Low-speed sailboat hull

Discussion in 'Hydrodynamics and Aerodynamics' started by laukejas, Oct 12, 2014.

  1. NoEyeDeer
    Joined: Jun 2010
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    NoEyeDeer Senior Member

    Well, even if the decks are full length you can adjust the amount of buoyancy under them by deciding where to put the ends of the tanks. But yes, you don't want too much volume there.

    The classic way of dealing with capsize in hiking dinghies is to swing your leg over the windward rail when the boat starts going over, and drop your weight onto the board. It's not as difficult as it sounds, and in combination with adequate side decks and tanks (and transom flaps) makes capsize recovery very fast indeed. You can stick the mast in the water and bring the boat up again and get underway, and the only bit of you that gets wet are your feet in the bilge water.
     
  2. sharpii2
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    sharpii2 Senior Member

    Wow! that's clever. I don't know if I could keep myself that together during a capsize. But I bet racers capsize a lot.
     
  3. tdem
    Joined: Oct 2013
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    tdem Senior Member

    I don't even get my feet wet 90% of capsizes!
     
  4. Richard Woods
    Joined: Jun 2006
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    Richard Woods Woods Designs

    No, few do, as its slower!

    Richard Woods
     
  5. laukejas
    Joined: Feb 2012
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    laukejas Senior Member

    Sorry it took me a while to write back. I'm deep in my studies now, so there was some catching up to do :)

    I see. Well, since I have very little experience, I didn't want to under-engineer. But if you're sure that 6mm is enough, than it's enough for me too.

    I actually think I managed to source decent plywood in my country. It is described as:

    "Water resistant birch plywood, from I to IV quality classes based on SFS 2413. Meets class E1 requirements, based on EN 13986. Glued with phenol-formaldehyde glues, meets CARB Phase 2 and Japan's 4-star requirements.
    Boil proof.
    Meets these requirements:
    EN 314 / 3rd class;
    BS 1203 / H 4 (known as WBP);
    DIN 68705 Part 3 / type BFU 100.

    Available in these sizes:

    1220 mm x 2440 / 3050 mm
    1250 mm x 2500 / 3000 mm
    1500 mm x 2500 / 3000 mm
    1525 mm x 2400 / 3050 / 3660 mm
    2150 mm x 3050 / 3340 / 3850 mm
    2500 mm x 1250 mm

    4, 6.5, 9, 12, 15, 18, 21, 24, 27, 30, 35, 40, 45, 50 mm."



    So, if this stuff is boil proof, then I guess it's more than sufficient for my project. I'm not sure about all these requirements, but if I take class I plywood, I guess it won't be that much different from marine plywood.

    I redesigned my boat with 4mm plywood for panels and vertical parts of the seats, while other parts are made from 6.5mm. Including transom.



    Sharpii2, thank you for the effort you put into that sketch and description. It was very useful. I actually liked it so much that I re-designed my project based on your sketch. It makes much more sense than my previous design. And It can be compatible with NoEyeDeer's suggestion to raise seats to the shear. Right now, I've left them below sheer, since sheer is slightly hollow, but if I make it straight, I can have seats up high. Not sure why I left sheer with that hollow, but most of dinghies have it, so it must have a purpose. Does it?

    I mean this hollow (in red):

    [​IMG]

    It complicated design somewhat. Maybe it saves some weight or something. Maybe it's just for looks, so that sheer follows the rocker or something.

    Is this really necessary, or can I leave sheer straight?




    Some more screenshots of this re-design:

    http://static.dyp.im/w2IG8Xs1LS/bc6103206dfb8c38820b6528874a59ea.JPG
    http://static.dyp.im/9FRAmpIsXR/4dce21e4f22273a9af72da084a0e75c1.JPG
    http://static.dyp.im/WrhsNlpU79/b448f89c52b66b6a95a5af68fcc8b246.JPG
    http://static.dyp.im/U7t1KUthhw/ec5e6b60548a9add163b8724621d5ce3.JPG
    http://static.dyp.im/p8tSblCs5v/f6421bc9e2a9dd1678c64292a6083b94.JPG
    http://static.dyp.im/WFt36IYgN7/ae7efeefcac5fcc346297656c3513fef.JPG
    http://static.dyp.im/yC8LCEBnz8/211b2b748ec5e5598fe65690436667bf.JPG

    So, apart from obvious changes, I rounded the bow deck to add structural integrity (a friend of mine told me that bends are much more strong that straights), increased the size of mast partner (better to be wrong on the good side).
    Weight now reads 23.5kg (~28kg with epoxy, glass, hardware and paint).

    I haven't yet spaced these parts on plywood sheets, but scarfing will be necessary only for bottom and side panels, so that's good.

    How does this look to you? Also, if you could comment on that plywood and sheer bend, it would be awesome :)










    The word "capsize" brings back a memory.

    Back in 2012, I accompanied my friend and fellow boatbuilder to a local regata in Rubikiai lake, Lithuania. His girlfriend also joined us. He brought his trailable dinghy named "Fortepiano", stitch&glue junk-rigger. I can't be sure, but it was around 2.2-2.5m long, and very wide.

    [​IMG]

    As we arrived on site, it soon became clear that the conditions aren't exactly most pleasant. Dark clouds and strong, cold wind haven't reduced the number of people on water, though.

    [​IMG]

    As local weatherman announced 30 knots sustained, several small boats capsized on the lake before regatta. My friend hesitated to go out on the water with a dinghy this small. It wasn't built for these kind of conditions. His girlfriend clawed on to him, refusing to let him try. I, however, was excited and encouraged him to go for it. After a minute or two, he decided to face the wrath. We began assembling the boat.

    [​IMG]

    As we assembled and rigged, other boats capsized in the background. My friend still hesitated a bit, watching them go over, but I was hooked on, and paid no attention.

    [​IMG]

    About twenty minutes before the start, the boat was ready. Although "Fortepiano" was designed for one person, my friend decided that in these kind of winds the performance would benefit two hikers. Cramped as it were, we managed to get on the boat, and after several groundings, leave the shore against the wind.

    [​IMG]

    The start gun went off, and we sped up along with other boats into unknown. The conditions got worse and worse. Our little boat was overloaded and sat low in the water. Waves crashed against the bow, and wind carried them inside. As my friend steered and sheeted, I was busy with a bucket, taking the water out.

    [​IMG]

    We couldn't exactly compete with cruising yachts, home built LASER replicas and factory made trimarans. Soon, we fell behind. My friend has made several adjustments some time before this race, and as a result of it, we had unmanageable weather helm. We had more involuntary tacks than planned ones. But, with white spray in our eyes and cold wind below our clothes, there wasn't a moment to rest. It felt like Cape Horn.

    [​IMG]

    The rudder was the first to give. Small door hinges weren't designed for such horrible weather conditions and abnormal weather helm. One of them broke clean in half. Stranded in the middle of the lake, we realized that we had nothing to fix it with. Not even spare rope. However, there was a chance. My friend's boat had no flotation tanks, so he placed large, 5 liter empty water bottles in bow and stern compartments and lashed them with string. Hesitantly, we unlashed some of them, and used the string to repair the rudder. We continued on. We refused to give up.

    [​IMG]

    After a while, the last hinge broke as well. With a heavy heart, we unlashed the last of our flotation bottles, and fixed rudder one more time. As we reached the middle of the lake, winds went crazy. We were about halfway to the windward mark, while other yachts were round it. "Tis not the victory, but the voyage that matters", we said. And continued to battle the nature.

    [​IMG]

    The long hours passed. Sad and tired, we saw that with a broken rudder, we have no chance of reaching the windward mark until nightfall. Filled with regret, we turned back to the shore. Downwind.

    And that was when it hit. A gust like I have never seen. We didn't even have time to react. It took our sail, and pressed against the water in a moment's notice. I remember flying out of the boat. The impact. The water.

    When I returned to my senses, it was all too clear. We didn't capsize. We broached. Right over the bow. With waves splashing into our faces, we gasped for air, holding to our dear lives. Luckily, we never neglected life jackets. I don't know if I'd be writing this if I hadn't wore one.

    And then the sheer panic overwhelmed us. All of our unlashed bottles broke free of the boat, and floated away. The sheer line was no more than 5cm above the surface of the water.

    "THE BUCKET!!!" - my friend yelled. The bucket was floating away too. Luckily, we managed to save it before wind took it away. So, we took turns, and while one was holding the boat upright, other was pouring water out with our dear bucket. The task wasn't made any easier by the waves, pouring all the water back in.

    A yacht approached. People offered us help. Wet and proud, we refused. So they took a picture.

    [​IMG]

    Finally, we managed to get the water out. We climbed in, and with our torn sail, drifted to the shore. With rudder completely broken, we couldn't even steer. Half of our sail was dragging in the water. Because the wind changed, we arrived at a very different beach than we have started from. Ashore, we tried to salvage some lumber to repair our boat enough to get back to the start/finish beach, but to no avail. Finally, a motor boat passed by. We swallowed our pride, and let her tow us back.

    As we, for the final time, drifted back to the finish line, everyone else has already finished 3 hours ago. As my friend's girlfriend approached to express her feelings, only one phrase could describe it.
    Hell hath no fury like a woman scorned.

    Later we learned that on that race, several more dinghies capsized as well, one keelboat was grounded, and another dinghy broke a mast. We realized that we got away pretty much unharmed. One of the lessons learned that day was that adequate flotation in a dinghy is a sacred thing, and we should have never sacrificed it for the rudder. It almost cost us the boat.

    It took us few more hours and an inadequate amount of alcohol to warm up. Although we lost, although the boat was broken, we came back safe. And as following races went on, we looked at these white sails from the safety of the shore. Happy. Alive to tell the story.

    [​IMG]


    The end
     

    Attached Files:

  6. NoEyeDeer
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    NoEyeDeer Senior Member

    Concave sheer profiles are mostly just for looks. They look boaty and make people happy. It doesn't have to affect where you put the seats. You could still have them at the sheer with that sheer, if you wanted to.

    And that plywood sounds fine, as long as the veneer quality is ok and voids are minimal. Birch is a bit heavier than some other species, like okoume, but is stronger too.
     
  7. laukejas
    Joined: Feb 2012
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    laukejas Senior Member


    I just looked up density of birch plywood. It is 700kg/m^3, compared to 450kg/m^3 which I designed for. Even if I use ONLY 4mm birch plywood, my boat, as seen in these sketches will weight 31kg. That is, not counting epoxy, hardware, paint, and so on. And I guess 4mm would be too thin for transom and seats.

    Any ideas how to lighten the boat by ~6-7kg apart from changing LOA? I doubt I'll find a source of spruce plywood...
     
  8. NoEyeDeer
    Joined: Jun 2010
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    NoEyeDeer Senior Member

    Ok, first question: how heavy is too heavy? What's your limit?

    You should be able to import gaboon/okoume ply to Lithuania, although it may cost you a bundle. You might (not sure) also be able to find poplar ply of reasonable quality, which is very light.
     
  9. laukejas
    Joined: Feb 2012
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    laukejas Senior Member

    I'll keep searching, but shipping items this big and heavy will cost me more than plywood itself...

    My limit is 30kg for the hull (not including spars, rudder, board, rigging, removable seat).
     
  10. NoEyeDeer
    Joined: Jun 2010
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    NoEyeDeer Senior Member

    Ok, so you might have to start optimising your structure for weight. Couple of things that stand out to me are the for'd frame in this shot - http://static.dyp.im/LOESlId2Fo/13e04b3aaa09f489ca597af467b26806.JPG - and the multiple stiffeners in this shot - http://static.dyp.im/rOZ8X8U4F3/8c1b5d15d2524fb4a46744f96984124c.JPG - under the seats.

    The frame seems pointless, given that the rest of the bottom is not supported anyway, and it would be a nuisance in practice. There's certainly no need for it to run up to the sheer, even if you want extra support for the bottom there.

    The stiffeners under the seats seem to be very close together, particularly on the vertical face which should hardly need any. If the seats are 6mm they shouldn't need any stiffeners at all. Even if you do need some, having them on the flat like that is wasteful of material. You could get the same strength and stiffness with much narrower bits that were slightly deeper.
     
  11. laukejas
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    laukejas Senior Member

    Thanks, NoEyeDeer, but these aren't the latest screenshots of my design. I updated it, and posted new shots of the design in my last big post. Here they are again:


    http://static.dyp.im/w2IG8Xs1LS/bc6103206dfb8c38820b6528874a59ea.JPG
    http://static.dyp.im/9FRAmpIsXR/4dce21e4f22273a9af72da084a0e75c1.JPG
    http://static.dyp.im/WrhsNlpU79/b448f89c52b66b6a95a5af68fcc8b246.JPG
    http://static.dyp.im/U7t1KUthhw/ec5e6b60548a9add163b8724621d5ce3.JPG
    http://static.dyp.im/p8tSblCs5v/f6421bc9e2a9dd1678c64292a6083b94.JPG
    http://static.dyp.im/WFt36IYgN7/ae7efeefcac5fcc346297656c3513fef.JPG
    http://static.dyp.im/yC8LCEBnz8/211b2b748ec5e5598fe65690436667bf.JPG

    As you can see, I removed the frame you mentioned. As for the stiffeners under the seat, if I remove them, total weight is 34.9kg, compared to 35.8kg with stiffeners. Only 1kg taken back. And I to cut down 10kg (down to 25kg, to allow extra weight for epoxy, paint, etc to come up to a total of 30kg).

    It seems not possible in my case. I'll try to downscale the boat.
     
  12. laukejas
    Joined: Feb 2012
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    laukejas Senior Member

    So, as I saw no other option to save as much as 10kg, I scaled down the boat by a factor of .8 (LOA went from 3.75m to 3m), then increased beam and rocker to compensate for design displacement (210kg).

    In my mind, this is not so bad, because:

    1. This boat is designed for extremely light winds, so having long hull with potentially higher hull speed is not very important;
    2. Less wetted surface (2.5m^2 compared to 3.1m^2 in 3.75m LOA version), also helps in low winds);
    3. With box and stern flotation tanks and side seats, it is still comfortable enough for two people, enough space to move and shift weight to trim the boat;
    4. Using same 4mm and 6.5mm plywood, hull will be stronger and more rigid;
    5. Easier to cartop single handed;
    6. Less materials, cheaper and easier to build.

    Or at least I hope so.

    Here is hydrostatics report from Delftship:

    http://www.boatdesign.net/forums/attachment.php?attachmentid=95967&stc=1&d=1416758086

    And here is linespan:

    http://static.dyp.im/VGfTlNsyeA/7fd0a79cfbaba249587001c093650fc1.jpg

    So, assuming I use this birch plywood I mentioned before, with density of 700kg/m^3, and some pine timber with density of 340kg/m^3, dry weight is around 24kg. With 6kg reserve for other stuff, it totals in 30kg, right on the limit. So I guess this could be perfect size for this boat.

    I also had to make sail somewhat smaller. It is now 6.9m^2. Still quite a lot for 3m boat, I think.

    Here are some screenshots:

    http://static.dyp.im/veZOFJgLlA/a690f2b003fde6b35d1e34e1e524f201.JPG
    http://static.dyp.im/vEhly1qT6k/59a311b0070fbd3681676569dd44501e.JPG
    http://static.dyp.im/Ug5Kva32U8/d26bc35709a67350003fe53c3dac9f8e.JPG
    http://static.dyp.im/bN8cFRTPk8/d0c938b70af8bf1de9a4237e58ecd2fe.JPG
    http://static.dyp.im/6BAaM9axd8/32022494018f33bd693a9f4e75c6c092.JPG
    http://static.dyp.im/Py3xTjOF58/539d5147e5efef3abcef8f237150a0c9.JPG
    http://static.dyp.im/qLbpB9ymzs/c32421b3846e7425cd641c66036bbabb.JPG
    http://static.dyp.im/kkWNqtNyFD/52a50a9ab396187bc5a05b4a2a89af61.JPG



    Sharpii2, NoEyeDeer, or anyone else reading this, how does this design look to you? Any potential flaws?
     

    Attached Files:

  13. NoEyeDeer
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    NoEyeDeer Senior Member

    It looks a bit funny. A bit more sheer towards the bow, and bit less towards the stern, would make for a much better looking boat. The proportions of the sail look a bit off too. I don't think a lug with those proportions would set very well. It'll be very prone to excessive twist. I'd suggest looking at the proportions for the lugs on the OzRacer and Goat Island Skiff. They set very well on any point of sail, and look good too.

    I did think of something else though. If this is primarily a light weather boat, why do you want side seats anyway? You'll never have two adults sitting as shown in light winds. You might have one to windward and one to leeward, but the one to leeward will have problems with head and boom interfering. They'd be better off sitting in the bottom of the boat.

    Not having the side tanks would save weight too.
     
  14. laukejas
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    laukejas Senior Member

    Thank you for quick reply!

    I will modify the sheer, then.

    I am not exactly sure what makes lug set well. Sharpii2 suggested that yard angle should not exceed 45°, and that 1/4 of it should be in front of the mast. OzRacer and GIS have much higher peaked yards, however.
    What makes lug set well, then? How do I design such sail (without blindly copying dimensions from OzRacer or GIS) that would set well?

    Well, mostly for comfort and structural rigidity. We'll be sailing long hours. I can make seats much lower, but if I remove them altogether, where do I place the daggerboard box? It will surely get in the way when sitting on that side if there will be no seats.

    If there are no seats, where would you place daggerboard box?

    P.S. By the way, these are balanced lugs. Do you know any good-setting standing lugs?

    P.P.S. And with seats low or non-existing, where do I fit removable center seat for rowing?
     

  15. SukiSolo
    Joined: Dec 2012
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    SukiSolo Senior Member

    Another weight saving mode is to laminate two very thin ply sheets around a lightcore. Say two 1.5-2mm sheets (try aircaft ply) either side of 4-6mm (+?) light spruce/pine/WR Cedar. this would give a reasonable thickness plus lighter weight. You can use 4mm for the transom but will need a frame to take all the loads plus give glue area to the bottom and side planks. In fact you should at minimum generously fillet this joint even if you use 6mm.

    My experience is that sitting areas require 5mm ply WITH reinforcing battens to retain integrity over time ie not crack and split. The battens can be light but you need the section height for stiffness.
     
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