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Old 06-23-2010
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Question Fast cat hull speed and overpowering

Hi all,

Complete newB here just wondering if someone can clarify a couple things that have been on my mind. I'll not be doing my first sailing course for a couple more weeks, but have been doing heaps of reading on sailing theory as I would like to walk into the course comfortable with the classroom stuff so I can concentrate on the actual fun bit. I guess it's also 'cause I'm a bit of a nerd too .

Jeff_H put it very well in a post I came across explaining how the accumulated forces of a sail can be thought of as having three active components; Drive- which is the component of the force that acts in the forward direction of the boat, Drag which is a component that acts to reduce drive, and Heeling Forces which operate across the boat. He then goes on to say the following: "In an ideal sense the goal in sailing is to maximize drive while keeping the other two components under control".

Overpower a monohull and you'll end up with the rails in the water, simple to understand. But how does this work on a multihull? How does a multihull skipper know when he is getting overpowered as he can't rely on the heeling degree. Is it a bit of a seat of the pants thing that a multihull skipper just has to get a feel for?

Another thing that I'm trying to figure out is how some of the faster cats can travel way above hull speed. I'm talking about 40 footers cruising well above 15 knots in a good blow. Not surfing, just cruising. Are these vessels effectively planing or is there something else at play here?

“Noah was a brave man to sail in a wooden boat with two termites.”
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Old 06-23-2010
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A multihull sailor has to take care not to get overpowered. This is one major reason multihulls reef for PEAK wind speeds rather than the average wind speeds. When a gust hits a monohull, it can often bleed ofb much of the excess force by heeling--something a multihull can't do. Some multihulls will have distinct warning signs of when they are getting ovepowered, others will not.

Many catamarans, especially those designed for the charter trade, are poor sailing craft and not much faster than monohulls. Others, like the Chris White-designed Atlantic series or the Melvin & Morelli designed Gunboat line, are very good and typically sail in the teens or higher.

I'd point out that these boats are not exceeding hull speed, but that their design requires a coefficient much higher than the 1.34 typically used for displacement monohulls.

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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

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