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Old 14 June 2007, 04:18   #1
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Country: UK - Scotland
Town: Ayrshire
Boat name: Raven
Make: Ribcraft
Length: 5m +
Engine: 150 suzuki
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Suz 6 hp propping

I currently have a 6 hp suz,the manufacturer only produce two props for this engine,"6 and "7.

Technically (after looking at the spec's)the only difference between the 4,5 and 6hp is RPM.

When this engine is on the back of a 3.4mtr sib,it will get the little boat up and on the plane (even with a chubster like me in it)

however even with the"6 prop, the engine used as an aux on my rib, simply cannot rev hard enough to produce the goods.

Does anyone know where a lower pitched prop for this small engine could be sourced?

Thanks,

IB.
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Old 15 June 2007, 15:24   #2
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I'd be interested in more info on this if you find anything out, as I have the same engine (Johnson 6hp which is a Suz painted white!) and the same problem.

One thing I can't completely get my head around is the "hull speed" which is the theoretical maximum displacement speed of the hull or something, I'm not sure if I could get more than the current 4.5kt (approx) by finding the extra couple of thousand revs, or if it would just use a lot more fuel! At the moment it makes very little difference to revs (or speed) on my aux if you have it at half throttle or full throttle, it just sounds a lot more strained at full throttle and presumably uses more fuel too, though I haven't measured it. You only get a fraction of a knot extra by using full throttle.
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Old 15 June 2007, 19:33   #3
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Originally Posted by BogMonster View Post
One thing I can't completely get my head around is the "hull speed" which is the theoretical maximum displacement speed of the hull or something, I'm not sure if I could get more than the current 4.5kt (approx) by finding the extra couple of thousand revs, or if it would just use a lot more fuel! At the moment it makes very little difference to revs (or speed) on my aux if you have it at half throttle or full throttle, it just sounds a lot more strained at full throttle and presumably uses more fuel too, though I haven't measured it. You only get a fraction of a knot extra by using full throttle.
That's pretty much what'll happen unless you have enough HP to push the boat onto the plane but a lower pitch prop will mean less slip at high throttle openings and should in theory (though I haven't tried it) mean that waves/swell on the nose won't stop you dead as easily.
If you think about it in terms of gearing it's like moving a Landy at tickover in 1st/2wd/HR at 2mph versus moving the same Landy at 2mph/3k rpm/1st/4wd/Low range. It'll be a lot harder for hills/bumps to stop the Landy in LR but it'll use more fuel.

At least that's my theory...
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Old 18 June 2007, 13:06   #4
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I see what you mean, the odd thing is though, if you open my aux to full throttle, there is really no appreciable difference in WOT revs between standstill and once you get up to 4.5kts - unlike on the main engine where the "stall speed" (if the boat is on the trailer for example) is about 2500rpm at full throttle (followed by a clang as a rock hits the prop, no prizes for guessing how I know that... ) then the revs rise as the speed starts to come up.
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