Hull shape impact on performance

Discussion in 'Multihulls' started by bluebox3000, Mar 20, 2014.

  1. bluebox3000
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    bluebox3000 Junior Member

    I'm looking at the design parameters for a catamaran and I'm wondering what the optimal hull shape is for a catamaran hull. The documentation suggests the most important factors are the length to beam ratio and the prismatic coefficient. Then for the boat overall the displacement and sail area.

    However the center of buoyancy and overall hull shape can be different given the above being constant.

    Would it be best to have the hull at its widest in center of water length or more like some catamaran at 3/5th point of waterline? Also with the above constant, how will a wide flat stern impact catamaran performance?
  2. gonzo
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    gonzo Senior Member

    The optimal hull shape is the one that satisfies the most requirements and creates the least compromises. You have to decide on what the parameters are and then design within them.
  3. Eric Sponberg
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    Eric Sponberg Senior Member

    I think you are going to get a couple of dozen opinions on this question from the same number of responders. But the answer is: There is no optimum hull shape. Each boat is different, and so the characteristics of the hull will be different to suit the needs of each boat.

    As with all boat design, the center of buoyancy and the maximum width of the hull will tend to be aft of amidships, and that's just because that is what has evolved over time and proven to be good hydrodynamically, as well as for weight balance because boats tend to be heavier toward the stern. But there are no fixed rules on these things.

    A wide transom can have a number of deleterious effects if not executed properly. If it is deep as well as being wide, it will create a lot of drag and the boat will be slow. Catamarans are displacement boats, generally, if you are talking about sailing boats. For power craft, most are displacement boats or semi-displacemetn, but there are some planing designs, particularly in racing powerboats. There are various types of bottom shape that can be considered, all have their own unique characteristics. Again, there are no rules.

    For sailing catamarans, a length-to-beam ratio of at least 10 on each of the amas is recommended for being easily driven. For planing power catamarans, this ratio may be less, but usually more than 6 or so, if I had to guess.

    For all catamarans, the narrower the hull, the more susceptible it is to weight changes fore-aft, leading to trim problems. And, at displacement speeds, higher prismatic coefficients, on the order of 0.7 or so are in order for lowest possible drag. That's just the nature of the physics of multihulls.

    So, in a nutshell, there are various guideline parameters, but nothing is fixed.

  4. YoungGrumpy
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    YoungGrumpy Junior Member

    Just being curious (I do not qualify to offer an opinion).
    What is the purpose? Are you designing or comparing designs? Is it performance only, no other considerations (type of construction, or, performance in what conditions)? And, what is the size range? 10-15 ft or 150-200 ft?
  5. bluebox3000
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    bluebox3000 Junior Member

    General curiosity. It is really around comparing designs as to why one would choose one "shape" over another given other factors are the same. My personal thoughts are around a 40 ft cruising catamaran. If one chooses a plan it is relatively easy to change some things during the building process. Some people extend hull transoms for a couple of feet without too much effort or choose alternative materials, sail plan and so on. However I think changing hull shape or fundamental structural components is a lot trickier. So for evaluating and choosing the right plan I think choosing the right "foundation" is more important than the things that can be adjusted to ones preference.
  6. YoungGrumpy
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    YoungGrumpy Junior Member

    There was an article I vaguely remember in a magazine couple of years ago about choosing a catamaran. The point I've took was, look for the below the waterline hull (or, how much will you load it) and the bridgedeck clearance. And the second one is making a more visible difference

  7. Richard Woods
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    Richard Woods Woods Designs

    was it this one that I wrote in Multihull International?

    The biggest factors affecting performance are the built in washing machine and the granite countertops.....

    After that its the length, weight (which implies a certain slenderness ratio)and sailarea. Then the wetted surface area, prismatic coefficient, angle of entry and the depth/BWL ratio. Wavemaking is proportional to BWL cubed, but go too narrow and you have to go deeper for the same displ, which increase WSA. So the ideal is a semicircle, thus depth is half BWL. But that's only good in flat water, which is why you don't necessarily want to have a day racing hull shape on an offshore cruiser, even if its faster

    A cruising boat will probably have a more Veed forefoot to reduce slamming. Which means less rocker which usually means its harder to manouver, but pitches less

    It's all a balance and as Eric (and Jim Brown) says, everyone draws their own statement of the multihull art.

    Richard Woods of Woods Designs
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