Page 1 of 3 12 ... LastLast
Results 1 to 35 of 73

Thread: New design: 18 foot centreboarder

  1. #1
    Join Date
    Jan 2006
    Location
    South of Butter Brook, Norway
    Posts
    800

    Default New design: 18 foot centreboarder

    Hi all. This thread will detail my next boat which Iím in the process of designing. She will be an 18 foot centreboarder. I am designing her for myself and hope to build her next year. She is an update on my previous boat and the changes Iím doing will try to correct some of the weakness I have discovered in my current boat after sailing here for some years.

    Iíd appreciate your comments and questions��Iím just a self-taught amateur designer, and there are no economical gains in this for me.

    First, Iíd like to detail my current boat, as she is the base upon which Iím Ďbuildingí
    Troll is a small lug rigged boat which I have camp cruised for some years now.

    Her particulars are as follow:

    LOA 5,1m
    Beam 1,6m
    Sa 9sqm

    There is no motor and the mast is lowered for rowing. The mast can be used as the center pole for a cockpit tent which I sleep under. She is V-bottomed with a deeper V in the forefoot, like a skipjack. She has buoyancy tanks in the bow and transom. One rowing thwart and large flat floorboards for sleeping and siting while sailing

    Her lines:
    TROLL.jpg

    Second picture shows waterlines at 20 deg. heeling:
    TROLL 20 krengning.jpg
    Under Iím listing the positive sides and the negative sides that I have found out after some years of use.
    Positive:

    • Open layout plan: The boat is basically open apart form one rowing thwart and the flotation tanks: this has proven flexible for sleeping rowing and sailing. I sit on the floorboards with my back to the rail, this gives a low center of gravity and good support for the back which is important for me.
    • Unsinkable: large buoyancy tanks are a good thing
    • easy build with good V in the front for rowing: The hull shaper required only a small area of double planking in the forefoot to take the twist in the bottom panel. I also think the sharp forefoot gave her a grip in the water which helped her keep her bow up to the wind while rowing.


    Another thing I have noticed after putting her shape into the software is that she probably trims a bit bow up when heeling, I donít know but I have read somewhere that this can help a boat to climb to weather.
    Negative:

    • Strange waterlines: The twist in the bottom panel results in extremely hollow waterlines. I donít think this is good. When the boat heels the effect is even more pronounced. The new boat must have better lines in the forefoot
    • Displacement distribution was wrong, the boat doesnít have enough buoyancy in the aft end.
    • No ballast and low freeboard results in low stability. This is particularly wearing when crossing areas where waves kick up.
    • Lugg rig was annoying and inefficient. I havenít written it of totally yet, but I think a gaff will be better.
    • Taking down the rig for rowing clogs up the interior.
    • Bailing takes time.
    • No place to get out of the wind
    • Not enough space for two ppl


    All in all, the new boat should have more focus in stability and more space. The freeboard needs to be larger and some ballast will be used, the rig will be kept standing. This means rowing is out, except for short distances in dead calm. And I will have to make provisions for a small motor, which is horrible but hey Iím getting old anyway.
    The new hull lines must be better designed. Volume distribution must be better and a round bilge must be used to get a nice wave friendly bow with nice waterlines.

    There is also some practical restriction to the design:

    Building space: here I have decided that 6m is the largest I can afford. So, the boat must be a little bit less than 6m.

    Size of my slip: The with of my slip is 2,1m. This restrict the beam of the new boat to 1,8m.

    Sails: Iím hoping to reuse my old mainsail. This might prove hard since itís a lug and I want a gaff. Also, I donít know if I will manage to get a large enough sail area. Anyway, I feel that with a boat of this size changing the rig later in the prosses is doable.

    Materials: I prefer materials that are easy to work with and easy to find locally. For this reason, I think I will omit for example lead for the ballast. Iím thinking of using water or possibly concrete instead
    Ragnar B.

  2. #2
    Join Date
    Jan 2006
    Location
    South of Butter Brook, Norway
    Posts
    800

    Default Re: New desing: 18 foot centreboarder

    OK, the first drawing is starting up!

    First a quick look at the sail plan: Iím thinking Iíll do a cat ketch. As mentioned before the mainsail will be taken from my current boat which is smaller, so the mizzen must be quite large to make up for the small area. However, I might find someone to take over my current boat and thereby not be bound to reuse the sails. Getting space for rudder, motor and mizzen in the transom area will be a challenge.
    YOLLIS SEGEPLAN.png

    The hull: here I have opted for a mix of flat bottom, strip planking and multichine. The flat bottom gives a lot of displacement in the rear which is good. My current boat is too skinny in the rear. The strip planked are in the turn of the bilges gives lower wetted area and a smooth forefoot. As beam is restricted, I put one hard chine in close to the waterline in order to improve stability. Another chine in the freeboard gives her a bit structural stiffness and some shape.

    Her particulars so far are:
    Loa 5,4m
    Beam 1,8
    Disp ca 650kg (fully loaded including one sailor)
    Ballast: 80 kg

    Letís have a look at the lines plan:
    YOLLIS LINESPLAN.png
    The layout is fully open with a self-bailing cockpit floor with flotation and stowing space under. The small shelter will be open in the back and have a hinged roof to give a bit more headroom. Sitting headroom with the roof raised will be just under 1m, enough for a grown person to recline back and get the upper body out of the wind and rain. A solar panel can be placed on the top, just enough to charge a battery for the running lights. Maybe there will be enough current for a small chart plotter.
    A quick look int Elements of Boat strength by Dave Gerr shows that she strip planking can be around 9mm thick provided it is glassed inside and out. However, Iím thinking the book gives too large scantlings given that the high freeboard makes her unrealistically large in the formulas in the book. This must be considered further. Keeping weight down is essential. Itís very easy to underestimate weight during the design phase and then overbuild during construction.

    For the bottom panel Iím considering 9 mm plywood and then 6 mm for the topsides.
    Attached Images Attached Images
    Ragnar B.

  3. #3
    Join Date
    Jan 2006
    Location
    South of Butter Brook, Norway
    Posts
    800

    Default Re: New desing: 18 foot centreboarder

    Time to look at the 3D model:
    YOLIS MODELL.gif
    The person sitting in the sailing position will have a backrest of 60cm height. This is quite high, 40cm would be enough. So maybe I can add a small seat of 20cm height. The seat can be made foldable to keep the interior as flexible as possible. The foldable seat would make the boat better for people of different heights.

    The down flooding angle is around 50 deg. I think this is quite high for an inshore boat such as this around 40deg would be enough maybe.

    For the two reasons mentioned above it could be interesting to lower the topsides. This would lower weight and make her look better. But with lower topsides I would have to raise the top of the shelter to get headroom, and a short tall shelter would look strange. We will se where we end up!
    Attached Images Attached Images
    Last edited by mizzenman; 08-14-2019 at 12:00 PM.
    Ragnar B.

  4. #4
    Join Date
    Jan 2006
    Location
    South of Butter Brook, Norway
    Posts
    800

    Default Re: New desing: 18 foot centreboarder

    Here are the inclined waterlines at 20 deg heeling.
    YOLIS VATTENLINJER 20 deg.png
    Iím quite happy about the waterlines in the bow area, the strip planked turn of the bilge gives a nice forefoot. In the midship and the transom we see that the waterlines are quite asymmetrical. Difficult to avoid with the beamy transom. Another thing is that she seems to be dragging her quarter when heeled over. I donít know how this will look when she is under speed and the stern wave build up. Probably worse than in the simulation, since the simulation is on flat water. Dragging the transom is not a good thing but most transomsterned small boats seem to do it. What is best, going for a clean run or enough volume in the aft end for stability? We will see where we end up further down the road.
    Also, with the beamy transom I might need double rudders. This would be cool but finding space for them and the outboard will be a challenge
    YOLLIS VATTENLINGER 20 deg_2.gif
    Attached Images Attached Images
    Last edited by mizzenman; 08-14-2019 at 12:05 PM.
    Ragnar B.

  5. #5
    Join Date
    Jan 2006
    Location
    South of Butter Brook, Norway
    Posts
    800

    Default Re: New desing: 18 foot centreboarder

    That’s how fare we are for now. Next steps are as follows.

    • model more structures to get a better estimate of the weight
    • remodel the hull to get CG over CF.
    • Check stiffness to see if the rig fits the hull. Maybe I can go for a taller rig, perhaps a jib headed main?
    • Check ultimate stability: with the relatively small internal ballast, getting knocked down is a reality. The large flotation chambers under the floors will increase the likelihood of turning turtle. This must be checked and prevented.
    Ragnar B.

  6. #6
    Join Date
    Dec 2003
    Location
    Sound Beach, NY
    Posts
    4,311

    Default Re: New desing: 18 foot centreboarder

    Very interesting.

  7. #7
    Join Date
    Nov 2008
    Location
    Schleswig Holstein Germany
    Posts
    747

    Default Re: New desing: 18 foot centreboarder

    Iīll be following with interrest.
    I like the idea of stripping the bootom with lappstrake on the sides.
    For my taste I would do the side decks a little wider, so hiking out is more pleasant.

  8. #8
    Join Date
    Jan 2006
    Location
    South of Butter Brook, Norway
    Posts
    800

    Default Re: New desing: 18 foot centreboarder

    Quote Originally Posted by Max F View Post
    Iīll be following with interrest.
    I like the idea of stripping the bootom with lappstrake on the sides.
    For my taste I would do the side decks a little wider, so hiking out is more pleasant.
    Interesting sugestion! At the moment I predict the exact shape of the side decks and the cabin must be reconfigured in the future since they are important for stability and flotation, particularly in a swamping. I want large open uncluttered spaces but maybe I have to give that up for more flotation!
    Ragnar B.

  9. #9
    Join Date
    Jan 2006
    Location
    South of Butter Brook, Norway
    Posts
    800

    Default Re: New desing: 18 foot centreboarder

    Quote Originally Posted by johngsandusky View Post
    Very interesting.
    Happy to hear that!

    I fear this will be a slow thread since smallets change to the design is quite time consuming!
    Ragnar B.

  10. #10
    Join Date
    Feb 2006
    Location
    Pennsylvania
    Posts
    8,876

    Default Re: New desing: 18 foot centreboarder

    Interesting project. Your requirements and thinking seem to parallel John Welsford's Long Steps project. I wouldn't rule out being able to row your boat, especially with the limited beam it will have.

    Here's John's sketch of Long Steps.

    -Dave

  11. #11
    Join Date
    Jun 2013
    Posts
    3,076

    Default Re: New design: 18 foot centreboarder

    It's hard to see that the stern is wide enough to need double rudders; it's narrower than that of many single-rudder racing boats.

  12. #12
    Join Date
    Oct 2018
    Location
    Vigo, Galicia, Spain
    Posts
    474

    Default Re: New design: 18 foot centreboarder

    Wow

    Wow, magnificent project, mizzenman, inspiring

    This is the spirit !



  13. #13
    Join Date
    Jan 2006
    Location
    South of Butter Brook, Norway
    Posts
    800

    Default Re: New desing: 18 foot centreboarder

    Quote Originally Posted by Woxbox View Post
    Interesting project. Your requirements and thinking seem to parallel John Welsford's Long Steps project. I wouldn't rule out being able to row your boat, especially with the limited beam it will have.

    Here's John's sketch of Long Steps.
    That is certainly a great boat! It wil be interesting to use for camparision. From the text on Welsfords blog it seems his coastline is more rugged than mine, this will affect the design. I dout I will ever be more than one hour away from a safe anchorage. Longs steps looks sleeker and with more ballast than I'm figuring so far. Also the flotation in the cudy is a great idea!
    Ragnar B.

  14. #14
    Join Date
    Jan 2006
    Location
    South of Butter Brook, Norway
    Posts
    800

    Default Re: New design: 18 foot centreboarder

    Quote Originally Posted by Chris249 View Post
    It's hard to see that the stern is wide enough to need double rudders; it's narrower than that of many single-rudder racing boats.
    Thats true, but I had asumed that the desiding factor was the proportion between the greatest beam and the beam at the transom, not the beam at the transom alone.
    Ragnar B.

  15. #15
    Join Date
    Jan 2006
    Location
    South of Butter Brook, Norway
    Posts
    800

    Default Re: New design: 18 foot centreboarder

    Quote Originally Posted by Juan View Post
    Wow

    Wow, magnificent project, mizzenman, inspiring

    This is the spirit !

    !Que bonito este cansion. Hojala mi barquito tendra la misma bellesa!
    Ragnar B.

  16. #16
    Join Date
    Aug 2015
    Location
    Austin, TX
    Posts
    456

    Default Re: New design: 18 foot centreboarder

    I like where you are going with this design, I am following along now. And thank you for sharing your review of the last boat you built, that was good information.
    "Yeah, well, that's just, like your opinion man"
    -The Dude-

  17. #17
    Join Date
    Sep 2018
    Location
    MO. USA
    Posts
    138

    Default Re: New design: 18 foot centreboarder

    Looks as if you have put a lot offorethoughtinto your project.

    I really liked "The Design Ratios A Naval Architect’s Dozen (or thereabouts)" it's a free primer on some basic principles of naval architecture for small craft. By Eric W. Sponberg Naval Architect BSE, PE (CT) CEng (UK) His comment on the center of flotation and moving it forward as the boat heels over is one of the Herreshoff’s tricks has shed some light on the ever changing attributes of a sailboat as it move through the water.
    I am also self taught and working on my Dellenbaugh Angle on a little sloop that I am designing. All though I'am not sure why I am chasing this number, for I already know that every one onboard will be climbing up the windward side in a thick air.

  18. #18
    Join Date
    Jan 2006
    Location
    South of Butter Brook, Norway
    Posts
    800

    Default Re: New design: 18 foot centreboarder

    Quote Originally Posted by Matt young View Post
    I like where you are going with this design, I am following along now. And thank you for sharing your review of the last boat you built, that was good information.
    Happy to have you here
    Ragnar B.

  19. #19
    Join Date
    Jan 2006
    Location
    South of Butter Brook, Norway
    Posts
    800

    Default Re: New design: 18 foot centreboarder

    Quote Originally Posted by John Howland View Post
    Looks as if you have put a lot offorethoughtinto your project.

    I really liked "The Design Ratios A Naval Architect’s Dozen (or thereabouts)" it's a free primer on some basic principles of naval architecture for small craft. By Eric W. Sponberg Naval Architect BSE, PE (CT) CEng (UK) His comment on the center of flotation and moving it forward as the boat heels over is one of the Herreshoff’s tricks has shed some light on the ever changing attributes of a sailboat as it move through the water.
    Yes, I only wish I had knew exactly how the sailing experience will change as the hullshape changes. Will the fatt transom boat be as sweet to sail as the slim transom boat? Will the extra stability make her more funn to sail? I wont know before I test here in real life and thats one of the things that makes this so exiting!



    Quote Originally Posted by John Howland View Post
    I am also self taught and working on my Dellenbaugh Angle on a little sloop that I am designing. All though I'am not sure why I am chasing this number, for I already know that every one onboard will be climbing up the windward side in a thick air.
    Yeah the whole dellenbaught anlge thing is a bit elusive to me. What are you designing?
    Ragnar B.

  20. #20
    Join Date
    Jan 2006
    Location
    South of Butter Brook, Norway
    Posts
    800

    Default Re: New design: 18 foot centreboarder

    Hi again!
    The last period has been spent plopping different material densities into Freeeship and deciding thicknes for different parts. So far, I have the following weight estimate.
    Total displacement of boat, equipment and one grown person: 645 kg.
    My original prediction was 670 kg. So far, Iím quite close. However, everything must be readjusted when we start to get more numbers on the stability.
    Also, we will see if I manage to build de masts as light as Iím hoping. Iím tempted to try a combination of wood and carbon fiber. We will look more at that later
    Object Metric tonnes
    Anchor + rode 0,015
    Ballast 0,084
    Battery 0,023
    Gassoline 0,025
    Deck 6mm 0,016
    Floor 6mm + glass 0,036
    One person 0,104
    Bulkyheads 6mm. 8 in total. 0,023
    Cargo 0,05
    Stringers, keel, strakes, stem 0,031
    Hullplanking 6mm and light glass above WL, 9mm and heavy glass below waterline 0,131
    engine 0,027
    rudder, daggerboard + daggerboardbox 0,029
    Mainmast 1,5cm wall thickness. including boom and yard + 5 kg of tackle 0,032
    Mizzen 1cm wal thickness. Including boom 0,01
    Twarts 0,007
    Sails 0,004

    The center of flotation and the center of mass balances out nicely after I put a large heavy battery under the floors in the cuddy. She will have a large AGM-battery and a solar panel on the roof of the cuddy. It will be mainly for running lights and reading lights. But, if I have enough juice I might try to install a small chart plotter, more as a toy than for any real need.

    I had a look at the dellenbaugh angle:
    New boat: 11,1 degrees
    Old boat: 15,2 degrees

    The new boat has a lower angle which means she will perhaps be stiffer, which is natural since she is larger. Anyway, I feel these numbers donít mean so much they are just fun to compare. The important stuff starts to happen when we look at heeling at 30 deg and 90 deg.

    Now for the SA/Disp ratio, I feel this is more important, here we have the numbers
    New boat: 17
    Old boat: 14
    This is interesting. The old boat has a low ratio, but she seldom feels under rigged too me. Possibly the reason is that Iím a sedate sailor. The new boat should be more sprity and at the same time feel safer with her large down flooding angle.
    Ragnar B.

  21. #21
    Join Date
    Nov 2001
    Location
    Seattle
    Posts
    26,807

    Default Re: New design: 18 foot centreboarder

    Why the gaff rig? My friends who do a lot of sail and oar cruising prefer the balanced lug, which allows them to quickly dowse the sail and strike the mast to reduce windage.

  22. #22
    Join Date
    Sep 2018
    Location
    MO. USA
    Posts
    138

    Default Re: New design: 18 foot centreboarder

    A couple of photos of a 1/4 scale model that I started this spring of a 16.5’ LWL & 27.5’ LOA Sloop, modified full keeled, cine spoon transom, Gaff or Marconi rigging /w a cutter option, with a warpable mast, or a small Lead Sled, Inspired by Herreshoff, Anker, and Cownishield. I know am stand on the shoulders of giants. I have done 12 pages of my own Detailed Drawings, The Tables of Offsets in Excel, and 7 pages of written Construction Specifications for this little lead sled. All I need no is to find a boat full of money somewhere and I can start the full size boat.

    IMG_4061.jpgIMG_4074.jpgIMG_4180.jpg
    Last edited by John Howland; 09-03-2019 at 02:56 PM.

  23. #23
    Join Date
    Jan 2006
    Location
    South of Butter Brook, Norway
    Posts
    800

    Default Re: New design: 18 foot centreboarder

    Quote Originally Posted by johnw View Post
    Why the gaff rig? My friends who do a lot of sail and oar cruising prefer the balanced lug, which allows them to quickly dowse the sail and strike the mast to reduce windage.
    The balanced lugg has two negative sides in my experience. First the sail draws noticeably better on the tack where the sail is supported by the mast. Number two is that the yard tends to bounce around in light air, which can make it frustrating to go to weather. These two things make me assume that the gaff is better. The luff is supported by the mast on both tacks and the yard is better supported.

    I’m not planning to take down the mast when I’m not sailing. It clutters up the boat to much and having a mast that is easy to lower would require that I found another solution for the cuddy.

    So, in short, I want better sailing and a more comfortable interior, but rowing will suffer.
    Last edited by mizzenman; 09-05-2019 at 08:09 AM.
    Ragnar B.

  24. #24
    Join Date
    Jan 2006
    Location
    South of Butter Brook, Norway
    Posts
    800

    Default Re: New design: 18 foot centreboarder

    Quote Originally Posted by John Howland View Post
    A couple of photos of a 1/4 scale model that I started this spring of a 16.5’ LWL & 27.5’ LOA Sloop, modified full keeled, cine spoon transom, Gaff or Marconi rigging /w a cutter option, with a warpable mast, or a small Lead Sled, Inspired by Herreshoff, Anker, and Cownishield. I know am stand on the shoulders of giants. I have done 12 pages of my own Detailed Drawings, The Tables of Offsets in Excel, and 7 pages of written Construction Specifications for this little lead sled. All I need no is to find a boat full of money somewhere and I can start the full size boat.


    She looks gret! I used to dream about long slender boats like that when I was a kid. I never got the chance to sail one so far.
    Ragnar B.

  25. #25
    Join Date
    Jan 2009
    Location
    northwestern Wisconsin
    Posts
    7,532

    Default Re: New design: 18 foot centreboarder

    Quote Originally Posted by mizzenman View Post
    The balanced lugg has two negative sides in my experience. First the sail draws noticeably better on the tack where the sail is supported by the mast.
    Interesting comment. That is exactly the same conclusion I've heard from a number of very knowledgeable, extremely experienced lugsail sailors. Apparently the "bad" tack actually results in better performance. But how is having a boat that does BETTER (in some situations) a bad thing?

    Quote Originally Posted by mizzenman View Post
    Number two is that the yard tends to bounce around in light air, which can make it frustrating to go to weather. These two things make me assume that the gaff is better. The leech [I think you mean the luff/leading edge?] is supported by the mast on both tacks and the yard is better supported.
    This seems contrary to my experience. For one thing, with adequate downhaul tension, the luff of a lugsail is plenty well supported. I wouldn't think the yard would move around any more than the gaff of a gaff rig (purely speculative, based only on my long experience with lugsails). Of course, in any rig, I've found in really light air, it helps to sit on the leeward side to keep the sail filled. It helps even more (for impatient types) to drop the rig and row to windward--much faster in a decent-rowing boat.

    By all means, build a gaff rig for your new design if that's what you want. I'd like to try a gaffer myself. But don't give up on the lug rig just yet--I don't think what you have experienced is typical once you have a well-tuned lug rig.

    Tom
    You don't have to be prepared as long as you're willing to suffer the consequences.

    www.tompamperin.com

  26. #26
    Join Date
    Jan 2006
    Location
    South of Butter Brook, Norway
    Posts
    800

    Default Re: New design: 18 foot centreboarder

    Quote Originally Posted by WI-Tom View Post
    Interesting comment. That is exactly the same conclusion I've heard from a number of very knowledgeable, extremely experienced lugsail sailors. Apparently the "bad" tack actually results in better performance. But how is having a boat that does BETTER (in some situations) a bad thing?
    My theory is that the performance of the lugg on the "bad tack" represents how the lugg should idealy perform on both tacks. and that a sail which has the luff supported on both tacks will perform as well on both tacks as the lugg does on the "bad tack"


    [/QUOTE]This seems contrary to my experience. For one thing, with adequate downhaul tension, the luff of a lugsail is plenty well supported. I wouldn't think the yard would move around any more than the gaff of a gaff rig (purely speculative, based only on my long experience with lugsails). Of course, in any rig, I've found in really light air, it helps to sit on the leeward side to keep the sail filled. It helps even more (for impatient types) to drop the rig and row to windward--much faster in a decent-rowing boat.

    By all means, build a gaff rig for your new design if that's what you want. I'd like to try a gaffer myself. But don't give up on the lug rig just yet--I don't think what you have experienced is typical once you have a well-tuned lug rig.

    Tom[/QUOTE]

    If I'm on the tack withe yard on the leeward side of the mast and the boat hits some wake, the air gets shaken out of the sail easely. I still have 3 or so yeras to experiment with my current boats so I have plenty of time for improvements. My next modification is to mount a line with a loop around the yard that can be tightened from deck to bring the yard closer to the mast.
    Ragnar B.

  27. #27
    Join Date
    Jan 2006
    Location
    South of Butter Brook, Norway
    Posts
    800

    Default Re: New design: 18 foot centreboarder

    Quote Originally Posted by WI-Tom View Post
    Interesting comment. That is exactly the same conclusion I've heard from a number of very knowledgeable, extremely experienced lugsail sailors. Apparently the "bad" tack actually results in better performance. But how is having a boat that does BETTER (in some situations) a bad thing?
    My theory is that the performance of the lugg on the "bad tack" represents how the lugg should idealy perform on both tacks. and that a sail which has the luff supported on both tacks will perform as well on both tacks as the lugg does on the "bad tack"


    Quote Originally Posted by WI-Tom View Post
    This seems contrary to my experience. For one thing, with adequate downhaul tension, the luff of a lugsail is plenty well supported. I wouldn't think the yard would move around any more than the gaff of a gaff rig (purely speculative, based only on my long experience with lugsails). Of course, in any rig, I've found in really light air, it helps to sit on the leeward side to keep the sail filled. It helps even more (for impatient types) to drop the rig and row to windward--much faster in a decent-rowing boat.

    By all means, build a gaff rig for your new design if that's what you want. I'd like to try a gaffer myself. But don't give up on the lug rig just yet--I don't think what you have experienced is typical once you have a well-tuned lug rig.

    Tom
    If I'm on the tack withe yard on the leeward side of the mast and the boat hits some wake, the air gets shaken out of the sail easely. I still have 3 or so yeras to experiment with my current boats so I have plenty of time for improvements. My next modification is to mount a line with a loop around the yard that can be tightened from deck to bring the yard closer to the mast.
    Ragnar B.

  28. #28
    Join Date
    Aug 2015
    Location
    Austin, TX
    Posts
    456

    Default Re: New design: 18 foot centreboarder

    For my Lug, I currently use 2 downhauls, one at at the boom tack and one some distance aft of the mast on the boom. I found that using these two, keeps the sail better behaved.

    I too would like to try a modern high peaked gaff, just curious.
    "Yeah, well, that's just, like your opinion man"
    -The Dude-

  29. #29
    Join Date
    Jan 2006
    Location
    South of Butter Brook, Norway
    Posts
    800

    Default Re: New design: 18 foot centreboarder

    So, since I now have all the weights in the model I can start get a look at stability. The first thing I want to use stability for is to check the dimension of the main mast. I want to try to get the weight of the mast as low as possible. Both to improve stability and as a challenge to myself. Iím not a structural engineer, and I canít find any literature on how to spec an unstayed mast, so Iíll have to go back to the basic engineering and find my way. Iíll analyse the mast as pure cantilever ignoring the axial forces, torsion and shear forces.
    Iím assuming that really thin walls can lead to problem of thin glue lines and possibly cracking of the wood. Iím thinking this can be counteracted by draping the mast in glass fiber or carbon. Carbon is lighter so than will probably be the best choice.
    First letís have a look at the righting torque needed to put the boat on a 30 degrees heel:
    At 300 I have the distance GZ between the center of flotation and the center of gravity at 0,281m
    So, for the hull alone, I have

    Mhull = Disphull x g x GZ = 500kg x 9,81m/s2 x 0,281m = 1516Nm

    Now I want to add some torque generated by the crew. Iím assuming two fully grown persons sitting on the rail will be the maximum crew.

    Mcrew = mcrew x g x (Beam/2 x cos(30) x GZ) = 200kg x 9,81m/s2 x ( 1,8m/2 x cos(300) + 0,281m) = 2080Nm

    Interesting that the torque generated by the crew is greater than the torque generated by the hull.
    OK, so I now have the total torque needed to put the hull 300 over:

    Mtot = Mhull + Mcrew = 3596Nm

    Since the main sail is about 70% of the total sail area Iíll use only that amount of the total torque do spec my main mast. Also, since the torque generated by the mast is working on the mast partners and not at the center of flotation I can reduce the torque further. Assuming the force is working on the top of the mast I get the torque at the main mast partners (This is conservative, the force is probably working somewhere at the middle of the mast by I donít know exactly where).

    Mmainmast = Mtot x 70% x (1 Ė Heightmastpartner/Heightmast) = 3596Nm x 0,7 x (1 Ė 0,5m/6m) = 2307Nm

    So far so good, now letís look at our material: wood.
    I think wood(pine) has a maximum tension of 87MPa under bending stresses. This probably warries with the moisture content of the wood, but Iíll just use the number I have. Now since I donít have perfect wood, since my mast profile will probably deform under bending and other sources of error, I need a safety factor. I think Iíll go for 3. I then get the maximum allowed tension in the wood

    sallowed = swood/n = 87MPa/3 = 29MPa

    The inertia that my mast profile must have to withstand the torque from the hull is:

    Equation A: Imainmast = Mmainmast x Rmainmast/sallowed

    I donít know the radius yet, so I must wait a bit with the inertia.
    The Inertia of a cylinder is given as

    Icylinder = (D4 Ė d4) x π/64

    Where D is the outer diameter of the mast and d is the inner diameter of the mast. Solving for the inner diameter I get

    Equation B: d = (D4 Ė Imainmast x 64/π) 0,25

    By putting equation A and B in to excel and playing around a bit with different outer diameters I landed at an outer dimeter of 11cm which gives me a wall thickness of 11mm. Greater outer diameter gives thinner walls and lighter mast
    I donít know if a mast with such thin walls will hold up in real life. But Iím willing to give it a try. If it breaks, I have learned something and if it holds up I can try to build an even lighter one��
    Ragnar B.

  30. #30
    Join Date
    Jan 2006
    Location
    South of Butter Brook, Norway
    Posts
    800

    Default Re: New design: 18 foot centreboarder

    Thinking of my requirement that the boat should not turn turtle its time to check what happens to the stability at heeling angles around 900. As you recall the boat has a self-draining cockpit. The double floors will have a lot of buoancy. She does not have side tanks. She has a ballast fo 80kg. we will now se if that amount of ballast is enough to keep her from turning over more than 900.
    Under you see the plot of her righting arm. Blue is for the loaded boat without crew. Orange is for loaded boat with one crew sitting in the cockpit floors on the center line.

    1.PNG
    As you see, down flooding happens around 55 degrees. The righting arm is negative from then on and stays negative even after the top of the mast and the yard enters the water at around 94 degrees. Conclusion: to fulfill my requirement to keep her from turtling I need more stability in the knocked down position.
    Now there are several ways to achieve higher stability. I want to avoid increasing the ballast since increasing the ballast will increase the weight of the rest of the boat as well, and I want to stay light.
    One option is to increasing the flotation in the area that is above the center of mass, around 0,5m above the keel. Another option is to reduce the flotation that is under the center of mass. This means lowering the cockpit floor. But this will perhaps cost me my self-draining. So, first step forward is to increase flotation above the center of mass.
    2.jpg
    Last edited by mizzenman; 10-10-2019 at 01:10 AM.
    Ragnar B.

  31. #31
    Join Date
    Feb 2006
    Location
    Pennsylvania
    Posts
    8,876

    Default Re: New design: 18 foot centreboarder

    Is the buoyancy of the spars in the calculations? That's often enough to stop a boat of this type from going completely over.
    -Dave

  32. #32
    Join Date
    Jan 2006
    Location
    South of Butter Brook, Norway
    Posts
    800

    Default Re: New design: 18 foot centreboarder

    Yes they are included, althought its hard to predict their contribution since they are free rotate. So far i have included the yard and the mast and left out the booms
    Last edited by mizzenman; 10-17-2019 at 05:49 AM.
    Ragnar B.

  33. #33
    Join Date
    Jan 2006
    Location
    South of Butter Brook, Norway
    Posts
    800

    Default Re: New design: 18 foot centreboarder

    OK. So as I said earlyer my next step is to add some side buyoancy tanks high upp in the hull. Side tanks. Bellow you se how I have cofigured them along the rail. They are placed above the center of gravity so their contribution to the rightningarm should be positive when the hull is 90 degrees inclined. You will also notice the the double bottom I have planned in order to get the self-bailing cocpit. This volume contributes negatively to the rightning-arm when the boat is lying down on her side.


    1.jpg

    Bellow you can se the new plot of the stability curve. If you compare to the previous plot of the stability, itís quite clear how the sudden drop in stability at the pint of down flooding has almost disappeared. I now have positive stability to 800 inclination, which is an improvement. But Iím still not at my goal: The volume of the spars is not large enough to keep her from turtling.
    I also see that my datapoints at +900 inclination look a bit strange. Better recheck my calculations there in case I have some error.
    2.PNG
    Ragnar B.

  34. #34
    Join Date
    Nov 2008
    Location
    Schleswig Holstein Germany
    Posts
    747

    Default Re: New design: 18 foot centreboarder

    Very interesting thread Ragnar!
    Wouldnīt the stability curve improve if the sidetanks go all the way down to the deck?
    It should also make the building easier.

  35. #35
    Join Date
    Jan 2006
    Location
    South of Butter Brook, Norway
    Posts
    800

    Default Re: New design: 18 foot centreboarder

    Hi Max!

    In order to get a positive rightning arm at 90 deg. inclination the center of flotation have to be above the center of gravity. Since my center of gravity is about 0,5m above the keel I need to add flotation above that point. Adding flotation below the center of gravity will actually affect stability negatively. Actually the problem in my design so far is that the double floors that I want to use to achieve my self-draining cocpit has a lot of flotation and this flotation is in the bottom of the hull, where it damages stability.

    You are probably correct that side tanks that descen all the way to the floor are easyer to build.

    Skjermbilde.PNG
    Ragnar B.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •