Best beam for Catamaran?

Discussion in 'Stability' started by sandersc, Jan 29, 2008.

  1. sandersc
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    sandersc Junior Member

    We are currently doing our own design catamaran. We are in the final stages now.

    It is a very sleek hull with lots of bouyance towards the rear. It started of as 8.4m, but it currently is 9.44m. The total beam however is 4,5m. I feel that this my be a bit narrow. What do you think , is it OK or what should the minimum beam be. Oh yes , it is a sailing catamaran.

    Cheers

    Sanders
     
  2. Pericles
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    Pericles Senior Member

  3. Fanie
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    Fanie Fanie

    4.5m ? Is that a canoe ? :D

    It will never be too wide (or too long for that matter) I started the boxy fisher at 8m x 6m and is currently at 10m x 8m. Still too small for all the fish I want to catch from it.

    You have to keep the mechanical stresses etc in consideration, the bigger it gets the more attention you have to give to this.

    I'm no expert on this, ok, I just know what I want and am still learning...

    Also look at http://www.boatdesign.net/forums/showthread.php?t=18851
     
  4. Ulf
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    Ulf Junior Member

    If you are going for speed and intended to fly a hull most of the time then the beam should be optimised for the intended sail area and wind strength, narrow enough to easily fly a hull and wide enough to give enough righting moment in strong winds. Apparently Reynolds Design experimented with different beams for their R33 and settled for a moderate 14 foot beam.
    If it’s a cruising cat then I agree with fanie, as wide as practical. But wider beam increase the risk of bridge deck slamming.

    Just my thoughts on the subject.

    /Ulf
     
  5. Fanie
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    Fanie Fanie

    Could it be that you can double the sail area if you double the width ? Which should be faster then ?
     
  6. Ulf
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    Ulf Junior Member

    The one that dosen't pich-pole =)
     
  7. Trevlyns
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    Trevlyns Senior Citizen/Member

    Hi all you Saffa guys

    A very basic ball park idea from Chris White’s “The Cruising Multihull” indicates a beam (centreline to centreline) of about 40% of LWL. I’d strongly advise getting hold of a copy as there is extensive discussion on beam as a lot of other factors come into play.

    What I do is compare statistics of similar sized vessels in a spreadsheet with a simple calculation of LWL divided by beam. You can quickly pick up the narrow and fat cats, average them out and see how yours compares.

    Missing South Africa but at least we don’t have power cuts over here! :)

    Best, fellas
     
  8. masalai
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    masalai masalai

    I have been influenced by application (cruising grounds - marina pen access, capability to sneak in to shallow protected spaces in a severe storm, passage making capability and so on)

    For me:- cruising tropics from India E & S to Samoa as a powercat..
    - - - - - - no marina, but under 25 ft to access most multihull liftout facilities
    - - - - - - 600 to 700 mm draught & motoring range of more than 2000 miles @ 10/15 knots cruise (calm seas no wind)

    Make your selections as to needs/criteria & then modify as per Trevlyns bearing in mind to select only within your design criteria. Many designs (sail in particular) seem constrained by marina berth access, making beam overall unduly narrow for cruising. Cats are not for people who accumulate stuff, as weight is the killer of performance and comfort of the designed bridge-deck clearance. Get plenty of time up sailing/motoring in cats around the size you want. Charter and pay the premium as experience is gained to sail only with partner & make a couple of overnight or 2 day non-stop passages - can you both handle the 2h on 2h off watches demanded?
     
  9. Pericles
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    Pericles Senior Member

  10. Trevlyns
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    Trevlyns Senior Citizen/Member

    Pericles!

    You left out two words at the end of the first line - ... to calculate!!:p

    Best!
     
  11. Fanie
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    Fanie Fanie

    Personally I don't like the skimpy 40% width from length. I would rather add the extra beam strength and make it wider, especially on such a small vessel.

    You could get away with narrower ratio on bigger boats. a 30m boat that is 12m (40%) wide is a WIDE boat, you get tired just by walking from port to starboard. It would be rediculous fir instance to make it 20m wide for many reasons.

    Small craft, wider ratio, large craft, narrower ratio.
     
  12. jehardiman
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    jehardiman Senior Member

    First, for drag, hull spacing has been shown to make no effective difference below a speed length ratio of 1.5 or above 2.5. Between those two points, the optimum seems to trend towards 0.4 due to interference effects. In that range, beamier cats are less efficient, but can carry more power in the rig. Additionally, due to the weight tied up in the crossdeck, very wide large cats become weight inefficient.

    There is no single optimization point, too many linked variables.
     
    Last edited: Jan 30, 2008
  13. Trevlyns
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    Trevlyns Senior Citizen/Member

    OK guys, haven’t even read this fully myself, but it’s been in my library for sometime now. Quite technical but I’m sure it will throw some more light on the topic in question.
     

    Attached Files:

  14. Fanie
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    Fanie Fanie

    I want the extra safety and space a wider setup offers, a 10m cat is relative small, making it narrow could really cramp it up.
     

  15. Trevlyns
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    Trevlyns Senior Citizen/Member

    Just another casual observation.

    Sometimes we conjure up these images (particularly in a discussion about beam, like this) that the wind is coming directly from the beam i.e. a beam reach. There, the greater the beam, the better.

    But hands up who knows, in yachting, we also beat to windward – or run with the wind on the aft quarter? Can we now see the 3d images thrusting the ends into the sea?

    There has to be a compromise on beam. That’s why I still advocate statistical comparison with similar sized yachts. These are designs that work in the real world, in all conditions.
     
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