tansomed vs double ended amas

Discussion in 'Multihulls' started by peterchech, May 28, 2011.

  1. peterchech
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    peterchech Senior Member

    I am redesigning the ama on my tacking outrigger canoe. It was originally designed as a shunter, so the ama is double ended. However i want to replace it now with a higher buoyancy one (it is currently %100 of unloaded displacement). I was on an f-27 the other day, and noticed that the amas were transomed, like most amas. My friend, who owns the boat, told me it is possible to bury the amas in an f-27. So why the transom? A submerged transom can cause a vacuum and slow the boat down. Especially in light of the apparent trend for low rocker amas and highly rockered vakas on trimarans, which can't help keep the transom out of the water...

    My current double ended low rocker ama just slices the water when it is over pressed, partially submerged. No transom when sumberged. So should i bother building a "modern" transomed ama or will a double ender work?
     
  2. garydierking
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    garydierking Senior Member

    There are differing requirements between a small outrigger canoe and an F27. The fore and aft trim of a small canoe will change radically depending on whether you have crew up forward or if you are sailing alone. A heavy skipper sailing alone and sitting aft will drag the stern of the ama. A double ended ama's drag is unaffected by this, but dragging a transom is a drag.
     
  3. peterchech
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    peterchech Senior Member

    Got it gary thanks... i do notice that alot. Guess ill keep the double ended shape then, just build in 6 more inches of sheer to the current, dory style hull. It is currently very narrow, about 5 inches wide at the waterline (though it rarely stays there) and 16 feet long. Maybe ill do ur ama nui instead. How many pounds displacement is that ama if i may ask?
     
  4. catsketcher
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    catsketcher Senior Member

    Different strokes

    Gday Peter

    One idea of a transom is to reduce drag. Strange as it may seem an immersed transom can have less drag than a double ender AT HIGH SPEEDS. If you want very straight aft buttock lines (like a planing dinghy or speed boat) then putting a transom on can help you keep volume to the ends and keep the buttocks straight. If you put a double end on you either need to reduce volume (which you may not want) or curve the aft run (which is often slow at high speeds)

    Your boat won't be that high speed so as Gary says the double fine stern is a good approach for many reasons.

    If you look through trimaran history you will see transom amas on great boats like Toria (Kelsall always liked transom amas) and modern ORMA style tris. Earlier Irens tris had a Shuttleworth style upsweep on the keel line which kept the amas wide aft but reduced volume. Now they just are big all over.

    You can find lovely boats with no transoms on floats too so it all comes down to the design spiral.

    cheers

    Phil
     
  5. cavalier mk2
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    cavalier mk2 Senior Member

    Newick uses mainly double ended amas. Some designers have taken into account the time spent is various speed ranges. An improvement in the speed range where you spend most of your time, usually lower, should increase your overall averages.
     
  6. peterchech
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    peterchech Senior Member

    Good point.

    What do you mean by "high speed" if I may ask? >10 knots? >15?
     
  7. keysdisease
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    keysdisease Senior Member

    What about some strakes or rails to provide some dynamic lift for like zero weight?

    Wouldn't weigh anything, would cost very little, would only come into play when hard pressed, might make the boat a little drier

    I believe it was "Twiggy" that had a designer mod lifting strake on the main hull after one pitchpoled and you sometimes see them on ocean kayaks.

    Steve
     
  8. peterchech
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    peterchech Senior Member

    I searched "lifting strakes" but they come up mostly in terms of powerboats. How would this be done on a skinny ama?
     
  9. catsketcher
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    catsketcher Senior Member

    It has been done on both Twiggies and other tris. A US designer did a powerboat like strake ama in the 80s. He built one boat (about 30ft) and I never heard of another. It was called the Warp or something. Strakes must be inefficient if you try to get them to develop lift as they are really very low aspect foils and so inherently bad news for lift. They would also be a bugger to make.

    Going for dynamic lift from strakes and ama shape is probably not the best idea. The best way to get dynamic lift is with an efficient canted foil. Even Dick Newick stopped using the new moon ama shape.

    Two or three Twiggies had a little foil thing on their bows - about 500mm out of the water. Ian Johnston put foam on the original one too. The problem with both modifications was that if they put themselves underwater they could actually trip the boat quicker.

    cheers

    Phil
     
  10. Corley
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    Corley epoxy coated

    I can recall Ian Farrier mentioning that one of the reasons for his transomed floats was because most crews tend to congregate in the cockpit and on the back of the floats particularly when racing and the extra volume is required to stop the boat dragging it's rear. It's also important in case you get caught during a tack with full sail up there is a remote possibility the boat could pitch over rearwards and the extra volume in the rear of the floats helps prevent that from happening. I must admit from an aesthetic viewpoint though a canoe stern ama can look very attractive.
     
  11. cavalier mk2
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    cavalier mk2 Senior Member

    A canoe stern ama can be made longer to preserve the volume aft and keep a flat run if you want, the only penalty is weight which shouldn't be much.
     
  12. keysdisease
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    keysdisease Senior Member

    I would envision a spray rail / lift strake similar to this one:

    http://www.tacomarine.com/item--2-1-16-x-1-7-16-Rigid-Spray-Rail--V21-0874.html

    Starting just below the deck at the bow and curving aft and down to just above the static waterline about 1/3 back from the bow.

    This would reduce spray and provide dynamic lift when pressed, more as the ama submerged and more lift came into play.

    Cheap, light, easy, might even work.

    Steve


     
  13. yipster
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    yipster designer

    i also recall Ian Farrier mention his boats plane, some stuborn forum members did not belive that and wanted him to prove that in extremes. pitty, otherwise he might still be around on the forum. so.. when a boat -or ama- planes than the hard stern gives less drag, yeah, not planing it might ad drag etc. but think thats one important consideration on transoms. came acros another post on amas with some good links, jim antrim you have to google for tho ;)
     
  14. peterchech
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    peterchech Senior Member

    Plenty of food for thought here... im def gonna do tortured ply here, which would make a round bulge difficult, but eliptical fine. I guess a symetrical double ender will suffice here... maybe extend the lengthfrom 16 feet to 20 to reduce the disadvantages of a double ender
     

  15. cavalier mk2
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    cavalier mk2 Senior Member

    On another note, a few years ago a F boat capsized bow over stern in gusty steep wave conditions. The boat jumped a wave with the bow up and the wind force on the wide webbing tramp nets flipped it backwards with one person losing their life. A net problem, not really the amas but it shows how many things need to be taken into consideration.
     
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