High performance small tri project

Discussion in 'Multihulls' started by frosh, May 13, 2007.

  1. Doug Lord

    Doug Lord Guest

    Frosh, I got out my copy of High Performance Sailing and studied the pix of the HSP and based on that and what you've said I think I'm wrong about your ama's. The way you describe "using" the ama's and the way Bethwaite obviously did means they'll rarely if ever be in the water and therefore their position is largely irrelevant.
    Yours look a lot larger than his as best I can tell.
    Looking forward to pictures.
  2. CT 249
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    CT 249 Senior Member

    Okay, I've found the old HSP article I did in '88.

    Dimensions - main hull 4.86m long, 0.46m wide. Main hull weight 27kg.
    Floats 2m long.....longer than I recall them being, actually. Looking at a pic I'd estimate that float beam was about 0.20-0.25 wide but my visual estimate is at odds with the rough scaling of the pic. Frank modelled them on the wingtip stabilising floats used on the flying boats he captained, including the Catalinas he drove in WW2. Total weight 79.5kg. Sail are 16.3 sq m.

    I was wrong about the floats having buoyancy tanks; the only positive buoyancy was their foam sandwich skin.

    The HSP does have a lot of flare forward to stop nosediving. You could stick the deck 6" under water and it would pop out. Normally flare is accepted as a bad way to "cure" nosediving these days, but it may work okay in the HSP as the hull is so slender. The volume does seem to be further forward in the HSP. Frosh's boat could be closer to modern no-nosediving cats (ie A Class) where the deck shape is so low buoyancy but low drag that the boat drives along easily, without "tripping", until it pops up.

    My rough estimate is that the bow of the floats are about half-way along the main hull, so Frosh's may actually be further forward.
  3. Doug Lord

    Doug Lord Guest


    The HSP MkXVII,p 187 of HPS, shows fairly long floats/amas but what is particularly interesting is the large amount of flare to both the main hull and ama/floats-definitely not 'wave piecing'-looks like the hulls might trip if pressed really hard.
    HSP DUO MkXVIII,p319, shows almost no flare to either ama/float or to the main hull. Ama's appear to be proportionately smaller than on MkXVII.
    I find it more and more of a stretch to call these boats (including my own 21 footer) trimarans since they are not designed to sail on the floats/amas. In fact, sailing on the "ama's" would be much slower on both Frosh's boat and the HSP's and on my foiler.
    How about monohulls with buoyancy pods? They almost fit the definition some use disparagingly of "monohulls with training wheels" but that
    is not accurate or meaningfull-these boats sail their fastest when sailed as monohulls with the 'pods" just there as backup(not training wheels). The configuration allows a high powered lightweight boat that is made easier to sail by having the pods and whose speed is not in any way dependent on them(other than for prevention of capsize to some extent both in light and heavy air).
    This type of boat is a natural as a monofoiler using just two foils. But whether it has foils or not the type is much more monohull than it is trimaran in terms of how it actually works-in my opinion.
  4. CT 249
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    CT 249 Senior Member

    Training wheels (as in, the ones on kids bikes) normally are just used as back up in my experience.

    I think there are problems with defining these boats as monos. Sure, they are designed to be sailed on one hull most of the time - but the very fact that you have the pods for capsize prevention changes the way you sail them, because the consequences of mistakes becomes much less.

    Secondly, I think it's torturing the language to say that something that gives a degree of effective buoyancy because of its displacement is not a "hull". Therefore these boats have three hulls, not one. They may be small, they are still hulls.

    Finally, there are other boats that have more than one hull, but are fastest when sailed on just one hull. The late F 40 and F28 tris sailed fastest on one hull, but they were trimarans.

    A modern fast cat, well sailed, sails on one hull almost all of the time, upwind and down. In a typical day around our cat club course, I'd say a top A Class sailor may have two hulls effectively in the water little more than a HSP sailor would. The fact that the hulls are canted shows that the boats are designed to sail on one hull.

    I think from memory, the top A guy (lightweight, ex world champ) was flying a hull from about 5 knots. The swiss lake cats fly them in less, I think. Does that make them monos too?

    I agree that these are not the same as a normal tri, but in my opinion that doesn't make them a monohull.
  5. frosh
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    frosh Senior Member

    The HSP and my incarnation

    Hi all, I have also checked out the photos referred to by Doug of the singlehander and doublehander versions of the HSP in Frank Bethwaite's "High Performance Sailing". These much more than my tri concept are basically optomised monohulls with lateral hulls purely designed to prevent severe punishment by immediate capsize for mistakes or sudden changes in wind strength. I see both HSP's as a type of hybrid between a mono and a tri, probably with more emphasis on the fact that it is a planing central ultra slim skiff, of minimum drag that is the commmonly only immersed one, when trimmed for max speed. It is obvious that both leeward and windward hulls should be kept clear of the water when trimmed to perfection.

    CT I think that your dimensions as in your posting where you recall the article you did in 88 must refer to the singlehander. The doublehander would be closer to 20ft long with quite short planing amas that are designed for emergency use to prevent frequent capsizing.

    My tri has more substantial amas (particularly in width and bouyancy), that are canted outwards at approx. 15 degrees, and designed for the leeward one to always be in contact with the water. This has allowed me to contract the overall beam quite a lot. The overall beam is only 54% of main hull length, while the HSP appears to be closer to 65% in the photo.
    My boat will be a true tri, and definitely not any version of a monohull at all, even though it sort of was, when it sailed previously as a Pacific Proa (starboard tack only). The ama will be pushed significantly into the water by heeling forces, I expect that possibly 100kg downforce might be acceptable at higher speeds. However I expect very minimal immersion of the ama when moving at speed. I also expect that this might be the quickest way to sail the craft rather than depower to attempt to keep the leeward ama clear of the water. An efficient sailboard shortboard does not produce much drag when moving rapidly as it is effectively optimised to plane with little wetted SA. I realise that there is a role from the de-weighting on the sailboard hull provided by the canted freesail rig which I cannot mimic in the tri.
    F. Bethwaite had a total of nineteen prototypes over 20 years, this one is my version 2. It has a different design brief to all FB's 19 versions, and it was my choice not to copy any of his versions at all.
    I believe that my main hull is possibly better than FB's best main hull from dimensions and shape, and also limited sailing experience already. The true unknown is the fully planing sailboard type amas (each with about 200kg total bouyancy). These are to allow the CB (dynamic) to be situated around 75cm to leeward of the main hull centreline, adding significantly to righting moment. I cannot determine from calculations, but hope that drag of the leeward ama, will be less speed robbing, than the reduction in drag on the centre hull combined with increased righting moment, will be significantly speed enhancing. Stability and resistance to further heeling should be fairly huge. This in turn will give us confidence to sail the craft really hard. BTW, I am still confident that the fairly aft ama position, will not lead to pitching forces tending to bury the bow of the main hull. Up till now there has been no indications that it will ever bury, but I could be wrong.
  6. Baronvonrort
    Joined: Jan 2006
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    Baronvonrort Junior Member


    I am not sure which HSP article you wrote or what number HSP you wrote about.

    Vanessa dudley and bob ross did a article and they have different measurements to you so do we believe vanessa and bob or you?

    Production version of singlehanded hsp
    Length- 5.08M
    total weight 84kg
    sail area 18.1 M^2

    It was monolithic e glass/polyester resin for main hull and floats.There was no foam sandwich in production version and it had blocks of styrofoam to prevent in from sinking rattling around in each hull.

  7. Chris Ostlind

    Chris Ostlind Previous Member

    Perhaps the confusion between different boats and quoted dimensions is in the quote above from Sam? That's a lot of boats under the same kind of name. It would be more than easy to have a decent degree of confusion surrounding the details... no matter the source.

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