Planing hulls for sailing cat

Discussion in 'Multihulls' started by Peter Brown, May 20, 2006.

  1. Peter Brown
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    Peter Brown Junior Member

    I have been looking at building a planing hull sailing catamaran of 9 - 12 meters with hard chine hull shape and approximately 1.2 meter hull width. I want a boat that has good load carrying capacity, fast reachning and running speeds and good volume in the hulls so you don't feel like you're in a trench. The hull shape would be similar to that used by Peter Joubert on his Big Bandicoot project. As there are no boats being built like this I ask the question. am I on the wrong tack with my ideas? I would appreciate your thoughts and design ideas.
     
  2. gonzo
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    gonzo Senior Member

    Yes, high load and planing capabilities are contradictory. Planing sailboats are light. With a cat, which has a very narrow bottom, light loads are even more critical because the lifting surface is rather small.
     
  3. frosh
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    frosh Senior Member

    Planing cruising catamaran?

    Catamarans rarely plane, particularly the cruising variety. However boat speed is not really subject to Froude's law linking waterline length and highest practicable displacement sailing speed, for very slim hulls.
    Therefore why the requirement for planing hulls?
    Your requirement for 1.2 m wide hulls seems very wide if this is at the waterline. Check out the Schionning Design website to see the proportions of his cruising range of cats: http://www.schionningdesigns.com.au/www/welcome.cfm
    I strongly advise that in the size range you are interested in you use a plan from a reputable and proven multihull yacht designer. The cost of materials and the hours needed for construction are far too high to risk a failure, and also a well known design has far better resale value. :)
     
  4. dougfrolich
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    dougfrolich Senior Member

    I don't think it worked that well.
     

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  5. grob
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    grob Senior Member

    The planing hull catamarans that have been designed, built and sailed have usually had stepped hull shapes based on the floats of water planes. I guess Yves Parliers boat is the most famous of these. They claim it worked very well under the right conditions until it crashed and burned.

    Gareth
     
  6. grob
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    grob Senior Member

  7. Peter Brown
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    Peter Brown Junior Member

    thanks for the advice. My reason for looking at wider/hard chine hulls is that when I raced a mosquito I would struggole on hard reaching and running while the paper tigers would fly past planing beautifully. I had a monohull cruising boat that was hard chined and down wind the boat would easily break out and achieve very good sppeds with good control. The wide hulls make them more livable and user freindly
     
  8. CORMERAN
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    CORMERAN Junior Member

    To: PETER BROWN
    Re: PLANING CAT

    We designed and built a planing cat - with relatively wide hulls - that
    carries a load well.
    Sea trials gave us 37 knots - WITH a payload.
    Our MERGANZER design goes about as far as you can go - we think -
    and still call it a catamaran.
    I concurr with " frosh " - in the endevour you are considering - some
    sage advice will save you a lot of pain AND money.

    OOPS !!

    I looked more carefully at your request.
    You are talking about a sailboat. The boat I'm refering to is, of course,
    a powerboat.

    However - all is not lost.
    I have also made a planing, sail cat - as well.

    -This is your problem.
    A sailboat needs to perform in light airs.
    The inherent contridiction is:
    - that more you design for planing - the worse it will behave in low
    wind conditions. (With a power boat, light airs, are of little concern.)

    Also, as others in this forum will agree: it's not at all clear if cats ARE
    in fact " planing " - at so-called planing speeds.

    So making a planing cat - as I found - is quite a challange!
     
  9. frosh
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    frosh Senior Member

    Peter, your analogy where you mention the Mosquito cat and Paper Tiger cannot be applied to a larger, heavier (even relative to surface area of hull bottoms) cruising cat.
    For a start the Mosquito is not the slower of the two overall; only under a very specific set of conditions.
    Secondly by putting another person on board both cats, the Paper Tiger would probably cease planing and go slower than the round bottom Mosquito.

    What this translates to is: cruising encounters many different sets of wind and water conditions, and, the cruiser being loaded with equipment and supplies will behave like an over loaded Paper Tiger.
    There is nothing wrong with a hard chine design cat hull. There are many professional designs done this way, but not for the purpose of achieving high speeds or planing, rather ease of construction.
    I still re-iterate my earlier advice; obtain plans of a well known design. Maybe James Wharram's range has something in hard chine that appeals to you. :)
     
  10. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    I think it's a common mistake to optimize for maximum speed, while we most of the time travel at much slower speeds. For cruising I would hink that a good average speed and comfortable motion is the most important.
     
  11. Peter Brown
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    Peter Brown Junior Member

    This is a very good discussion and I thank all responses. When racing off the beach cats you looked at the SA/V ration of the hull and tried to minimise the SA. Having sailed on many bigger cats I find that they have the deep U section hulls, but beacuse of the load being carried are well down in the water so the SA/V ration is poor and the boats perfromance no better than a fast displacement boat for the length of the hull. The narrow hulls make for poor accomodation so I thought why do it, why not have bigger, roomier hulls that do not sink when loaded for cruising. This would give good displacement type speeds and then I thought why not have a boat which can plane. The extra bouyancy would stop the tendancy to nose-dive when being pushed hard. I realise that too much volume would give a harsh ride, but surely there must be some middle ground where you can achieve good volume in the hulls and good reaching and running speeds. As the boat is intended for long term cruising I am not concerned at high upwind speeds so long as the boat has good capabilities on the wind, but fast off the wind.
     
  12. frosh
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    frosh Senior Member

    Peter, probably it is not designed to plane, but I found this Australian design on the web. 30 ft. long by a maximum beam of each hull of 1.35m which should provide a fair bit of room. Kits are available as well.
    http://www.cunningcat.com/catindex.htm :)
     
  13. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    I see that wider hulls will be better for accomodation, but it will slow you down most of the time. Cruising cats has relatively small sail areas because of safety (I think). So going faster is more a question of how much sail you have the nerve to carry, and the danger of a capsize.
     
  14. nero
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    nero Senior Member

    The solution I went along with is of Shuttleworth's lightbulb style hulls. I have 1 meter at the wl to 13.9 meters long. Displacement is at 7,000 kg. The hull is not designed to surf but to let waves pass without pitching. Well that is the theory. In another few years I'll see if it floats.

    I half of one hull built and the impression in the interior is "wide". At eye level there is 2.25 meters of width at mid ship.

    The "lightbulb" shape can be thought of as two hull superimposed. The bottom hull is for least resistance and the top hull is the safety that comes in to play when waves get big.

    Having no catamaran sailing experience, everything I note is theoretical or stuff I've read from others.

    And yes it is a gamble to put $30,000 into an experimental design. If all goes wrong, I'll float it on a small lake and sell it as a house boat. smile
     

  15. CORMERAN
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    CORMERAN Junior Member

    To Mr. Peter Brown,

    yes a balance can be found.

    The planing multi - hulls I've designed and built have proved this.
    However, the specifics go into the area of copyright and pages of data
    and - with many clients - weeks of intense debate.

    I'm afraid you are at a point where spending some money - with experienced
    people - is virtually unavoidable..........
     
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