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Old 05 April 2010, 15:38   #1
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Crusing speeds

I am looking for some honest answers here please regarding small ribs . I have a Valiant Dr 490. Cruses away very happly at 18 kn. Push to 20 KN and the rid can get severe. I am wondering if 18 kn is a resaonable crusing speed for this size of boat wish is used off the west coast of Ireland. I am experminting by adding ballast to the front of the rib to try to increase the crusing speed and thus increase the wetted area of the hull. This will also have the effect of moving the centre of bouyancy


TSM
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Old 05 April 2010, 17:38   #2
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Originally Posted by two stroke mick View Post
I am looking for some honest answers here please regarding small ribs . I have a Valiant Dr 490. Cruses away very happly at 18 kn. Push to 20 KN and the rid can get severe. I am wondering if 18 kn is a resaonable crusing speed for this size of boat wish is used off the west coast of Ireland. I am experminting by adding ballast to the front of the rib to try to increase the crusing speed and thus increase the wetted area of the hull. This will also have the effect of moving the centre of bouyancy


TSM

My old Zodiac 420 used to cruise at around that. However when on your own without passengers you may find it easier to go a little faster.

In that boat I used to carry a spare 10 litres of fuel up front, there was a very significant change in attitude when I used it up, so I would advise putting a bit of weight up there will help, but not too much for fear of becoming a submariner!!!

Hope this helps,

Gotchi
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Old 05 April 2010, 18:04   #3
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I reckon you're about on the money there Mick. When Parkesey had his Valiant 490 he was cruising around the same speed comfortably. He kept up with me at higher speeds but the passengers couldn't take it.
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Old 06 April 2010, 04:22   #4
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Yep,

Sr4 / Suz 30 18knt = Comfortable , 22knt (max) = not.
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Old 06 April 2010, 07:20   #5
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I also have a theory thatthe force that a hull has to withstand is a funtion of the square root of the speed i.e. at 20 kn the hull has to widthstand approximatly 19% more energy than at 18 kn.
F = M*V*V
F= force
M = Mass
V =velocity
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