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Old 06 March 2007, 13:22   #1
Country: UK - England
Boat name: SOLD
Length: under 3m
Join Date: Jun 2004
Posts: 794
Help on Prop size

Have just changed prop from stanard 23 black S/S to a 22"Yamaha salt water polished S/S pro went out on flat water W.O.T 6,000rpm max trimmed up with four people and nearly a full tank 43kts max
BHG tell me that realy i should get the engine to rev no more than 5800 rpm
so i thought that a 23" would be perfect but they only make 24"
i havant had time to try the 24" but is there a rule of thumb i/e more prop less rev,s etc, as i thought that if we where only two up with say a 1/3 of a tank
would i start to over rev ?

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Old 06 March 2007, 16:25   #2
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Nos4r2's Avatar
Country: UK - England
Town: The wilds of Wiltshire
Boat name: WhiteNoise/Dominator
Make: Ballistic 7.8/SR5.4
Length: 7m +
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MMSI: 235090687/235055163
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This is just a personal opinion, but your engine has a Wide Open Throttle rev range recommended by the manufacturer. Personally I'd keep going up in pitch with your max load of people/gear onboard til you're about 100-200 rpm above the minimum recommended RPM at WOT.
Then dump then on a beach for 10 mins and try it out solo at WOT and see what your revs are.

You shouldn't (in THEORY) be overreving set up like that and you should gain speed and use a bit less fuel.
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Old 06 March 2007, 17:08   #3
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Country: UK - England
Town: Berkshire and Devon
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Length: under 3m
Engine: Mercury 5hp
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Whoa! Slow down Roy! When you started down the route of swopping props, what was your reason for this? And what were the performance stats, ie what speed were you achiheving with the standard 23" and what revs was the engine reaching at that speed?

BHG are fairly reasonable in what they say, as from Yamaha's web site Full throttle operating range: 5000~6000rpm.

Sounds like you need to try the coarser prop (24") as 43kts with the 22" is far too slow for your RIB and you've probably hit max revs too early.
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