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Old 12-06-2012
jackdale
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Re: Course to steer at a given speed.

Quote:
Originally Posted by seanpatrick View Post
A while back, I was reading the chapter in Bowditch dealing with dead reckoning. I was especially interested in the section dealing with course and speed made good. Well, last night on my lunch break (I work nights), I decided to solve a quick problem I made up:

"If I were on a ship which was 10nm due East of port, with a set of 360° and drift of 3 kts., and I needed to arrive in port in exactly one hour, what course and speed should I use?"

I figured the answer to be 253° at 10.5 kts. But then I started to wonder: That's all well and good for a vessel under engine power, but what about a sailboat? I know the way to figure course to steer at a given speed, but wouldn't turning a sailboat into the current (as in my example) actually slow the speed through the water enough to change the necessary course to steer? Or is this error so small as to be negligible? Or is there some other way to factor that in?

Regards,
Sean
You answered the questions as you should. In order to maintain a speed made of of 10 knots, you would need to have a boat speed (knotmeter) of 10.5 knots. In theory, it does not matter if you are a powerboat or a sail boat. In fact a powerboat on flat water can maintain a speed; a sailboat subject to wind speeds, helming ability, trim, sea conditions, etc.. is much more difficult.

BTW - am average boat speed 10.5 knots in a sailboat is really rather quick.

If you really want to get it right, you also need to account for leeway.

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