Underwater hull form for sailing catamarans

Discussion in 'Multihulls' started by Pericles, Jan 7, 2008.

  1. Pericles
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    Pericles Senior Member

    Having spent hours and hours and hours sat here at my PC, looking at every catamaran available to view on the net, I feel it may be time to ask a number of questions.

    Are cylindrical shaped, round bottomed hulls with rocker the almost ideal shape for wickedly quick catamarans? Robert H, Perry writes thus about the Gunboat 62, "The hulls show an L/B of 9.0 when I scale the individual hull beam at 2.1 meters. L/BOA is 2.2 and 28 feet of beam buys you a lot of stability. The canoe body rocker shows the hull depth quite shallow forward with the forefoot knuckle above the DWL. This indicates that the longitudinal center of buoyancy may be aft, which suggests the boat needed volume aft to float the accommodation and mechanical weights. The D/L is 77.62 if I use the “max load” displacement." Read article below.


    He describes the hull shape as "canoe", which I take to be "Slim semi displacement hull form". This is a difficult shape to achieve with marine ply unless the builder uses Kurt Hughes' very clever system of "Cylinder Mold".


    I have read and re-read most of the Stitch and Glue and/or Cold Mould (Mold) books available and theoretically, feel confident to build something that has a reasonably fair chance of floating.:D

    On the other hand, S&G works with twisted and tortured marine ply in various thicknesses, which combined with CM techniques, can create hull forms that excel at full planing speeds. So, the most critical question remains to be answered. Could a true monohedron underwater hull form such as this, http://www.bateau.com/studyplans/DE25Cabin_study.htm?prod=DE25Cabin be extended beyond 25 feet to 50 feet? Please disregard the necessity of correctly building and strengthening the shape. I am interested in your opinions about the suitability of the underwater hull shape in a catamaran when powered by sail. Apologies if this subject has been discussed previously, but searching the forum has not yet revealed it to me.

    I appreciate that the "hump" is an important consideration, but modern catamaran rigs deliver prodigious thrust.

    All opinions and advice welcome without prejudice.


  2. brian eiland
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    brian eiland Senior Member

    Hull Lines

    I have to be very brief right now as packing for trip to Thailand.

    If you have access to it you should look at a couple of articles in Seahorse magazine that discusses the hull lines of the "Club Med" catamaran designed and built by Multiplas

    The Race
    Club Med

    After the spaceship shape of the Pete Goss’s strange catamaran, many observers were a little disappointed with the ‘conservatism’ and monastic simplicity of Gilles Ollier and Yan Penfornis’s three catamarans.

    Loïck Peyron, skipper of sistership Code One, disagrees: ‘Conservative? From the exterior, maybe. The innovations are there but you cannot see them. Only results talk!’ And the numbers talked immediately: 34.7 knots on the day of the official launch. One month later, Club Med smashed the previous 24-hour record with a new mark of 625.7 miles at an average of 26.07 knots.

    As a member of the Ocean Passage Committee, I was fortunate to be dispatched to San Salvador to check Club Med’s time after her new record on Columbus’s old track. Along with sails I had enjoyed with Dalton’s team in Brittany, the brief 400- mile journey on to Miami was helpful in better understanding the power of the beast. During my first watch at the helm, approaching Nassau, I was literally amazed by the immediate reaction of the rudders. The boat is a true bicycle, so responsive to a slight touch of helm! The same pleasure as steering a Dragon. There was nothing to compare with PlayStation. In seconds you jump from a peaceful 20 knots to 27-28 knots with ease.

    Fred Le Peutrec, the Tornado helmsman, was ‘locked’ at the wheel during the 24-hour record. Adding some last miles in the final hour, he regularly pushed the speedometer over 37 knots! ‘On the polars, we know the boat will easily do 650 miles a day, maybe 700,’ predicted Gilles Ollier. As an example, they sailed from Villamoura in Portugal to Portofino, Italy at 25 knots average speed!

    What makes the design so fast?

    See Seahorse February 2001 for the remainder of this article

    ....remember her Club Med
  3. Guest625101138

    Guest625101138 Previous Member

    I have a lot of faith in what Michlet/Godzilla produces.

    If you just want to go fast without any thought of tacking around buoys then the hull form is very slender, straight rocker, fullness carried fore and aft to maximise volume while minimising WL beam, canoe stern to avoid transom drag, almost flat bottomed and hull beam to depth ratio around 1.4.

    I have attached the underwater shape (blue hull) of a 12m cat, displacing 6 tonne overall and optimised for 20kts. It shows the features described. WL beam is 693mm and draft is 469mm.

    This is the sort of hull Godzilla produces whenever you go well beyond the hull speed. Total drag on the boat at 20kts is 5.2kN.

    These hulls can be approximated using hard edges for sheet construction without much loss in performance. I have attached an example. This hull is optimised for speed just over hull speed so the ends are not as full.

    Nothing complicated in this shape. Is easy to make. You could get a rounded bilge by just rolling sheet forming parallel sides for 80% of the length and fit shaped ends.

    Rick W.

    Attached Files:

  4. brian eiland
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    brian eiland Senior Member


    The problem with those hull shapes is there is NO resistance to pitching, and that will KILL all the drive from your sailing rig.
  5. Guest625101138

    Guest625101138 Previous Member

    These are intended to show the underwater section only for the easiest driven hull. How you get them into that trim and keep them there for as long as possible presents a range of challenges.

    My own experience and observations with these types of hulls is that they tend to wave pierce. They are adverse to lifting/pitching. If anything they run the risk of pitchpoling. So in the limit you need some lifting ability in the bow above the waterline to climb up over a wave rather than driving through it and maybe getting tripped up.

    The closest you see to these sort of lines are the outriggers on the big tris. When they are going at full tilt the entire displacement is carried on the lee outrigger. They are not quite as full in the bow as the blue one I have shown but their design is still embryonic.

    If you look at the underwater section of some of the more efficient power cats they have very simalr shapes to what I have shown. So why wouldn't powerful sailing cats head the same way.

    So for a fast sailing cat you could design each hull to carry full displacement. Set the initial trim bow up so that at design speed it trims level with one hull on the verge of flying.

    Rick W.
  6. Richard Atkin
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    Richard Atkin atn_atkin@hotmail.com

    Quote "The problem with those hull shapes is there is NO resistance to pitching, and that will KILL all the drive from your sailing rig."

    Brian, I have been designing a cat (with a huge amount of help from Rick Willoughby). With my main focus being good straight line speed in light air, I have gone for a slim canoe shape without much flaring above the waterline, and barely any rocker at all.

    Have you had much experience sailing this type of hull? Your comments on pitching contradict Ricks comments on pitching. I am less concerned about pitchpoling as I don't intend to take big risks in strong winds.

    - Richard
  7. rob denney
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    rob denney Senior Member


    I disagree. Those are pretty close to the lee hull shapes on Harryproas (l/b 20:1, no rocker, semi circular sections, becoming U at the ends, prismatic 0.8) and they pitch far less than any catamaran according to all the people who have sailed on them. They are also very fast.

    They do not turn very quickly, (not a proa problem), which is one reason why they are used on trimaran floats, but not on main hulls. The other is that they have more than optimal wetted surface, which is a disadvantage in light air. Again, not a proa problem as the boats are so light that wetted surface is way below any similar length cat or tri.


  8. Freenacin
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    Freenacin Junior Member

    That's my understanding of it. More rocker = more pitching, but faster tacking, less rocker = less pitching, but slower steering response.
  9. farjoe
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    farjoe Senior Member

    Is faster tacking really due to more rocker?

    I have a small hobie like cat with lots of rocker but no dagger and it tacks like a pig.

    On the other hand I have another slightly bigger cat with a lot less rocker but a sizeable dagger and it very rarely misses a tack.

    With regards to the comment of increased skin resistance in previous mails, is this really an issue for a cat which can easily compensate by putting up more canvas?
  10. Fanie
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    Fanie Fanie

    I think you have to take in consideration the water you are going to sail on. Those shapes would work on flat water but as Brian said, I too have my doubts if you have swells.
  11. Richard Atkin
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    Richard Atkin atn_atkin@hotmail.com

    If you focus below the waterline, it seems logical to me, that more hull volume at the ends, and less in the middle, will equate to less pitching (assuming the weight is centralised). What you do above the waterline is another factor, but has nothing to do with rocker.
    Am I wrong?
  12. JCD
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    JCD Follow the Bubbles!

    Hello all,

    There are always these contradictions by one or the other. I see rocker as a leg on a "rocking" chair. Smooth transition aft and forward "dampening" but a hell of a lot of hobbyhorsing or "pitching".:confused:

    Then this dam gunboat comes along with a rocking chair for a keel line and they claim it works for reducing pitching.:confused:

    :confused: Why can't we all just get along and agree:confused:

    I think this whole thing is a conspiracy by the nautical community to confuse us.:D

  13. Guest625101138

    Guest625101138 Previous Member

    Assuming both boats either have jibs or not, the keel will make a substantial difference. It will allow you to track higher to the wind (less leeway) so you are tacking up through a smaller angle.

    One trick I soon learnt sailing a small cat without a jib was to reverse the angle of rudder once it was in irons and started going backwards. Some people just find it easy to go well past the windward mark and jibe. Probably not a lot of difference in time.

    A deep keeler with a lot of weight and soft underwater lines will really pinch up on a buoy. You can make way maybe 20 degrees to the wind. A cat has to sail fast. Once you lose speed they are awful to turn irrespective of hull rocker.

    Rick W.
  14. Richard Atkin
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    Richard Atkin atn_atkin@hotmail.com

    J, perhaps the rocker creates some lift at the bows at high speed....maybe 'reduced pitching' is the wrong definition. Maybe not. I agree with you....there is so much contradictory information.

    When I run into debates like this....I listen to those who can speak from actual experience, rather than "knowledge". So far, those who have had personal experience, tell me that less rocker means less pitching....even in waves.

    Maybe, in the future, other people with personal experience will tell me something completely different. If that happens, I will buy a gun and blow my brains out all over the wall. (just kidding).

  15. Pericles
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    Pericles Senior Member


    A rocking chair rocks because it's placed on a hard surface. A boat is cradled in water and thus the hull is not rocked by waves because the bow and stern are shaped to reduce the effects of the waves. A designer could take this concept to its final form which is a ball. A ball does not pitch end to end. The converse is that a telegraph pole will pitch alarmingly when floating in water.

    As boat design is always a compromise, the best designers such as Morrelli & Melvin are able to take the clients requirements and produce amazing vessels, hence their Gunboat series. Length is good and their hulls are cylindrical in form with maximum beam 60% aft of the bows. This hull form minimises wet surface drag, so, for example, the narrower and deeper the hull form, the greater the drag. Your Green Lantern design will have greater drag pro rata than the Gunboat series for this reason.

    A catamaran designer, by virtue of the two hulls, is not constrained to produce a hull that stays upright by itself. This means the hull forms are optimised as long, narrow, displacement shapes. I was toying with the idea of using an elongated and narrowed planing hull form that could be built with sheets of marine ply and combining S&G techniques with Cold Mold systems.

    A V bow, flowing aft and becoming cylindrical and then rising smoothly to a slim canoe stern should work quite well for an underwater shape. Not as great as the Gunboat, but doable with ply panels shaped to create the desired form when stitched together. I don't know whether you explored Kurt Hughes' site, but his use of preformed ply composite panels cut to shape and joined together, will also figure in my planning. It's worth the time.

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