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#11
posted to rec.boats.cruising
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Sail-powered SWATH catamaran
Thank you everyone for all the responses, and humor, and a special
thanks to Jeff for the Team Philips link. Definitely a thumbs-down response to my suggestion. I agree it is an unusual combination. Perhaps I should have emphasized the wave-piercing aspect rather than make mention of SWATH. I'll try and explain further. Reserve buoyancy is important of course, but it is a handicap when a large amount of that buoyancy has to be pushed thru the water. By dedicating only an adequate/optimal amount of reserve buoyancy to the hulls (let's call them amas) allows them to efficiently do the tasks they are intended to perform, i.e. stay afloat and progress thru the water. For a limited class of vessels, I believe it is possible to make a better arrangement without pushing empty buoyancy thru the water, and thus improve performance. Such a vessel might be a coastal cruiser. Hence my post in this forum. My suggestion was for 100% buoyancy per ama, providing 200% of the total displacement of the boat, this being adequate (just) and still confers stability and sea-kindliness. Elevating the main hull by 5 or 6 feet and in a central location provides the remainder of the reserve buoyancy (but only when the vessel finds itself in conditions that need it). Here's a very basic schematic: m m Side view: a ---------------- a s cabin s t ---------------- t 2 amas - 0000000000000000000000000000000000000000000000 Because the buoyancy in the amas is minimal but adequate, the wetted surface area of the amas combined is not excessive. Consider each ama as a 3 ft diameter "cylindrical" at 80 ft long, with little rocker, and 50% immersion, equates to approx 750 sq ft wetted surface. This is only marginally more than an 80 ft monohull (670 sq ft) but its 60+ sq ft keel makes them almost equivalent in this regard. So wetted surface area should not be a drawback (in this length range). Total buoyancy for these amas amounts to 16+16 = 32 tonnes. This allows an operating displacement around 15 tonnes, allocated equally to amas and cabin contents/superstructure (5T each), thus keeping CG low (but 4 ft above water). Frontal area is a mere 6 sq ft per ama, making an excellent wave-piercer. This is quite an improvement over say a similar catamaran, since less water is pushed aside and the shortest distance is thru the waves, not over them. Lateral resistance of the amas amounts to over 160 sq ft, and although shallow, there should be little need for a keel. (This also makes for good directional stability, though could present some difficulty in turning). Two masts perhaps, allowing smaller sails for easier handling by crew. Thanks again, Sailnut On Oct 24, 9:53 pm, DSK wrote: wrote: 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.?? Please explain further. Perhaps you mean the reduced volume available for reserve bouyancy? Apart from this pre-requisite, I see no serious drawbacks to creating a superior performance and wave-piercing catamaran.You don't think that the lack of reserve bouyancy, and the concomitant stability, is a "serious" drawback? Everybody else does. Fresh Breezes- Doug King |
#12
posted to rec.boats.cruising
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Sail-powered SWATH catamaran
...Perhaps I should have emphasized the wave-piercing
aspect rather than make mention of SWATH. ... A narrow water plane and light weight are key to high speed in catamarans. The A class cats have been leading the way in very low volume, very fine hull shapes. They are very much wave piercing designs. The new C class "Object 2" by Steve Killing is a good example of the current state of the art in this. Many cruising cats have been designed with very fine bows and narrow water planes often expanding at a distinct chine to get reasonable volume in the living spaces. Some have even had wave true wave-piercing bows extending beyond the main hulls. However, the trend in dedicated cruising boats has been to towards wider, simpler hulls. When Alexander Simonis was designing the Moorings 4500 he did a study and found that Fn's rarely exceeded 0.6 in the cruising fleet and concluded that ideal cruising cat hulls should place more emphasis on wetted surface and load carrying and less on wave drag. His hulls ended up being around 1:8 length to beam and quite full and round. The advantages of simple tooling, high volume and good load carrying are hard to argue against in a dedicated cruiser. In the racing fleet very light, slender, wave piercing hulls have proven themselves. Sadly, there seems to be comparatively little demand for cat designs that take the middle ground. -- Tom. |
#13
posted to rec.boats.cruising
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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! -- Qhiron Message Origin: TRAVEL.com |
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