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Thread: Sail-powered SWATH catamaran

  1. #1
    gpclarke
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    Default Sail-powered SWATH catamaran

    Several navies have introduced SWATH catamarans and found benefits like
    high speed, stability, and fuel efficiency. Worthwhile advantages, yet
    no sailing versions of the SWATH concept seem to have emerged. Anyone
    know of any examples sailing anywhere?
    Reducing the scale of these military vessels down to regular sailboat
    sizes would create a very tender boat since each hull would be only 50%
    buoyant. So beefing up to perhaps 100% in each hull would be a first
    design step. Apart from this pre-requisite, I see no serious drawbacks
    to creating a superior performance and wave-piercing catamaran.
    Anyone care to differ, ... or offer further design refinements that
    might help make this the catamaran of the future ?

    SailNut.

  2. #2
    Bill Kearney
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    Default Sail-powered SWATH catamaran

    I see no serious drawbacks to creating a superior performance and wave-piercing catamaran.
    Somehow wave piercing and sails aloft seems pretty unlikely combination.

  3. #3
    tsmwebb
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    Default Sail-powered SWATH catamaran

    So I envision ever-increasing sailplans requiring ever-more-massive rigs to control them spiraling ever-outwards until the physical structure of the vessel itself is incapable of containing them, all just to travel slower than a rowboat.
    I think you're right. I made some napkin calculations (and did a
    little mitchlet modeling) on this idea seven years ago and came pretty
    much to the conclusion that you'd need to do something pretty radical
    to make a true swath cat move in light air. The wetted surface is what
    really does one in. A friend of mine did a compromise design with very
    long bulbs down in New Zealand and he was happy with her, but she was
    always a motor sailer and is being refit to be a mostly motor whale
    watching platform. The real advantage of swath is that it makes sea
    kind boats. At some speeds they may also be efficient but only where
    wave drag dominates and only if you can get the displacement far from
    the surface. I've noticed with the local swath boat (Navtec) that they
    create a pretty energetic wave train, but she's a comfy boat in most
    weather. As others have noted you'll also need to think a bit about
    stability if using sail plans that have a heeling moment (really big
    kites might work). Foils plus some kind of reserve bouncy (ie. hulls
    near the surface) seemed likely candidates to me but both add drag.
    There were lots of other issues with making the thing work and making
    it maintainable (eg. access to the hulls, control surface linkages
    &c.). And it would be a bugger to build and expensive... Seems like
    the kind of boat project that keeps folks from getting out and
    cruising...

    -- Tom.

  4. #4
    Novice Member Qhiron's Avatar
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    Default Sail-powered SWATH catamaran

    cool ideas, guys, very interesting considerations. Here's something I'd love to get your input on. (tsmwebb: no way this swath is slow!) Remember the crossbow/sling design thing, water speed record... years ago. Bill, submergence gives stability. Sailnut is right, if dimensions work. Thanks, Q

    It's pretty clear that swath design parameters focus mainly on submergence, buoyancy, drag and obviously wave amplitude considerations. What are best design considerations for max. wave amplitude? Somebody must have figured that one out. Certainly in all the literature available, there should be design parameters which define how high the main hull rides above the lowest possible draft. This, then should offer guidelines for ideal beam to draft to length parameters, right?

    So, what if we turn this design-configuring swath definition around? What is the ideal beam to draft to length oa, for an ocean-going, sail powered swath cat? Oh, if only Hobie could be here!

    Then I'd like to use this to scale relationship to size a new hull design to the max. allowable strength parameters of the construction materials proposed. Submerged, aluminum, or steel construction (ballast) with a composite carbon fiber and stressed-skin membrane hull design above water, for weight/mass reduction. Stressed membrane maximum shear strength should provide an ideal maximum dimensional size of such a vessel. Now a bunch of you are scratching your heads going: wow....

    Got any suggestions? Here's mine:

    LOA: 145-160 ft
    Beam: 48 ft (yeowsa...)
    Draft: 16 ft DW, 6 ft- littoral
    clear-to-hull air plane: 6ft-16ft
    Mast (s) Hgt. 180-220 ft. (depends upon geom. config.)
    usable deck/main hull: Loa: 110 ft., beam: 36 ft, height 16 ft

    obvious secondary propulsion systems are diesel/electric, in pontoons.
    With the large surface area of such a design, eco-PV solar panels and
    rigid wing composite sails for a quad, cross-braced mast system is proposed

    In the luxury, gigayacht designs I have yet to see anyone suggest this. I understand the probs with cats, structural torsion, cantilever, huge moment arm loads, and obviously beam width in harbors, (who'd want to come in?) but it seems that for ideal ocean going stability, platform area and largest sail/mast design configurations, this kind of swath cat design has huge advantages.

    There are two other parameters in the initial design configuration layout I'd like to propose for this kind of radical design:

    1. Adjustable draft depth, swing/hinged, or hydraulic/telescopic pontoons, or
    2. air-inflated, water ballast rigid structure pontoons.

    i.e. when shallow draft required, submerged pontoons are air buoyed, raising entire structure higher off water plane. With conditions of deep water sailing, pontoons are partially ballasted with water, the inherent structural mass increase of pontoons adding stability and inertia-resisting wave modulation under high yield wind conditions.

    As in any sailboat, less mass is better for increased performance. With the moment arm of a large beam, huge mast lengths and max sail areas can be achieved. What are the limiting factors for such a design? Manoeuverability? Turn radii, ouch?

    I assume the optimum hydro-dynamic submerged pontoon shapes, scaled, will be the Ohio class boomer sub hull shapes. Put enough money into those shapes, no?

    So, where are my major probs? I need a big job to get this started. LOL!

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