Sailing vs power catamaran beam

Discussion in 'Boat Design' started by zimbodave, Apr 27, 2016.

  1. zimbodave
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    zimbodave Junior Member

    Please would you guys help me out here.

    Sail powered catamarans tend to have a length to beam ratio somewhere around 2:1
    Power catamarans tend to be much narrower.

    Why is this?

    I understand that sailing cats need the stability but I think it's such a shame that power cat's lose that mean purposeful look because they are narrower.

    I love the stance of sailing cats but somehow it gets lost when they are designed for power. And think of all that room that's lost in the process.

    Is there a compelling reason for power cats to be less beamy and carrying a wedding cake? I think they would look so much nicer if they more closely followed the low windage more beamy approach of sail?

    Are there engineering reasons? Please enlighten me :)

    Cheers

    Dave
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    You may find this useful.
     
  3. Rurudyne
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    Rurudyne Senior Member

    I would think it has quite a lot to do with many power cats having planing hulls.

    If having displacement hulls or planing hulls seems like it should make no difference for extra drag caused by having hulls too close together, and a low power displacement hull powercat has just as much reason to want to avoid this drag as a sailboat would; but, I'm not sure what that has to do with a boat meant to go fast pushed by powerful engines.

    Also, if the hulls were widely spaced like the (usually displacement) hulls of sailing cats they would seem to be more vulnerable to roughness from how their hulls might interact unfavorably with waves at speed should they meet up with them non-simultaneously.
     
  4. ElGringo
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    ElGringo Senior Member

    After reading the thread that Ad Hoc referred to, (and several others) I don't recall reading anywhere that the balance fore and aft is mentioned. On a ready to go power catamaran, is there a balance to shoot for? Like maybe 60% aft or something?
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    It is all speed dependent. Basically the faster you go the further aft the LCB/LCG. But that does not influence the hull spacing, or stability per se in the context of the question by the OP.
     
  6. ElGringo
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    ElGringo Senior Member

    Yes, I should have asked the question in a new topic. I am still wondering if there is a starting point with a new boat to get the best balance.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Handsome is, as handsome does, Dave ! You might mention what boat size, and desired cruise speed, you are thinking of.
     
  8. Rurudyne
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    Rurudyne Senior Member

    Just had a thought: if a powercat is planing doesn't that mean that the length of the immersed hull is effectively shorter than at displacement speeds? Would that then mean that the effect separation of the hulls, relative to their "length", would be greater than at displacement speeds too?

    I suppose it would depend on if hull waves from a planing hull spread out differently ... but I kinda suspect that they don't.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Sailing cats are wider to give stability under sail, and to mimimize inter-hull interference effects at lower speeds in light airs.
     
  10. zimbodave
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    zimbodave Junior Member

    LOL....what size, now that is the question.
    I want to build a (predominantly) solar powered displacement cat.
    It would be nice to have a cruise speed of around 8-10 Kn this suggests a LWL of about 100 feet. However, common sense and my pocket have dictated somewhere closer to 60 feet. (And the size comes down steadily, the more I research this) So not really sure at the moment. That will be something the NA decides when the time comes.

    At this point I am only really interested in why displacement power cats tend to be so much narrower than displacement sail cats.
    It seems a shame to make them so narrow.

    Is one reason for making them narrower to encourage an easier ride? I would think the wider a cat becomes, the more righting force each hull would have thereby encouraging a rough unforgiving motion?
     
  11. ElGringo
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    ElGringo Senior Member

    My idea of the perfect cat is a Stiletto 27 with the sail rigging removed and the hulls modified for outboard motors. Hang a pair of 20hp motors and enough gas for about a 300 mile range and I would be happy.
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    It is not rocket science really.

    The wider the hulls, the more structure. The more structure the more weight. Weight kills speed.

    Thus the simplest way to keep the weight down, and hence to achieve the desired speeds, is the keep the structure weight down i.e. keep the beam as narrow as possible.
     
  13. HJS
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    HJS Member

    The greater width means also that the power catamaran get too great stability, which makes it very inconvenient in a seaway. This leads guaranteed to seasick for most. :mad:

    That's why they do not run passenger catamarans on some exposed routes. This applies mainly cats mowing in so-called displacement speed or semi-planing speed.

    Therefore it is better to choose a trimaran.;)

    js
     

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  14. Ad Hoc
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    Ad Hoc Naval Architect

    The picture of the trimaran that you posted, is actually a whole lot worse.

    The aft WPA is very large as such any stern or stern quartering seas shall pick up the stern and with little to no buoyancy and WPA up fwd, it shall either nose dive or broach.

    If you are that obsessed with a trimaran, then you need stabilised monohull, not a trimaran. Your picture is nothing like such a vessel. An example of one is here.

    I think you need to understand the effects of buoyancy, WPA and added mass on natural periods of motion before making such wild claims.
     
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