Beam to length ratio for power catamaran

Discussion in 'Stability' started by Carlazzomark, Jul 12, 2022.

  1. Carlazzomark
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    Carlazzomark Senior Member

    Should the beam to length ratio of a power cat be the same as a sailing cat (about 50%), or can it be narrower?

    Thanks
     
  2. fallguy
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    fallguy Senior Member

    It is a decent standard for a cruiser, but for a planing hull; weight might get up awful high.

    Need more context for anyone to answer well.
     
  3. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    The beam to length ratio has to do, above all, with the stability of the boat. In a catamaran this 1 hull beam to hull length ratio has less to do with stability, which is more influenced by hull spacing. Therefore the answer to your question is, in my opinion, little to do with each other.
     
  4. Alik
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    Alik Senior Member

    Depends on speed, first of all. The close the catamaran to hump of resistance, the wider it should be.
    Then, depends on requirements for space, accommodations, etc.

    Generally yes, powercat can be (and usually is) narrower than a sailing cat.
     
  5. Carlazzomark
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    Carlazzomark Senior Member

    Sorry, I should have offered more detail.

    This will be a very small boat, 12-15’, and no more than 9 hp. The hulls are displacement. It is for lake use for two, possibly three, people. Top speed perhaps 10 mph.
     
  6. fallguy
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    fallguy Senior Member

    The most common problem I have seen with cats this size is the displacement is often insufficient for the load. And so, your main objective (in this context) would be to make sure the available displacement is sufficient for the plan versus making the boat wider and then heavier.

    I recall more than one small cat builder who later regretted hulls too small..
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    It can be what ever you want/design it to be. It all depends upon the duty!

    Then it'll make little difference.

    Indeed... forget "performance" in the power context, given your length and expected available power.

    However, given your length, and the fact it is a 'displacement' hull form... you are at the prismatic hump at around 6knots, which means 9hp engine is likely a waste....as it wont get 6 knots.
    Either forget 9hp and go for less, like 6-7hp max, or increase the length of the boat.
     
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  8. messabout
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    messabout Senior Member

    For the use that you have described, it is likely that a monohull will be a better choice. A 14 footer will handle the 9.9 nicely and can push the boat into the low twenties. The mono will have more people space and will be more comfortable than a small cat would be.

    Single engine cats can have problems with the convergent waves from the hulls. At certain speeds the wave conflict between the hulls will make the engine very unhappy....and sometimes have the boat become unpleasantly and even dangerously wet.
     
  9. sailhand
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    sailhand Senior Member

    Interesting comments here. I have been designing, building and testing small displacement cats since 1996. I have sold heaps of plans for these small boats and have many happy customers. I use one for my tender and you can see this dinghy on youtube doing more than ten knots with a 3 hp, search magic carpet 3.5 with a 3hp. It is 3.5 metres long and 1.75 wide, exactly 50%. It can comfortably deal with two people and groceries fuel etc. . Essentially the length and beam of your hulls tends to dictate your displacement. Put simply the general concept is bigger boat bigger load. If you want speed in a displacement cat you really should keep the waterline beam length ratio of each individual hull above 10 to 1. 10 ft hull 1 foot wide on the waterline. This is a really simple explanation and does not include hull shapes etc. If you want to carry a particular load in a small powercat its probably a good idea to calculate that weight and then calculate your hull size, ie load carrying capacity using the 10 to 1 minimum rule. You will easily achieve 3 average people along with minimal gear in a 14 footer in a lightweight displacement cat. They are however not the best hull shape to carry high loads at this length so will not tolerate overloading and still maintain your target speed. Goodluck, cheers
     
  10. Scuff
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    Scuff Senior Member

    Do you have a link to your website/plans?
     

  11. sailhand
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    sailhand Senior Member

    No I am retired and not interested in maintaining a website. You can view my dinghy on youtube and there is an email address on there. I have a bit of an ongoing problem with email addresses at the moment which will hopefully be sorted soon. I see no reason a slightly larger version wouldnt suit your needs. We have 7.8m x 3.5 metre catamaran doing 19.8 knots, thats right so close to 20 yet so far, with 4 onboard and full fuel with a pair of 30s. Thats a lot of boat in a hurry for 30s. Hull shape in displacement boats is critical. In my opinion, and thats all it is an opinion, planing hulls are less critical in hull shape. I used to barefoot waterski when I was a kid and I can assure you my feet arent an elegant shape but they got me planing at around 37knots if I remember correctly, I was 210 pounds then. Get the hull shape wrong and youll be stuck with adhocs formula speed = square root waterline length x 1.3, and at 14 feet thats definitely closer to 4 knots than 14. Cheers
     
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