Tris and Proas - realtive lengths of vaka and ama

Discussion in 'Multihulls' started by Anatol, Jun 1, 2015.

  1. Anatol
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    Anatol Senior Member

    Hi all,
    I see some argue for amas as long as or longer than vaka, in a tri or atlantic proa. I seem some of the speed demon machines are like this, or at least with ama bow fwd of vaka bows. Could someone explain the logic of this, both for an ama with lateral resistance and for one without? For the same bouyancy, I see a longer form has less cross sectional area but more wetted surface. But ther must be more to the conversation than that!
    thanks.
     
  2. Jetboy
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    Jetboy Senior Member

    In my estimation - the float or ama length is dictated by many of the design goals. First it must have sufficient flotation. Second, it's hull form should be sufficiently long and narrow that it has relatively low wave form resistance. With these two parameters you tend to end up with the form similar to most catamaran hulls as striking a balance between length, volume, and efficiency.

    As compared to the vaka/main hull, often the same length is used because it makes sense for shipping and visually it seems appropriate. Moreover most folding or trailerable trimarans have physical space limitations. Usually the floats need to nest underneath the side of the cockpit/cabin where space is made as the hull form has a concave curve. In doing so the cross sectional area is limited, meaning to increase volume, it's hard to do without making the float longer.

    For larger ocean race boats that are not concerned with folding, my guess is that longer hulls make for more stability and reduce risk of pitch poling. That would allow safer ability to venture into higher speeds.
     
  3. upchurchmr
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    upchurchmr Senior Member

    Amas have been lengthened on sailboats to prevent "tripping" or pitchpoling when on a reach.
    There have been a few examples where the ama bow extends fwd of the Vaka bow for the same reason.
    The mast has also been moved aft on extreme day sailors to further provide more resistance to pitchpoling.

    Amas are extended aft to keep the stern from sinking when on a close tack.

    On the other hand you could look at Gary Baginet's foil stabilized trimarans which do none of these things.
     
  4. Anatol
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    Anatol Senior Member

    > Amas have been lengthened on sailboats to prevent "tripping" or pitchpoling when on a reach.

    OK, I guess I was dimly aware of that. Pitchpoling is something one would like to avoid, no doubt ! :) But isn't that more a matter of bouyancy up front than length?

    >On the other hand you could look at Gary Baginet's foil stabilized trimarans which do none of these things.

    like Sid? nice. I've also seen some tris with Bruce foils. I can't see how a foil midships will prevent pitchpoling. How about a small bow mounted lifting foil, like a chine runner in reverse?
     
  5. upchurchmr
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    upchurchmr Senior Member

    The ama fwd bouyancy is part length and part size.
    The farther fwd the less size you need in cross section shape.

    Sid doesn't prevent pitchpoling using the Bruce foil (many people don't know it by this name). In that case it is the lift of the foil and the bouyancy of the bow that combined to provide the resistance. But I believe Sid has a rudder foil also, which holds down the back - therefore keeps the bow from dipping.
    Distribution of the weight in the boat will also help. More weight aft keeps the bow up.
    It really is not one thing, it is the combination, as well as the nut connecting the tiller to the boat (skipper) who controls a lot with tiller and sheets.
    Better question Gary if you want a real answer.
     
  6. P Flados
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    P Flados Senior Member

    I believe Sid operates in both "Foil assist" and "Full foiling" mode.

    Except for Dave Keiper's Willawaw, use of foils instead of relying on ama displacement is "new".

    For both cats & tri's, there is much interest in this "new" approach. The foil option greatly improves stability. For example you can look over the videos of G4 competing in open water racing http://forums.sailinganarchy.com/index.php?showtopic=157244&page=8

    Foiling also has promise of speed not available in displacement mode.

    However, getting a foiling solution to work and meet all needs is a challenge.

    Getting a passive (non-wand) "fast foiling solution" was not even thought of as a real possibility by most until they saw the AC72 boats going at it. After the recent realization that foiling can work, lots are trying, but only a few seem to be succeeding.

    "Foil assist" only can do quite a bit and is not that hard to achieve if it is part of the initial design. However, many do not want to put in the effort for assist only mode when they see the promise of full foiling. However, all three choices (this includes displacement only) can be good choices depending on what the boat is intended to do.
     
  7. Anatol
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    Anatol Senior Member

    P Flados
    thanks for your reply.

    >"Foil assist" only can do quite a bit and is not that hard to achieve if it is part of the initial design

    I'm with you on 'foil assist'. So if one were designing an atlantic proa with foil assist on the ama, with a goal or both lifting the (leeward) ama and preventing pitchpoling, I'm assuming a couple of inclined 'J' foils fairly close to each bow (?)

    The possible downside is substantial lateral resistance in the ama, complicating steering and increasing forces on cross beams. Any opinion on this ?
     
  8. lucdekeyser
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    lucdekeyser Senior Member

    Why not add a "pacific" proa to the conversation? Just put the longer vaka where the ama otherwise is and reduce the design to its utmost simplicity like with the Bullet List
     
  9. Anatol
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    Anatol Senior Member

    lucdekeyser
    I'm very aware of the atlantic/pacific debate. I'm not opinionated. I'm just interested in seeing where I can take the atlantic type. In any case, the Harrys are not pacific proas.
     
  10. P Flados
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    P Flados Senior Member

    I am not sure how to make this work for a proa. I have thought about it a lot and even tried some on the water experimenting. I did not have much luck. Foil shape, foil AOA control and foil steering all become very complex if you try to use just 2 "always in the water" foils. More than 2 foils with foils that remotely retract/extend would be complex, but could duplicate what is used on a cat or tri.

    For foil assist, C foils up front plus rudder T foils seem to be a relatively mainstream solution now. This choice should allow foil placement where things work good for helm balance. Just having the foil action greatly adds to stability and reduces the pitchpole threat. A big key to this concept is providing adequate structural strength for maximum potential foil loads. Smaller amas are probably possible when relying on foil assist, but I do not recall anyone really trying to put a lot of effort into refining this concept.

    I also do not recall a lot of success making a more J pattern foil provide a big assist function on a cat or tri. It might work for a narrow range of speeds, but I think that there is generally just too much lift available. This is where a small AOA change can launch the boat at higher speeds.

    Also boat size and boat mission really matter. It all has to work together when trying to deciding on ama size/shape and the planned "foil assist" contribution.

    Full foiling is a different story. How to balance out lift (up front vs. further back), control ride height and still make it properly balanced for lateral forces seems to be a big part of the current "learning curve".

    Based on what we have seen with the cats, I would bet that an uptip L or J foil on a Tri needs to be pretty far back for heave stability.
     
  11. Anatol
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    Anatol Senior Member


    Thanks for your interesting reply.
    "allow foil placement where things work good for helm balance."
    If I understand correctly, this would imply retracting the fwd foil?
    "Just having the foil action greatly adds to stability and reduces the pitchpole threat."
    could you explain how this works? if ie , there is one C foil extended, somewhat aft.
    "adequate structural strength for maximum potential foil loads. "
    good point!
     
  12. P Flados
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    P Flados Senior Member

    Anatol,

    For a tri/cat, the C foil is usually just aft of the forward beam. This normally works out for a standard rig.

    For a proa, you can try to duplicate a boat that is just a tri with one ama hull removed. However, a tri does not normally have the mast at mid boat. The best options I have seen for improving placement of rig center of effort are either two rigs or a roller furling jib on both ends.

    For most rig choices, the center of effort for the sail ends up no where near where you really want it on a proa.

    Using a simple rig (that ends up more aft than desired) and going for optimum (for balance) foil / rudder placement can be done with a complex solution. This is typically more than 2 total boards with only one "dagger" and one "rudder" in the water at a time. Alternately some proas have boards that can be moved to a different trunk during a shunt.

    If you want a high performance proa and you want only two boards, you probably end up with a more complex rig. Getting this all sorted out for a high performance boat is very hard. I do not recall any claims of a builder that was truly happy with the results.

    For something closer to a "fast cruiser" comprises can be made that are more satisfying.

    The proa layout that seems to have your interest has big advantages with respect to allowing most boat weight to be placed where it helps RM the most. However, the shortage of "success stories" using this layout should tell you how hard it is to make it work out.

    For a smaller craft, I am convinced that tri can be built with much of the theoretical advantage of the Atlantic proa while allowing more optimum placement of the rig & foils. It would be a single beam tri with a sliding beam. It would have smallish amas and would rely on lots of ama foil assist.

    The stability improvement is just part of the bargain for any boat with good balance and plenty of "foil assist" up front and an inverted T rudder in back. Normally waves and wind can easily get "in phase" when pushing hard such that an ama nose is pushed under at just the wrong point. With foil assist, a big fraction of overall boat lift is moved back to the foil and it is moved to be subsurface. Also, in a puff the boat tends to speed up. This greatly increases the "foil assist lift fraction". The T rudder also plays a part in greatly reducing vertical motion at the back of the boat. This all just works together to reduce "hobbie horse" motion.

    The above is just a discussion of reduced boat motion. Reduce pitchpole risk is helped by the reduced motion, but it is also helped more directly. With lots of foil assist, a boat can continue to just smoothly slice forward (instead of either deep diving or having the nose "pop" up) when a hull nose goes just under the surface.
     
  13. Anatol
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    Anatol Senior Member

    Thanks for your long post! Especially the explanation of foil effects, pitchpoling etc - very helpful.

    "I am convinced that tri can be built with much of the theoretical advantage of the Atlantic proa "

    Lets just assume that, however crazy, I'm interested in trying to make an atlantic proa with cat schooner rig work - ie - sail well and be easily shuntable short-handed - preferably without jibs (thought I accept they may be necessary) and with a minimum of foil-fooling and rig-marole (heh) at shunting.

    Strength wise - my logic is to put everything on two substantial crossbeams - mast, rudder/foil, and ama, locked into a fairly light ply vaka via substantial bulkheads.

    With two foils on crossbeams on lee of vaka, I should be able to lock the fwd one and use the aft as a rudder. This woulds be separate from small lifting/stabilising foils on the ama.

    sound viable?

    "RM" ?
     
  14. upchurchmr
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    upchurchmr Senior Member

    You are reinventing Cheers by Dick Newick.
    Of course its feasible - with attention to detail.

    I think you can still buy the plans.
     

  15. Anatol
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    Anatol Senior Member

    yes, maybe. I know Cheers. I'd like to think I'm doing something *a bit* different. Maybe its just the satisfaction of following through the design challenges myself. I'm also interested by an old Australian Atlantic - About Face.
     
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