loads and failure modes for small trimarans

Discussion in 'Boat Design' started by DriesLaas, Feb 7, 2011.

  1. DriesLaas
    Joined: Aug 2009
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    Location: South Africa

    DriesLaas Weekend Warrior

    I'm starting the design of a small (18 foot) tri. I would like to get some input on the loads these craft see, and typical failure modes experienced.

    For instance, I suppose the maximum load on the aka is equal to the righting moment of the boat just as the main hull's bottom leaves the water. Now combine that with twist as the bows of the leeward ama buries into a wave, and you have some serious stresses appearing.

    But this is all conjecture, so your combined wisdom will be much appreciated.

    If I look at that near capsize of Sodebo, I get much more interested in sorting this out for my own boat, because although the two boats are a universe apart, the seas on which the sail are the same.
  2. Doug Lord
    Joined: May 2009
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    Location: Cocoa, Florida

    Doug Lord Flight Ready

    18' tri-food for thought

    There is a lot to the question you ask that needs more info. For instance, most small tri's aren't designed to fly the main hull(which is a shame). If they were designed to fly the main hull would the design approach the square or over square configurations of larger tris? And if so would the boat be designed, say, for two people on a trapeze? You can see how the RM can go sky high. Then with all that power, how do you manage the boat in pitch?
    But there is another problem in design, particularly in small tri's: if you opt for a beachcat killer max power tri, square or over square how do you get the main hull to fly in light air? If you don't you have a problem-speed/performance wise:
    Say you had an 18'X 18' tri with a 400 pound all up weight(achievable with very small amas) then the RM of the boat is about 6375ft.lb with two 175lb crew sitting in the center of the boat. Say you had 280sq.ft of sail on a 31' mast and that the CE was at 12.4' . That would mean that the boat would not fly the main hull until the sail had 1.8lb per sq.ft pressure! That's about the same as the maximum wind pressure an F18 cat can sail in with two big guys on the twin traps(without any depowering/reefing).
    Now, that identifies the design quandary of a small MAX Tri designer: it is very difficult to come up with a design that can use the power of a square configuration and still be able to fly the main hull in relatively light air. Flying in light air is really important since most venues in the US average 10 knots or less. The new AC 45 cat is designed to fly a hull in 6 knots and yet is able to be sailed in 30 knots. If you go narrower you're giving up the natural advantage of a tri-at least of a max power tri. There are numerous other solutions but many(not all) of them require that such a design is depowered.
    I hope this gives you something to think about-I have no idea whether you are considering a daysailer type(fairly low power) or a beach cat killer. Either way you'll have a lot of fun!

    PS-for a rough understanding of the engineering involved(required) you can "pick" your maximum righting moment and look at other multies with the same RM to get an idea of what would be required structurally. Unfortunately, as best as I can tell there are no 18' tri's designed for max power and very, very few if any designed to fly the main hull. An 18' performance tri would be an expensive project and it would be well worth the investment in the expertise of a marine engineer/naval architect-not necessarily to do the design for you but to do the engineering you may not be qualified to do. Good Luck!
  3. idkfa
    Joined: Sep 2005
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    Location: Windward islands, Caribbean

    idkfa Senior Member

    Can't help you with absolutes, but if you make the crossbeams (and attachments) stiff enough so that she does not twist, then you've exceed the bending loads also. ie. if she is rigid enough, then she is also strong enough (chances are). The twisting strain is your main concern, to test, support her on three points (and not four/six as normally done) and if the twisting is minimal, then you're fine...

    Think thin-walled box-sectioned crossbeams with generous radius curved attachments...

    Or you can go with the usual Al sections and have the usual flex, bob-stays will help.

  4. DriesLaas
    Joined: Aug 2009
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    Location: South Africa

    DriesLaas Weekend Warrior

    I want to have my cake and eat it

    Attached is a sketch of the concept.

    I'm trying to build a comparatively quick boat, which looks like a micro-cruiser from the outside, but is light enough to compete against some larger (preferably monohulls, just to see the looks on their faces) boats if the opportunity arises.
    Everything not needed on the inside structurally, will be made as removable modules to lighten the boat up for day-sails and racing (eg a rudimentary galley box, porta-potti, floorboards, batteries for house-loads, it all goes on the dock for race day.)

    I will keep the two pilot berths as an integral part of the structure.

    I have a donor rig in the form of a prindle 18 rig, which really drives every other parameter. (220 sq feet upwind SA)

    The length has reduced to 17 feet, and at this lwl I can get a L:B ratio of 8.2 and still carry the race displacement of 880 lbs.

    The challenge will be to build the whole structure for about 550 lbs, which is why I need to really understand the loads and failure modes these boats experience.

    I agree that the one approach is to look at similar successful boats and copy, which is the most practical way of doing it.

    My other problem lies in the ambition to do some coastal sailing with this same boat, and believe me that our coast between say Durban and Richard's Bay, or between Cape Town and Saldanha, is absolutely treacherous. Another very good reason to understand the loads well.

    Attached Files:

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