Prismatic for a trimaran?

Discussion in 'Multihulls' started by InetRoadkill, Aug 1, 2019.

  1. InetRoadkill
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    InetRoadkill Junior Member

    I'm having a little trouble finding guidelines for the prismatic on the main hull of a trimaran. I've read you treat the main hull in much the same manner as a monohull. But since the trimaran is often finer than a monohull of the same length, as well as lighter, and can exceed hull speed, I'm wondering if the prismatic works the same. Right now the main hull sketch I have is 45' long and has a prismatic of 0.64. The guesstimated displacement is 13,500lbs. L/B is 9:1.
     
  2. oldmulti
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    oldmulti Senior Member

    Prismatic's are the same concept for mono' or multis. It basically picking the best prismatic for the speed and length of your boat. If your boats average speed is about the square root of your waterline eg 6.5 knots on a 42 foot waterline then a PC of 0.55 - .58 will work. If you think your tri will average 8.5 knots or 1.3 times the square root of the waterline then a PC of 0.64 will work well. Do further research into speed length ratio's and associated PC's then be realistic about the average speed you tri will do. 8 knot averages are realistic for a well designed modern tri.
     
  3. InetRoadkill
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    InetRoadkill Junior Member

    Since tris routinely exceed their hull speed by a significant margin, doesn't that mean an adjustment to Cp is needed or do you go ahead and design for lite breeze performance and just let it power thru when in fresher winds?
     
  4. oldmulti
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    oldmulti Senior Member

    All yacht design, like every thing else, is a compromise.. You optimize for the conditions you think you will meet on average and have a boat that is not fully optimized above or below those conditions. EG An older lagoon design was never going to hit 15 knots unless it was surfing down a big wave so they designed there hulls for 6 to 10 knots with prismatics below 0.6. Non foiling C class cats rarely sailed below 10 knots so had prismatics of above 0.65. Please read about average performance of cruising boat. The best averages cat and tris of about 45 feet get is about 200 miles per day unless they are Gunboat type machines where they may go 260 miles a day. Your designing for 8 to 10 knots average not the mythical 25 knots quoted by Piver et al. A pure racing tri is different story but even those only do PC' of about 0.65. You design for average performance not peak performance. You design and build the structure for the worst conditions you meet though not the average conditions.
     
  5. Doug Lord
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    Doug Lord Flight Ready

  6. Manfred.pech
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    Manfred.pech Senior Member

    CP is just a math feature of design. Sailing in a seaway it is not constant and changing every moment. The hull is in the water and out, it is diving and lifting or on foils and the angle of attack varies.
    More of interest is the bottom shape with concave and convex areas to help the hulls to get foiling or to get lift or suction (Farrier main hull with
    swept up stern) when it is needed.
     
    Last edited: Aug 4, 2019
  7. oldmulti
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    oldmulti Senior Member

    Look over at the Multihull Structure Thoughs tread page 13 for the main hull lines of a very fast 42 foot tri that is shaped for simple building. This is from a designer who designs fast boats.
     
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  8. rxcomposite
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    rxcomposite Senior Member

    It all depends on the arrangement. Are you designing to a large fat central hull with thin outer hulls or is the 3 hulls all same size and length?

    Here is from Leo L.
     

    Attached Files:

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  9. latestarter
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    latestarter Senior Member

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  10. Manfred.pech
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    Manfred.pech Senior Member

    Thank you for cooperation. M.
     
  11. InetRoadkill
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    InetRoadkill Junior Member

    I was playing around with Free!Ship and I noticed it reports the Cp for the forward half of the hull and aft half of the hull along with the overall Cp. This raises the question whether you should try to balance the Cp both fore and aft, or does it matter?
     
  12. oldmulti
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    oldmulti Senior Member

    What do you want to achieve? If you want to minimize pitching you need the hull to have some asymmetry fore and aft. By doing asymmetry you get differing CP's fore and aft. No great problem in having differing CP's. Your real design issue though is getting the center of buoyancy of your floats slightly forward of your main hull center of buoyancy to minimize pitching and then matching that to the float hull shape over the heeling range. This takes a good designer or an experienced person to get it right on a high performance tri. Look up John Shuttleworth stuff as he had an article on it some time ago.
     
  13. Manfred.pech
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    Manfred.pech Senior Member

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  14. DCockey
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    DCockey Senior Member

    How does "floats slightly forward of your main hull center of buoyancy" minimize pitching?
     

  15. oldmulti
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    oldmulti Senior Member

    Attached Files:

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