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Old 26 August 2006, 02:16   #11
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Country: UK - Wales
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Boat name: DynaMoHumm/ SRV/deja
Make: Avon8.4, 5.4 & 4.777
Length: 8m +
Engine: Cat3126 Yam 90 &70
MMSI: 42
Join Date: Dec 2003
Posts: 6,556
some boat eh!

Quote:
Originally Posted by Rogue Wave
The hull is a pretty good design and it will compare favourably with other ribs in it's class. you should be seeing at least thirty knots with your rig and closer to 70 with a 70hp.
should read 40 with a 70 hp

if you are seeing 5,500 revs and going very slowly then I would definitely check out the void
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Old 26 August 2006, 14:10   #12
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5500rpm and 12 knots? Doesn't sound right at all. Usually, if you are unable to get up on plane, the motor will bog down well into the 4000's, probably even lower (don't ask how I know this.)

Does the speed (as low as it is) seem to be linear with the rise in rpms, or does the thing accellerate to 12 kts and stay there while revs rise?

I'm thinking possibly a spun prop hub. If you have a buddy with the same engine, see if you can borrow his prop for a bit.

A waterlogged hull is a possibility, but I would think that if the overall weight was the problem, the revs would be much lower.

Dunno.

jky
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Old 26 August 2006, 15:27   #13
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Country: UK - Wales
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Boat name: DynaMoHumm/ SRV/deja
Make: Avon8.4, 5.4 & 4.777
Length: 8m +
Engine: Cat3126 Yam 90 &70
MMSI: 42
Join Date: Dec 2003
Posts: 6,556
you may well be right!
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Old 26 August 2006, 17:02   #14
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Boat name: imposter
Make: FunYak
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Engine: 2 stroke YAM 20 HP
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Quote:
Originally Posted by mrlimbeng
The theoretical speed will be approx 40.7 knots.

Does the weigh of my RHIBs will affect this speed? as it seem that my boat does not goes that fast at all.
Assume the weigh of the rhibs is 331kg , how will it affect the speed if i add more weight.

Thanks for the help.....
Remember this is the theoretical speed with NO slip on the prop etc. Realistically 10% is the best you would get so about 35knots (as others have said from experience) is about the best you can expect. BUT you should be getting much more than 12 knots. jky and RW both have practical suggestions as to what may be wrong so hopefully one of them will fix it.
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