High performance small tri project

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

  1. Paul Scott
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    Paul Scott Senior Member

    I think someone tried it with a catamaran (Quadramaran?) configuration using 4 hulls up in Vancouver BC. Harris? Can't remember if it was a planing concept or no. Many years ago. Was supposed to be interesting in waves.

    Paul
     
  2. PI Design
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    PI Design Senior Member

  3. frosh
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    frosh Senior Member

    Bottom hull shapes.

    Hi PI, A huge amount of thought, sketching and planning went to the idea that a central hull needed to be optimised to plane efficiently at low to moderate angle of heel, either towards port or starboard. I think that I have hit on something extremely close to optimum. Don't want to say too much at this stage for a whole number of reasons. I will let the forum know after real on the water testing. Remember it has sailed already as a tacking outrigger!
    Any suggestion that 3 IC's joined together proves that a crude tri can plane might be true, but is entirely irrelevant to what I am attempting to achieve.
     
  4. CT 249
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    CT 249 Senior Member

    The only time I sailed the boat, I found that the floats worked as designed; that is, they would develop the planing lift to play a major part in keeping the boat upright. The flex in the alloy beams allowed them to skip without burying and they didn't slew the boat when they hit hard. The small size meant that they were training wheels; you couldn't cruise the boat along like a "regular" small tri such as the Supernova. However they did make the boat vastly more manageable than it would have been without them (increasing roll moment probably helped too).

    The problem with increasing the float size, Frank believed, was that when you did capsize, a large float was impossible to sink and therefore the boat inverted and stayed there. That was part of the reason for the small floats. These had two parts; an outside shell that had ports so that it could flood (allowing you to sink the float when recovering from a capsize) and a small airtight section. When sailed normally the ports did not take up water.

    The boat could be hard to recover, at times, and then it would pop up.

    Frank said he modelled the hull on board design. IMHO, I thought that the concave stern sections were not the right way to go as boards with concave sterns were not as succesful; the concave was up the front where planing lift is generated. However, Frank does his homework and I may be wrong in applying windsurfer ideas so literally.

    One thing that strikes me is that the IC actually works like the windward float of an HSP; the plank is a high-aspect planing surface and when you're going fast and drop in to windward, it normally just planes along.

    In some ways the HSP was just a big Canoe, with a giant fixed plank. Damn fine boat.
     
  5. Paul Scott
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    Paul Scott Senior Member

    CT, the HSP floats rotated like Gougeon's Adrenalin and Victor T floats?

    Paul
     
  6. CT 249
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    CT 249 Senior Member

    Paul, they probably did rotate to some extent because the fore-and-aft separation of the light alloy tube crossbeams was quite small. The main flex, however, seemed to be up and down, which seemed to have a similar effect in allowing the float to give with the seas.
     
  7. frosh
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    frosh Senior Member

    Planing bottom shapes

    Frank said he modelled the hull on board design. IMHO, I thought that the concave stern sections were not the right way to go as boards with concave sterns were not as succesful; the concave was up the front where planing lift is generated. However, Frank does his homework and I may be wrong in applying windsurfer ideas so literally. QUOTE - CT 249 - DISCUSSING FRANK BETHWAITE DESIGNING THE HSP.

    I have done some sailboard design and construction myself for shortboards, apart from my tandem sailboard.
    I built a bottom shape "form" or you might call it "mold".
    I then built 2 boards from this mold, one for me and one for my son. Although over engineered (to last forever, which I expect they will), and about 10kg each, when state of the art would be closer to 6kg. they have performed above expectations, according to myself and son. (We have a huge number of sailboard sailing hours under our belts, and betwen us have owned around 20 different boards).
    The description of the bottom would be as follows.
    Bottom spring or rocker is long and gradual in the mid sections to bow. Small tail rocker in the final 30 cm.
    Slight concave in the very first 40cm from bow. From there going towards stern, dead flat until around 75cm from stern. From this 75cm ahead of stern a slight V is introduced almost impercepible to the naked eye at first, then gradually becoming more pronounced and gradually increasing in size all the way to the very stern. Strictly speaking it is not a flat panel V at all, but has some convexity when you put a straight edge on the bottom, from side to the middle line.
    It was almost certainly good instinct from extensive observation, and reading on my part, and good luck also, but I have been amazed how well this design performed, and handled. We both agreed that is was the most user friendly shortboard to foot steer and gybe that we had ever owned.

    The real point of the discussion above, and I apologise for it's length, is that I used the same "mold" to build my trimaran amas also, before finally trashing the mold, as it was taking up space I desperately needed. What is interesting is that it seems very different to F. Bethwaite's concave tail design that he modelled after successful sailboard design.
     
  8. Paul Scott
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    Paul Scott Senior Member

    frosh, did a 9.5' kind of close to what you are describing- more v in the tail for foot steering, and a bit more concave in front. Although it was spectra, and LEAKED. Good while it lasted, which was not for long! I would call this shape classic. Or epic.

    Paul
     
  9. frosh
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    frosh Senior Member

    Hi Paul, our shortboard design was based somewhat on the STARBOARD carve model. About 2.6m long and 61cm wide, it was designed for early planing, and maximum manouverability, in the open sea in all wind conditions. It had bevelled rails which gradually disappeared towards the tail. Had 1.5mm marine ply incorporated into bottom and deck (like the wood STARBOARD), although they only use a very thin wood veneer. It also incorporated carbon, dynel, in strategic places, fibreglass and epoxy. These are probably the most indestructible short boards ever built. Probably 3-4 kg over optimum weight, but it doesnt feel that it slows them at all. Can hand them on to future grand kids in very likely perfect condition. Fins were hand made and slotted thru a casette like daggerboard case. Each fin cost a few $ each in materials, but many hours work. Could install and un-install a fin in 1-2 seconds, very handy.
     
  10. Paul Scott
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    Paul Scott Senior Member

    frosh, my idea was to keep the water accelerating all the time at speed. Never give the water a break. Was that a concern with your board design?

    Paul
     
  11. frosh
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    frosh Senior Member

    Hi Paul, I don't really understand your question. Please re-phrase, and I will try to answer re the design philosophy.
    Sam
     
  12. Paul Scott
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    Paul Scott Senior Member

    frosh- when I did the board, it was in a series of boards that had the following idea; that at planing speeds, the flow was kept in constant acceleration, so that the flow didn't want to separate very much from the bottom. Or in other words, keeping the boundary layer thinnish. So the tactic was to shape the bottom so the flow was always being speeded up. At first we started out with a concave bow and added a gradual v, hoping that the flow would be accelerated from the middle (or top) of the concave, out from the v to two concaves which ran back to a flattish, or v'd stern, and let whatever de-acceleration happen in the wake. Then we went to what you are describing. (Although we usually went with tucked under rails to the edge of the concaves underneath. Held on to waves better.) My question was whether this was your theoretical approach? The constant acceleration idea that is. There was another idea that came from Sweden, that you wanted the middle cross sevtion totally turbulant, and the edges were where the weight was supported and the flow was smoother- looked kind of like this; __-------__ with square edges/sides. The square concave was usually about 1/4 to 1/2 inch deep. I tried one that looked like a cubist Hickman Sea sled- complete crap! It worked, but the envelope seemed kind of narrow. Well, really narrow. And the bow stayed down. Veeery entertaining. Hope this is a bit more coherent.

    Paul
     
  13. frosh
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    frosh Senior Member

    Hi Paul, I am struggling a little to fully follow exactly what you were trying for in your design but I will attempt to answer as follows:

    The water flow being in "constant acceleration". Unlikely that this can actually be achieved, and if so why? Is it anything to do with Bernouli's principle?
    "The pressure of moving air explains the upward lift on the wings of aircraft. When the air flows faster over the top on the wing then below, there is an upward push called lift".

    When designing a shortboard it is always designed with planing as the only possible scenario. Apart from limitations of the rig and sailor, the main issue is wetted surface reduction, and control of the board via the feet. Any sort of concave bottom or the _------_ shape (also a form of concave) will increase hydrodynamic lift at marginal planing speeds but is actually slower at rapid planing speed.
    Tucked under rails were the most common treatment for rail shape as it was a good compromise between a board that could be easily manouvered and had moderately sharp rails for clean water release to minimise water drag on the hull.
    Starboard were the ones who introduced a variation of the tucked under rail with a sharp bevel ( instead of a well rounded arc), at around 45 degrees and about 15mm in width.
    It was believed to allow good manouverability with the advantages of a hard sharp rail as used in the quickest slalom boards. I thought that the idea was probably going to work as they claimed. I would say our opinion of the performance of the finished product bore this out as a definite advance over the tacked under rail for most types of sailboards.
    My aim was for the board to plane early, mainly due to some bow concavity and more than average width in the middle region. The flat bottom here was for the best compromise for max planing with minimum drag. At higher speeds there should be no bow nor mid sections ever in the water and only the last 60cm. of tail approx. This area needed to be slightly veed to avoid pounding in chop, to allow some small degree of softening the ride through rough water. It also aided rail to rail transitions when initiating a gybe, as the tail section was easily rocked over from one slightly vee'd panel to the other. As I stated in an earlier post these panels were vee'd but very slightly convex.
    I believe that this facilitated slightly better control and ride as the bottom orientation to the water was always slightly changing. This explanation when read with my previous posting pretty much covers the whole design philosophy of my shortboard.
    The fact that I have use the same bottom mold for the tri amas was partly a matter that it was convenient, and that I believed that for planing amas, many of the principles would be the same.
     

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  14. Doug Lord

    Doug Lord Guest

    tri

    Frosh, from that perspective combined with your previous drawing I'd agree with Tom that the "ama's" are too far aft. Looks to me that the dynamic center of lift of the amas would be behind(or at best lined up with) the CG of the overall boat+ crew; is that right? And thats not even considering a desirable forward positioning of the lift to help counter rig induced pitching. Many tri's are designed so that the CB or center of dynamic lift or both of the ama's are ahead of the CB of the mainhull for this reason. I'm interested in your thinking here...
    I'm curious about the overall beam: the load on the planing hulls would be reduced and their effectiveness would be greater with more overall beam. I'm fairly sure the HSP was proportionately wider. What is your rationale?
    Just as a suggestion ,why don't you restart your topic under "multihulls" and add these sketches so everything will be in one place.And maybe you could post some pictures there too.
     

  15. frosh
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    frosh Senior Member

    Float fore and aft position.

    Out of all the posters on this forum only one (to our knowledge) has actually sailed the Bethwaite HSP, and that is CT 249 (Chris Thompson). I am truly encouraged by his quote above. My concept is definitely not in the design style of the Farrier tris'. I have unashamedly based by boat in the HSP which was fascinating to see just sailing in a photo, which I have seen. I have contacted Bethwaite design to attempt to obtain such a photo. If I can, I will post it on the forum.
    In the the 80's when the HSP was created in it's final incarnation, and was sailed by a journo with Julian Bethwaite, they claimed a top speed of 23 knots on the day; their first outing together on the HSP, remember no lifting foils. This was a phenomenal performance.
    Doug, the main hull shape was heavily influenced by the recent Int. Moths prior to lifting foils such as the "Hungry Tiger". However I wanted a boat that had higher power to weight ratios, and less loading on each square foot of hull bottom. Further the HT Moth was very canoe like in design, emphasizing low drag in high speed dispacement sailing. I was determined to allow my design to break out easily from displacement to full on planing at moderate wind strength. I modified the bottom shape accordingly. It works, I have sailed in 15 knots plus of wind a few times, and there is no doubt to me as skipper that the main hull clearly lifted and very noticably increased in speed at the same time. Being a proa we got the best performance with the slim ama to windward and flying clear of the water.
    Essentially what we had was a 5.6m x 40cm beam Moth for 2 crew with main sail and large jib. and it was for long periods sailing as a ultraslim monohull.
    Problem was that the concentration levels demanded of 2 guys, each controlling one sail with high power to weight was too demanding and swims were fairly frequent. Frequent recoveries from capsizes meant early exhaustion for the crew. Hence the re-evaluation of the project to maintain or improve on the original performance and reduce dramatically the possibility of capsize. Also I realized that it needed to handle identically on either tack as under the mental pressure in strong winds, my brain did not fully adapt to the different configuration and sailing requirements needed on each tack. (Being completely different). Also I have 2 assymetric kites that I have never had the confidence to launch yet due to various instability issues. The new tri arrangement will pop a kite every time that we steer about 110 degrees off the wind. This is when we expect to see the spectacular performances I have been dreaming about. Obviously it is unknown quantity now!
    You have to consider my boat as a giant planing Moth for two, rather than a conventional tri of any previous style out there.
    Now about OVERALL BEAM:
    The bigger the better for some designers, but loads on rigs and hulls increase hugely as the rigs become much more powerful which seems the logical way to go when one has increased the beam to around square. Remember that I had built essentially a 2mm thick marine ply main hull even with the arced bottom shape, and lightweight WRC framing. I didn't know how much force the whole boat could take without breaking up. My belief is that a small optimised rig on a low drag ULTRA LIGHTWEIGHT boat would perform as well as boat twice as heavy with a huge rig. The other factor was that unlike all typical tri's, righting moment would come from a very active crew hiking out to the max rather than the leeward ama providing buoyancy derived lift to resist sail heeling forces. My Overall beam is 10 ft. for a hull length of 18 ft. 6 inches. With one crew on a trapeze wire, structural loadings from the windward stay can dramatically reduced, and that is also an important part of the design philosophy. One more experiment is whether The James Wharram gaff rigged soft wing sail can function on a high performance boat. So far, testing has shown that it probably can, but much more work is to be done. Also we have recently stiffened the tiny section round section carbon mast with adjustable triple diamond stays. Without the diamonds the mast would be falling away far too much losing most of the rig power. No photos yet, but they will come! At that stage I will move the tri part of this thread to a new thread.
     
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