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Old 17-06-2019, 07:05   #31
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Re: Calculating power requirements for catamaran.

Originally Posted by Pauls View Post
Let's keep this simple. SA/Displacement is a ratio of power (sail area) to displacement.
Let's keep this simple. While power is somewhat dependent on sail area in some conditions, it does not mean the same thing at all. Both point of sail and wind speed have a lot to do with power of sails.
In heavier conditions, power is more dependent on righting moment (ability to carry sails), not maximum sail area.
Originally Posted by Pauls View Post
SA/D varies greatly, depending on the targeted use of the boat. You know what you want your boat to do. Look at the SA/d of comparable boats designed for similar use, and use that information to make a decision on the SA/D for your design. This is simple. And it is reality based. Making comparisons based on real world performance is a very solid, realistic approach.
Exactly, whenever similar boats having suitable SA/D is available for comparison. I don't think there are many with only 100 sqft of maximum working sail area for each tonne of weight. There are many with more storm sail area than that.
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Old 17-06-2019, 12:19   #32
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Re: Calculating power requirements for catamaran.

That's the point of comparing SA/D to similar vessels of similar performance levels. The proposal of 100 sq ft of sail area to 2000 lbs results in a SA/D of 10.07. I would consider that an absurdly small number.

SA/D is a basic value used in designing the rig of a boat. For good reason.
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Old 22-06-2019, 00:31   #33
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Re: Calculating power requirements for catamaran.

Originally Posted by dustman View Post
The beam would be 20', and the schooner rig would have a low CE.

The Wharram wingsail is not a true rigid wing sail and is reefable, supposedly fairly easily.

I'm also trying keep the masts pretty short and keep weight to a minimum. More sail means more beefy structure and more weight, more weight means more materials and more rigid materials. All this equates to increased cost and complexity and reduced ease of use and safety.

I want safe, simple, frugal, even if it means I get around a little slower. But I really don't feel like if I stay in the tradewind belts that 200 miles a day is a stretch. But, alas, I have little first hand experience, so here I am to make sure I'm not getting ahead myself.
It seems to me you want your hulls to be like a giant beach cat, but with an Optimist sail...? You say there will be no accomodation in the hulls, but if you build a pod or cabin on deck between the hulls, this will add to your windage, strength and construction issues. Wharrams, Woods, and Harry Proa all use the hulls for accomodation (as do all other production cats, albeit with huge central cabins as well).

If you are serious about your design, build a 20' version first to test in the real world. Otherwise build or buy a small Wharram and get experience with that. There are a number of Tiki 38s cruising the world, some posting on Youtube. They seem to get very good daily runs in the right conditions, and are safe, easily handled boats. Why not build one of those and add modifications to make it your boat?
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catamaran, displacement, length, power, sail area, speed

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