Small Trimaran; Ama design?

Discussion in 'Sailboats' started by science abuse, May 27, 2010.

  1. science abuse
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    science abuse Junior Member

    Ihis is for a 16ft sailing canoe project.

    In the interests of reduced drag, I'd always subscribed to a steep-angled hull profile for the ama's, due in no small part to its popularity amung employed boat designers.

    But a thought crept into my head: a steeper shape dives deeper, and youhave increased surface area interacting with the water when heeled over, which means more drag.

    Has anyone used an ama design that planes rather than "cuts"? Something with a somewhat flat bottom that becomes more effective as the boat leans on it harder? this would also open up the option of a smaller ama, as planing would (to some degree) displace the eed for bouyancy. Would I then need a centerboard keel, since the amas would be slipping across the top of the water?

    "Sailrocket" seems to employ something like this under its wing, though that craft is asymetrical and keeled. It also seems as though the ama under the wing isn't loaded, since the wing seems to be meant to provide vertically neutral force.

    Thanks in advance!
     
  2. Fanie
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    Fanie Fanie

    Hi Science,

    The same as a deep ama, a flat surface one will also have a lot of surface cantact. The right ratio between depth and width should give optimal.

    You have to exceed a certain speed for a hull to plane, depending on the weight also.
     
  3. science abuse
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    science abuse Junior Member

    This is true. The theory is (as long as I'm handling the boat right) the load on the ama will increase right along with speed. As she heels over with increased windload, speed will also increase with said windload.
     
  4. Doug Lord
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    Doug Lord Flight Ready

    SA, Frank Bethwaite designed some high performance small trimarans with relatively short ,amas with a flat back end that planed. His amas were designed ,primarily just to aid static stability but as you can see in the picture could help a bit at other times!
    My personal opinion is that, on the type of boat you're considering, a narrow14/1 ( Length divided by beam) ama hull might work best since it is unlikely that a standard canoe hull will go fast enough to have an effective planing ama.
    Now, if you had a powered up international canoe type boat then a planing ama might be worth considering if you didn't want to use the sliding seat, trapeze or rack. The IC has about 100 sq.ft. of sail on a 17' hull-some also use spinnakers.
    Once you decide on the sail area you can experiment very easily by using something like a carbon or much cheaper aluminum tube
    and carved styrofoam "hulls" to find out what works best with your boat. You can use a thin piece of plywood as a center "keel" with styrofoam on either side, hand shaped. Probably won't even have to glass the styro until you've found a shape you like. This allows very inexpensive experimentation.
    Good Luck!
    ---------
    Click on image, then click on resulting image for max size:
    HSP
    kayak with single aluminum crossarm-these amas appear too short to me(higher drag than longer/narrower)
     

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  5. science abuse
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    science abuse Junior Member

    Sail area is, if I recall, 82 sq ft. She'll be sloop rigged, and I'm not planning to play with jibs or spinnackers just yeat. I need to make sure the mast can handle the mainsail first. :)
    The mail hull will be Coleman Scanoe:
    http://www.northwestcampers.com.au/Scanoe.JPG

    It's not a traditional canoe design, though I'm not sure it's hull is different enough to merit a change in philosophy.
    Perhaps I should take a note fromt he professionals:
    [​IMG]
    Not flat, but not sharp; Rounded with a pointy bow.
     
  6. Doug Lord
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    Doug Lord Flight Ready

    I think the best solution is to use styro, wood and aluminum and test some experimental ideas-both "planing" and skinny and combinations. you could initially just test on one side. Simple,effective and you'll get what works best with your boat. Slightly time consuming but better than designing and building the wrong shape. A suggestion: go with as much cross arm clearance and beam as you can-the more beam the smaller the ama has to be.
     
  7. Fanie
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    Fanie Fanie

    I think that flat hull or near flat hull was designed to plane ! Hardly a comparisson as to what you can get out of a canoe speed wise ! Unless you put an outboard on it that is ;)

    Dough, that 2nd picture of yours - I can see 2 of those ama's and the small sail just so you don't have to row. You can just use the oar to steer with :D
     
  8. Doug Lord
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    Doug Lord Flight Ready

    ==============
    I agree! Thats why I said this in my first post: "My personal opinion is that, on the type of boat you're considering, a narrow14/1 ( Length divided by beam) ama hull might work best since it is unlikely that a standard canoe hull will go fast enough to have an effective planing ama."

    There are all kinds of fun ways to experiment with this....
     
  9. Fanie
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    Fanie Fanie

    Best way is probably to get out there with some fishing gear :D
    Bugger sea trials, it's got to withstand the fish trial ;)
     
  10. Doug Lord
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    Doug Lord Flight Ready

    ============
    Thats great!
     
  11. science abuse
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    science abuse Junior Member

    The nearest lake for "sea" trials is a 45 minute drive, however balsa scale modeling isn't out of the question.

    I'm actually going to be using foam throughout the boat. There will be decking fore of the step, to help keep water out. I'm a firm believer that there should be no place on a boat that you cannot step, so I'm making it good and sturdy. Here is the longest deck rib, 18oz and in fashionable pink:
    [​IMG]

    The logic of widening the beam is sound, however the aluminum aka's weigh more than the foam of the amas, plus she should come about easier with a narrower beam. I'll need to find a compromise.

    For some scale, below is the same boat with my "diving" rig on it. I wanted added stability for getting in and out while freediving, and I set them at 6ft to put the oarlock pivots in the perfect spot:
    [​IMG]

    The amas were the first I'd ever made, and my first time using foam and fiberglass. I found that the plywood was unnecessary. They weigh more than double what they need to weigh at only 8ft long, but I'm pretty sure they could pierce the hull of most Bayliners. :p
     
  12. DrCraze
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    DrCraze Junior Member

    Put the flat side of your amas out so you get a little lift while you're heeled over. Might give you a little added resistance to leeway as well.
     
  13. science abuse
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    science abuse Junior Member

    So applying wing foil physics to the ama in the water?
    I'm actually trying to come up with ways to lengthen tose amas and strip the wood from them. As they are, I could rotate them around and try each surface. If I spun them 90deg, I'd have the psuedo-flat bellied Ama that I mentioned in the first post. I'm a bit concerned that they're too small for that much sail.

    Follow up question: what is the logic behind the bow disign of modern multi-hulls? seems upside-down compared to conventional designs.
    [​IMG]
     
  14. Guest625101138

    Guest625101138 Previous Member

    I have attached another view of BMW Oracle's hulls. It gives a better idea of the proportions. You also need to note the curved dagger boards.

    Note the the amas are longer than the main hull. This gives them low drag at fast displacement speeds. They will give some lift at very high speed but they have enough buoyancy to carry the full displacement.

    The central hull does not play any role hydrodynamically when the boat gets going. So the amas are the important hull from a sailing perspective. The dagger boards provide vertical force component as well as resisting leeway.

    You can make very simple amas using flat panels. Make them large enough to carry the total displacement plus about 20% reserve. Ensure the beams are high enough to not clip wave tops and are aerodynamic in section.

    There needs to be a bit of rocker in the bow to give bow up at speed. The underside of the hull should also be flat to improve dynamic lift. Avoid flaring the bow out to the deckline. In fact if anything you want the deck very narrow at the bow and a "V" shape so it will rise easily through waves. The amas will wave pierce and you do not want them to bury deeper.

    Obviously if an ama is the only hull in the water then it is best to fit the rudders and dagger boards to the amas rather than the main hull.

    Rick W
     

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  15. Fanie
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    Fanie Fanie

    Some good comments Rick.

    Science, that is my kind of cat. Just check the width out. If you fish with your buddies you can be out fishing all day and be glad when you meet up at some stage eh :D

    It sucks to be so friggin poor eh :(

    Personally I like big ama's. If I was you I'd forget the racing sailing, it will require expensive materials and I don't know if such a speedster would be achievable with standard stuff in such a small size. Instead concentrate more on something practical and pleasurable, without being a sloth either.

    The main requirement however is wind.

    I like that pink btw. It's going to bring out your femine side some :D
     
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