The softer wood sands away quicker than the harder, which is why you get those waves, and why it’s so hard to sand fir smooth!
    Yeah, this was the first time plywood scarf trial. I thought that the belt sander could do the job, but wasn't feeling good about the surfaces when I epoxied them up.

    When speaking of the blade, are we talking router with a jig?

    Thanks again!

    Eric
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    Billy Currington

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    Default Re: Exterior ply for trailer sailer?

    Quote Originally Posted by Canoeyawl View Post
    Generally speaking, materials are the cheapest part of the boat.
    I always thought that honor went to the owner!

    But perhaps to your point, why mince dollars in critical elements? The question of 3 plys vs. 5 plys is an important question, especially with 1/4" (6mm) material. I'm looking forward to testing the samples. Still developing protocol.

    Thanks Canoeyawl!

    Eric
    Last edited by FishoutaFlorida; 07-18-2018 at 02:25 AM.
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    Default Re: Exterior ply for trailer sailer?

    Quote Originally Posted by amish rob View Post
    Oy! Especially a sailboat!

    Peace,
    Robert
    I keep coming back to exterior sheets. Looking forward to testing these samples!

    Thanks!
    Eric
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    Default Re: Exterior ply for trailer sailer?

    Quote Originally Posted by FishoutaFlorida View Post


    Yeah, this was the first time plywood scarf trial. I thought that the belt sander could do the job, but wasn't feeling good about the surfaces when I epoxied them up.

    When speaking of the blade, are we talking router with a jig?

    Thanks again!

    Eric
    A hand or power plane will do it. Tack one sheet on top of the other set back the length of the scarf and the lower one aligned with the edge of the bench and the two sheets act as your jig.
    It really is quite difficult to build an ugly wooden boat.

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    Default Re: Exterior ply for trailer sailer?

    Indeed, very interesting thread

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    Default Re: Exterior ply for trailer sailer?

    Quote Originally Posted by FishoutaFlorida View Post
    OK, back at it.

    The boil test was intended to be the deal breaker in material choice, but that was not the case. Other than noticing a weight gain in the sanded pine sample and some minor checking on the DF sample, all was similar.

    So on to the scarf test. I don't think this is a test to determine which material to use but is perhaps more of a test to determine the type of joint to use.

    Scarf or butt with a backing?

    So I built two scarf joint samples, one with the 11/32 sanded pine and the other with 1/4 DF. 12:1 taper.

    DSCF7249.jpg

    Belt sander with 60 grit gave me this.

    DSCF7252.jpg

    Planted wide board and weights to hold it all together.

    DSCF7254.jpg

    Creating the 12:1 tapers was a bit easier that I thought (although the ply limits in the 2nd photo may not be showing the most consistent taper), hopefully the joint construction will work out. It is certainly a skill based on observations and experience.

    Anyway, I've stuck them back in the garage and will probably let them sit there till the weekend. I'll be dreaming up a testing protocol up until then.

    Thanks for all observations, questions and comments, they all help.

    Later,

    Eric
    The face grain direction is important. You made the fir sample sideways. You should see a big difference between panels tested in different directions. You should make another fir sample with the face grain perpendicular to the scarph. The comparison between the samples with different orientation will be interesting.
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    Default

    The Hartley 14 is a really cute little boat. Good luck with it.


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    Default Re: Exterior ply for trailer sailer?

    Quote Originally Posted by MN Dave View Post
    The face grain direction is important .
    For "bendability" perhaps, but in regard to strength the difference is insignificant, if at all. In either direction the wood is joined long grain to long grain, which is all that matters. Either will result in the same properties of a plywood sheet. Now if one were to join two panels with the grain of one perpendicular to the other, that would be a different kettle of boiled plywood....

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    Default Re: Exterior ply for trailer sailer?

    Raoul Jackson swears by exterior ply too - well soaked in epoxy and sheathed.
    I use a belt sander for scarfing ply - buggered it up a couple of times, but its fine.
    Wet out both faces and let the epoxy soak in before adding your final gluing epoxy.
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    Default Re: Exterior ply for trailer sailer?

    Greetings all,

    Quote Originally Posted by Peerie Maa View Post
    A hand or power plane will do it. Tack one sheet on top of the other set back the length of the scarf and the lower one aligned with the edge of the bench and the two sheets act as your jig.
    Thanks Nick, I think that a hand plane with sandpaper finish will be the prescribed method.

    Indeed, very interesting thread
    Thanks pandes, hopefully beneficial too!

    Yar. Prezactly. I usually add a sacrificial board under the edge of the lowest piece/sheet to allow the feather edge to be supported without having to eat into the bench. I like a power plane to do the grunt work, followed by a low angle block plane on an angle. I hope you like sharpening!

    Incidentally, I poo-pooed sanders earlier in the thread, but the reality is they work fine with a COARSE grit and steady hand. I’ve cut many scarfs in ply with a low speed angle grinder and 36 grit paper, but I’ve shaped untold number of things with said tool, so I feel very comfortable using it.

    Comfort and skill with the tool is often as large a factor as the tool itself in these cases. Practice will always improve your technique.

    Go slow, use the lines made by the plies as guides, and check your work often. Have fun.
    No power plane yet, plenty of hand planes though. Multiple recommendations for doing the same thing speaks volumes!

    Using the 60 grit on the belt sander seemed to be a good choice, but it was Sunday afternoon and I was concerned about noise to the neighbors and rushed the process a bit. When I put the pieces together they looked like they fit very well. But those ply waves end up being hollows...

    Thanks for the reminder of why I do this, to have fun! I appreciate each minute spent with the wood, but sometimes the challenges can override that and I need to be reminded why I'm doing this!

    In fact, I'm starting to look for symptoms of "analysis paralysis" around here. But I'll ride it out to the end!

    Muchas Gracias Rob!

    The face grain direction is important. You made the fir sample sideways. You should see a big difference between panels tested in different directions. You should make another fir sample with the face grain perpendicular to the scarph. The comparison between the samples with different orientation will be interesting.
    This is one of the reasons I appreciate feedback so much. I hadn't even noticed that before, importance elevated because of just 3 plys. Thanks Dave!

    The Hartley 14 is a really cute little boat. Good luck with it.
    I think so too, thanks a lot Phil!

    So my takeaways are:
    • Stack up plywood as done here for scarf joint
    • Orient the face grain of the plywood longitudinally
    • Rough out taper with hand plane
    • Finish with coarse sandpaper
    • Take pictures of cute boat
    • Have fun!


    You guys are great.

    I'll cut test strips and test them this weekend. Keep those observations, comments and criticisms coming!

    Eric
    Last edited by FishoutaFlorida; 07-18-2018 at 11:39 PM.
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    Default Re: Exterior ply for trailer sailer?

    Quote Originally Posted by Dusty Yevsky View Post
    For "bendability" perhaps, but in regard to strength the difference is insignificant, if at all. In either direction the wood is joined long grain to long grain, which is all that matters. Either will result in the same properties of a plywood sheet. Now if one were to join two panels with the grain of one perpendicular to the other, that would be a different kettle of boiled plywood....
    Yes and no. For sheathing where you are worried about bouncing off a rock, grain orientation may not matter -- if you can pin down all the variables. It is hard to say how much more flexible a small boat would be if the plywood was sideways. For dagger boards and frames, where directional strength and stiffness are important, it makes a major difference, especially if the plywood is thin. For thin 3 layer test specimens, the results may be way off. Thicker panels with many thin layers will behave differently.

    The test should include some pieces without a scarph. It doesn't matter if the panel breaks in the scarph as long as the load is about the same with and without a scarph and the break doen't follow the glue line. If the fiberglass reinforced butt joint is used, then the break should not be in the joint, but it should be at least as strong as the sample without a joint.

    Since there are already some samples with the grain running the 'wrong' way the difference in load to failure would be interesting if the OP has a way to measure it.
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    Default Re: Exterior ply for trailer sailer?

    Quote Originally Posted by Canoeyawl View Post
    Generally speaking, materials are the cheapest part of the boat.
    Well said indeed.
    Never understood people that use crappy big box materials and then have to spend extra materials to protect limitations.

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    Default Re: Exterior ply for trailer sailer?

    Back when I was in a motorcycle club the pithy observation went like this: " What's the cheapest part on a BMW? The rider".
    I see there is some carryover to boats.
    Steve

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    Default Re: Exterior ply for trailer sailer?

    Quote Originally Posted by MN Dave View Post
    The test should include some pieces without a scarph. It doesn't matter if the panel breaks in the scarph as long as the load is about the same with and without a scarph and the break doen't follow the glue line. If the fiberglass reinforced butt joint is used, then the break should not be in the joint, but it should be at least as strong as the sample without a joint.
    True Dave, when I was testing them, the DF always broke along the grain. But maybe more importantly, I should have included some pieces with a butt backed joint as the plans call for. When it gets down to it, it's not just a question of what material to use, it's also a question of which method I should use. I didn't really consider that until yesterday. Oh well.

    So here are the sawed samples of the sandeplywood:

    DSCF7256.jpg

    And here is the DF:

    DSCF7259.jpg

    And last, but not least, my highly technical testing device composed high quality components with sensitive measurement capabilities:

    DSCF7267.jpg

    I'm beginning to think that I could continue on this search for the perfect wood and jointing method, so I'm going to finish this up. I tested 6 samples of each material and really did get some good feedback on the material and joints.

    More later,

    Eric
    Last edited by FishoutaFlorida; 07-22-2018 at 02:08 PM.
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    Default Re: Exterior ply for trailer sailer?

    Keep 'em in suspense my Dad always said.

    So you can see in the earlier cross-section photos that the scarf joint on the sandeplywood was ok, but the DF really didn't come out like I thought it should. Need to try the planer/sander combination in construction I think. Anyway, here are a couple of representative shots of the breaks. Remembering the test rig, my had was always right on the joint.

    Here is the sandeplywood:

    DSCF7271.jpg

    And here is the DF.

    DSCF7270.jpg

    The sandeplywood failure was in the outer ply on the tension side. By feel, this was the stronger material, but it was also slightly thicker. No joint failures.

    The DF failure was similar with failure on the tension side. Again, by feel, this was the weaker material by a lot. Which sort of makes sense due to the alignment of the out plys grain, and the failure of that outer ply was right along the grain. With the ply grain oriented as it should have been, material strength would have been much greater. One joint failure...figured that there would have been more. Maybe there would have been more had I done that scarfing correctly.

    So I started into this little test sequence to get some good feeling for the material I should use for the hull of a 14' Hartley Trailer Sailer, then I started wondering about the method, scarf or butt backed? How did I do? Here are some of my thoughts on the matter:

    1. Apply the KISS principle at all times.
    2. Pay attention to other's observations.
    3. Be flexible.
    4. Keep overall goals in mind.

    But what wood will I be using?

    Never understood people that use crappy big box materials and then have to spend extra materials to protect limitations.
    Not to sure that is the case in this situation. I'll be using glass on the outside of the hull regardless of material. But I think that for my first build, that I'll be using a 6mm 5 ply Joubert Okoume for the hull and a 9mm, 7 ply for the transom. Big price differential but I wasn't happy with the DF's checking and spliting and the sandeplywood wasn't going to do it either.

    With regard to the jointing method? Not too sure on that one just yet. I also wasn't happy with my DF joints and am leaning towards the prescribed butt backed joint.

    Eric
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    Default Re: Exterior ply for trailer sailer?

    Where is wizbang 13 when you need him? he could sum up the next paragraph in about 6 words.

    The point of a scarph joint of a butt block is to get a smooth curve without a leak. These boat designs are more stiffness driven than strength driven. You stagger joints because joints are weak. Scarph joints can be almost as strong as the wood, but if you avoid bashing rocks, you will never break anything. In other words, almost all boats are overbuilt and can tolerate a lot of damage and rot before they fail. It is more important to get a smooth curve than the strongest possible joint. This is not to say that you shouldn't try to make the joint as strong as possible. It is just that a fair hull is more important than a few weak spots. If you couldn't tolerate a weak spot, exterior ply would be an abject disaster and marine ply would be sorely in need of improvement.

    Don't get me wrong. I applaud the effort that you have made. It is valuable. I will be critical, but only on the technical aspects. No disrespect intended.

    OK, good job. 'A' for effort. The scarph joints, well, there is room for improvement. Not to worry, the improvement will come quickly with experience and your obvious willingness to learn. The glue line is too thick. The angles look different on either side of the joint and the angled surface looks uneven. The edges and glue lines in this picture all have to be straight. The waviness tells you that the surface isn't flat. Variations in width of the layers tells you that the angles are all over the place. Filled epoxy is very forgiving, so while there is a lot of room for improvement in the workmanship, there won't be a huge improvement in strength until you turn the grain direction 90°. I think I said before that the strength of wood is about 10 times higher in the grain direction than across the grain.

    I will suggest the Payson butt joint one last time. I get the impression that you're not interested, so I will not push it again. The strength will be similar to a scarph and there is no planing. The whole thing will be glassed over in the end. The most difficult part is turning the piece over to glass the second side. You can do both sides at once. If you lay out the glass and resin for the back side of the joint on a strip of wax paper or plastic film, you can set the plywood on the strip and apply the top side glass.

    I would recommend the okoume, but remember that okoume isn't rot resistant. We use it because it is less expensive, lighter, smoother, prettier and it bends well. We studiously avoid thinking about the durability. We argue for the best of everything and still compromise. When we do worry about durability, we bite the bullet and get the stiffer, stronger, heavier and more expensive meranti. Fir checks unless you glass it. It sounds like you are glassing everything anyway.
    Management is the art of counting beans. Leadership is the art of making every being count. --Joe Finch

  27. #27
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    Default Re: Exterior ply for trailer sailer?

    Some information posted on another site regarding thin plywood.

    "I did some crude experiments some time ago, with sheathing the plywood I am using to build my boat and have been meaning to put the results on the site and was reminded of this in today’s thread on lauan plywood.

    My plywood is 4mm 3-ply, the outer layers are about 1mm each and the core about 2mm. It was described as Far Eastern marine ply, stamped BS1088.

    When I got it, it was obvious that it was very much stiffer parallel to the face grain than perpendicular.

    I was wanting to assess how to improve stiffness at least weight/cost.

    I cut strips parallel and perpendicular to the face grain and used various treatments one side only and also on both sides giving 16 strips in total.

    They were:-
    a) untreated
    b) 3 thin coats of unthickened epoxy
    c) 1 layer 81grm/m^2 plain weave one side only
    d) 2 layers of 81grm/m^2 plain weave one side only
    e) 1 layer of 162grm/m^2 open weave one side only
    f) 1 layer 81grm/m^2 plain weave both sides
    g) 2 layers of 81grm/m^2 plain weave both sides
    h) 1 layer of 162grm/m^2 open weave both sides

    all the cloths were finished with peel ply which was removed prior to testing. Where only one side was clothed the strip was tested with the cloth in tension.

    I tested the strips for stiffness before adding the treatments and the average stiffness parallel to the face grain was approximately 7 times the stiffness perpendicular to the face grain. The strength was about 2.75 greater parallel to perpendicular.

    The results for the improvement parallel to the face grain were


    treatment _______strength____________stiffness
    a)__________________ 1 _________________ 1
    b)__________________ 1 _________________ 1
    c)__________________ 1.1 _______________ 1.2
    d)__________________ 1.1 _______________ 1.2
    e)__________________ 1 _________________ 1.2
    f)___________________ 1 ________________ 1.75
    g)__________________ 1.3 _______________ 2.15
    h)__________________ 1.1 _______________ 2.35

    The results for the improvement perpendicular to the face grain were

    a)_________________ 1 _____________ 1
    b)_________________ 1 _____________ 1.1
    c)_________________ 1.7 ___________ 2.1
    d)_________________ 2.3 ___________ 2.5
    e)_________________ 2.4 ___________ 2.45
    f)_________________ 2.8 ___________ 2.7
    g)________________ 4.35 __________ 4.4
    h)________________ 4.3 ___________ 5



    Given I only used 1 sample per spec. it is subject to random variations that a proper testing system would resolve but I think the results are reasonably consistent and do give an idea of what is happening.

    Where plywood spans in 2 directions the spacing of its supports will determine which direction is critical for failure.

    My conclusions are
    1) that epoxy coating on its own provided negligible benefit in terms of strength/stiffness.
    2)Where the stress parallel to the face grain is critical, sheathing one side is ineffective and sheathing both only improves stiffness with not much better strength.
    3)Where the stress perpendicular to the face grain governs e.g. a multi-chine canoe where the ply tends to span between the chines, significant improvements in strength and stiffness can be achieved.

    Once you go to thicker plywood with 5-ply the difference between parallel and perpendicular will be greatly reduced so the above analysis probably would not apply. Similarly for 3-ply with thinner outer layers and a thicker core.

    It was comforting to see how far you had to bend ply before it went bang."

    P1010030.jpg

  28. #28
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    Default Re: Exterior ply for trailer sailer?

    Quote Originally Posted by MN Dave View Post
    Don't get me wrong. I applaud the effort that you have made. It is valuable. I will be critical, but only on the technical aspects. No disrespect intended.

    OK, good job. 'A' for effort. The scarph joints, well, there is room for improvement. Not to worry, the improvement will come quickly with experience and your obvious willingness to learn. The glue line is too thick. The angles look different on either side of the joint and the angled surface looks uneven. The edges and glue lines in this picture all have to be straight. The waviness tells you that the surface isn't flat. Variations in width of the layers tells you that the angles are all over the place.

    I will suggest the Payson butt joint one last time. I get the impression that you're not interested, so I will not push it again. The strength will be similar to a scarph and there is no planing. The whole thing will be glassed over in the end. The most difficult part is turning the piece over to glass the second side. You can do both sides at once. If you lay out the glass and resin for the back side of the joint on a strip of wax paper or plastic film, you can set the plywood on the strip and apply the top side glass.

    I would recommend the okoume, but remember that okoume isn't rot resistant. We use it because it is less expensive, lighter, smoother, prettier and it bends well. We studiously avoid thinking about the durability. We argue for the best of everything and still compromise. When we do worry about durability, we bite the bullet and get the stiffer, stronger, heavier and more expensive meranti. Fir checks unless you glass it. It sounds like you are glassing everything anyway.
    Thanks for the info Dave, that is exactly what I'm looking for!

    Your points about my scarf cutting are absolutely correct. I still need a lot of practice and using a different technique will undoubtedly help. Ergo the thought about going back the designed joint. I've looked at the Payson butt joint finally and it could be a contender. Thanks for bringing it up again, sometimes it takes once or twice for me to wake up.

    And with regard to meranti, I've had a tough time finding it locally. May need to look a little harder. The okoume is about 3 times the cost of the DF, so if meranti is even more expensive...

    Thanks again and keep 'em coming!

    Eric
    God is great, beer is good and people are crazy.

    Billy Currington

  29. #29
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    Default Re: Exterior ply for trailer sailer?

    Quote Originally Posted by oldcodger View Post
    It was comforting to see how far you had to bend ply before it went bang."

    P1010030.jpg
    Now that is a testing program! Thanks for sharing oldcodger, perfect time and place!
    God is great, beer is good and people are crazy.

    Billy Currington

  30. #30
    Join Date
    Jun 2007
    Posts
    616

    Default Re: Exterior ply for trailer sailer?

    Quote Originally Posted by FishoutaFlorida View Post
    The okoume is about 3 times the cost of the DF, so if meranti is even more expensive...
    What grade of DF is this? I'm in central NJ and I recently needed a sheet of 3/8" marine ply. Locally DF ACX is about $51 a sheet and meranti is $65 and okoume was about $80. Are you looking at CDX? That may be a big mistake.
    Last edited by Dusty Yevsky; 07-23-2018 at 11:45 AM.

  31. #31
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    se pa (Bristol PA)
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    Default Re: Exterior ply for trailer sailer?

    CLC is a good place to get marine ply. in Annapolis not too far. https://www.clcboats.com/shop/boat-b...-cedar-strips/

    Or, take your chances.. all this "testing" just proves what most of us know.
    Denise, Bristol PA, Oday30, Anchor Yacht Club, On tidal Delaware River. my current project; http://forum.woodenboat.com/showthre...0-Ducker-Resto

  32. #32
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    Jun 2007
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    616

    Default Re: Exterior ply for trailer sailer?

    I'm fortunate that ML Condon is about a 40 minute drive for me. All sorts of marine timbers there..

  33. #33
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    Default

    Quote Originally Posted by Dusty Yevsky View Post
    I'm fortunate that ML Condon is about a 40 minute drive for me. All sorts of marine timbers there..
    Well that sounds like North Jersey to me! I love ml Condon it's where we got White Cedar and Sitka Spruce years ago,
    Denise, Bristol PA, Oday30, Anchor Yacht Club, On tidal Delaware River. my current project; http://forum.woodenboat.com/showthre...0-Ducker-Resto

  34. #34
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    Default Re: Exterior ply for trailer sailer?

    Quote Originally Posted by Dusty Yevsky View Post
    What grade of DF is this? I'm in central NJ and I recently needed a sheet of 3/8" marine ply. Locally DF ACX is about $51 a sheet and meranti is $65 and okoume was about $80. Are you looking at CDX? That may be a big mistake.
    Well Dusty, it is an ACX DF from Lowes. But my memory isn't working well, because I remember $25/ sheet of 1/4" and my review shows $37.62 so let me edit my sentence. "The okoume is about 2 times the cost of the DF,..." The cost of 1/4" Jourbert Okoume is $75.xx around here. Thanks for taking the time to send your thoughts!

    Quote Originally Posted by DeniseO30 View Post
    CLC is a good place to get marine ply. in Annapolis not too far. https://www.clcboats.com/shop/boat-b...-cedar-strips/

    Or, take your chances.. all this "testing" just proves what most of us know.
    Thanks for the heads up about CLC Denise, I think that they are a quality house. But their plywood prices are higher, locations adds shipping costs, I can't pick my pieces and Annapolis is about as far away from RAT beach as you can get and still be in the continental US (really only matters on orders over 100# because they have a flat rate for that).

    I also appreciate that my "testing" may be telling folks something they already know. But as I mentioned earlier in this thread, sometimes you need to knock more than once to get my attention.

    Lots of folks out there believe that for a trailer sailer like this, a good quality exterior plywood can do the job. So I set out to see what I could do with exterior ply. Not having the boatbuilding experience just multiplies my questions and reminds me of the definition of assume. This program showed me that I didn't like 3 plys, wondered about durability of okoume and had to come to terms with the price.

    Since I already planned to glass the exterior, Hello okoume!

    Now what about the jointing method? Scarf, butt backed or Payson? Keep those comments coming!

    Back to the boat now,

    Eric
    God is great, beer is good and people are crazy.

    Billy Currington

  35. #35
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    Default

    if I were able and willing to build anymore I would only be using traditional build methods be they canoe, kayak, runabout, skiff, or one I kind of fancy is an electric launch but none of it's going to happen. Lol.

    scarf em up! Even done half a---ed they look better then blocks!

    Maybe I'm cast from a different mold but this shopping around over dollars and cents and not willing to drive a couple hours to get materials but willing to drive a couple hours to use the boat, all seems kind of silly to me.

    Even high-priced materials are still the Lesser part of the build, cost wise.
    Denise, Bristol PA, Oday30, Anchor Yacht Club, On tidal Delaware River. my current project; http://forum.woodenboat.com/showthre...0-Ducker-Resto

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