Small Tri's under 20', any mention of foils is banned..

Discussion in 'Multihulls' started by waynemarlow, Jan 13, 2015.

  1. Doug Lord
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    Doug Lord Flight Ready

    Pitchpoling will always be an issue with too wide trimarans with too much sail area not using foils to create pitch resistance. The question is :"whats too wide and whats too much sail area?".
     
  2. waynemarlow
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    waynemarlow Senior Member

    Interesting question, all the 30 - 50 ft Tri's sucessful modern designs that have made it into production, seem to be in that 75% width to length, using about 150% - 180% of volume to over all weight, almost central hull length amas. I'm sure Doug will have better details on that than I but I'm using a broad stick Stat on that, correct me if I'm wrong ( and I'm not including racing tri's in that )

    But at 20 ft we have a whole different world where the crew is likely to almost weigh as 1/2 much as the boat. In playing around with say crew weight at 140kgs at about 1.2m from the centre line one can see the effect on the righting moment.

    I asked the question earlier about 4.5m or 5m overall beam without anyone really coming back to answer, am I getting the feeling that width maybe more of a problem in things such as Marina berthing, trailing, sea way handling etc than going optimum width to gain max RM
     
  3. Doug Lord
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    Doug Lord Flight Ready

    Power to the People

    There is a design conundrum in using all the power that a trimaran platform can provide that you don't have so much on any other type of boat. It is unsafe to use that power with no other modifications mainly because of the consequences off the wind. Solving the problem opens up a rather large performance window which hasn't been explored very much on small under 20 trimarans. The fact that very few, if any, small tri's have been designed to use all the power possible with the ultimate trimaran platform is unfortunate and a major reason why small tri's are often talked about as being slower than small cats. But there is a monster* in there just waiting to be turned loose by innovative design and the benefits will accrue to daysailer types as well as to well designed weekenders.It's a whole new branch of small trimaran design that is ,as yet, untapped.
    *a good, kind, easygoing yet very fast monster
     
  4. upchurchmr
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    upchurchmr Senior Member

    Doug you just can't be the slightest bit polite can you?
    First its that cutsie s**t with the f**l, then you don't even fake it.

    You got your own thread to address f**ls.

    One question, if foils are the answer to capsize then why did they not help in the America's cup, and why did the multiple capsizes happen in the 45's?

    Sorry, I really don't want to hear the answer here. Please answer on your own thread.

    Oh, another question I don't want you to answer. Why did the previous AC where there was an over square tri not capsize?

    My apology to Wayne, I can't seem to follow my own advice.
     
  5. Doug Lord
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    Doug Lord Flight Ready

    Here are the answers to the questions you don't want answers to. There are some aspects of trimaran design which Wayne brought up--sail area and beam- that can't be properly addressed without consideration of all aspects of modern technology especially f---s.

    ====
    * USA17:
    Length-113 ft (34 m)

    Beam -89.9 ft (27.4 m)
     

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  6. 2far2drive
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    2far2drive Senior Member

    Im with you. He just cant quit... he will say the word or blank it out... proselytizing it to no end. Doug, you are not welcome here, seriously.
     
  7. Corley
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    Corley epoxy coated

  8. Gary Baigent
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    Gary Baigent Senior Member

    Sorry, Doug is correct, the AC45s did not have lifting f**ls; they were conventional catamarans ... to those who don't want to hear a reply.
    However I agree Doug can overdo the f**l unmentionable thing but ... I'm afraid, he is again correct; the future of high performance sailing is going to gravitate around f**ls of differing designs. Any swinging d**k can see that.
     
  9. upchurchmr
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    upchurchmr Senior Member

    Gary,

    I agree he is correct about the utility of lifting f***s.

    Sorry I made an obvious mistake about the 45's.
    Please address the 72 somewhere else.

    I, like others, just don't like getting smacked in the face everytime I come to this thread.

    I personally will probably not go to lifting foils due to the expense, and the irritation from Doug. Petty? maybe, but it is a real reaction.

    How would you like me to talk about motorcycles are faster everytime Doug says foils?

    Again I'm going to try not to respond. Sorry to everyone. It was a bad day at work.
     
  10. upchurchmr
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    upchurchmr Senior Member

    Oh I see Doug answered.
    Pitty he is finally on my ignore list.
    Petty?? Probably.
     
  11. Gary Baigent
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    Gary Baigent Senior Member

    Ahem, well back on topic, no hydrof***s trimarans.
    Now if you want a simple, uncomplicated and lightweight tri (without mentioning the unmentionables) the Gerald Holtom concept is the way to go.
    Am posting the image of his tri again. So you can study it. Notice the triangular float is set at an angle so it fools the water with the trickery of higher flotation. The only lighter multihull concept than this is the proa ... and then not by much. In fact both designs would probably be of equal weight. So if you want a 20 foot tri without unmentionables, this design would be hard to beat in terms of construction simplicity, yet provide very high performance.
    The other image is of course, a much larger boat (and not suited for this thread) Misguided Angel ... built in the late 1980s by a crazy Kiwi. It worked okay except the designer thought the angled floats (plus attachments) would be adequate for stopping leeway ... but this was not the case, boat needed a central vertical dagger. Also the windward float (and its unmentionable) dragged; boat needed beam dihedral to lift it clear - like the Holtom design.
     

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  12. rapscallion
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    rapscallion Senior Member

    I like the Catri approach:

    Wider beam, smaller amas, with angled straight boards on the amas for lift and stability. I have had the pleasure of sailing on a catri 27 in 6' waves and 20 knots of wind...and that boat was stable. With a boatspeed of 15 knots, I could set my beverage on the deck and not worry about it spilling.

    The other nice thing about the catri is the winglets on the stern of the amas seem to help a great deal with pitchpole. We pushed the boat hard and it was always stable.

    Like Doug, I believe there is a monster trimaran waiting in the shadows. I'm just hoping for a T20 design to take off and rival the thistle, lightning, flying scott and scow classes. I want a fast, fun, multi class that is as accessible and the classes previously mentioned, with out the required acrobatics of beach cat racing.
     
  13. AnthonyW
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    AnthonyW Senior Member

    Back to the ama question on small tris

    I have a question I have been meaning to ask, and it is driving me nuts. Ama volume %. What is this precisely based on - one or two amas, and volume to which weight. Have looked this up in loads of boat building construction books, read it in countless articles, but there is no clear definition. Would appreciate a nudge to get to the lightbulb moment.
     
  14. AnthonyW
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    AnthonyW Senior Member

    Speaking of ama volumes...

    Not being an engineer - but I would have thought whilst long elegant ama's on a small tri look great, surely there should be a cross over point where the wetted surface area for a small volume should direct one to something more bulbous for so small a size? So many of the popular small tris in my mind have long thin outriggers. Look elegant - but is this optimal? Or is it a function of weight distribution with a crew of more than one?

    I would have thought amas along the lines of Randy Smyth's Sizzors design would be more in order. Otherwise unless one is flying two hulls, the cat would surely always have the edge only dragging one hull.

    Speaking of which - Sizzors is quite remarkable. For those wanting to see a trimaran that is small and extreme - do look it up. Sadly it looks like an expensive beast to replicate for an accessible class, and no doubt requires some experience.

    (I don't know if it has unmentionable appendages - but for the sake of global peace lets pretend it doesn't and not mention the war.)
     

  15. upchurchmr
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    upchurchmr Senior Member

    Anthony,

    At light winds you don't need an ama at all.
    The wider beam means you can use your body weight to balance the force from the sails.

    In medium winds you will reach a point where you can't continue to completely balance. The drag will be from both the main hull and the ama, gradually shifting from mostly main hull to ama as the wind increases. You are probably sailing as fast as a monohull can go max. So the trade off is wetted surface compared to wave making. As you go faster the wave making gets more important. When do you see a cross over? I don't know.

    But if you have a short stubby ama at high wind speed, it will certainly be causing lots more wave drag.
    Additionally you will not have the bouyancy aft while headed upwind, and you won't have the bouyancy fwd when reaching.

    Lack of bouyancy fwd makes you susceptible to pitch poling. Which you will only want to do once. Especially if the boat is harder to right than a beach cat (some people never learn to do this).

    The problem is you need the boat for a wide range of conditions, and most want to survive the worst.

    Choose your preferred best sailing conditions.
     
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