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Old 07 June 2016, 07:16   #1
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Choosing the right propellor

Hi,

I currently have a 5.3m RibQuest with a 2010 90hp 2 Stroke Yamaha.
Prop is 17'' aluminium standard Yamaha.

It is running at 5500rpm at 42mph top end. Acceleration is great but some cavitation when cornering hard.
Any thoughts on improving the top end? I've set the engine fairly high on the transom.
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Old 07 June 2016, 07:18   #2
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Personal thoughts are trying a 19'' Stainless
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Old 07 June 2016, 09:35   #3
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But it is great speed.
What is WOT range for this engine ?
Not idea what you would like to achieve if RPM is in WOT range.
Jump from 17" to 19" can lower RPM to 5200 so speed can be the same or even lower

If cavitation is the problem you can lower engine but MAYBE top speed be a little smaller in such case.
If you lower engine MAYBE RPM be a little less than now
but it require some test.

I lifted up engine and top speed and RPM were higher but cavitation and grip forced me to put again engine to lower position.
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Old 07 June 2016, 10:15   #4
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Something seems to be wrong.
I do have XLS showing max theoretical speed based on achieved RPM, gear ratio, slip
If your engine is 90A gear ratio is 2.0

RPM 5500
pitch prop 17
gear r. 2
slip % 3
knots 38,39421667
km/h 69,10959

You can not go faster use 17" prop than 38 knot (69 km/h)
Even if you have 3% slip which is really incredible low ... (usually 5-10%)

If we replace 17 to 19 in this equation we can have (and lower RPM to 5200)
you can (theoretically ) have 40 knots:
RPM 5200
pitch prop 19
gear r. 2
slip % 3
knots 40,57057333
km/h 73,027032

This is all about math and physics. Not sure if spending much more money on 19" prop is really worth 3 km/h more. But again this is simple equation showing max possible.
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Old 07 June 2016, 10:28   #5
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Originally Posted by Ribquest5.3 View Post
Any thoughts on improving the top end?
Do you think you'll often have any need to go over 40 knots? Not many people with 5.3m boats are regularly sitting that fast for long periods. Props are all about compromise, yours sounds pretty good.

Quote:
I've set the engine fairly high on the transom.
thats probably why you are getting ventilation in tight turns. Are you trimming down/in for the turn?
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Old 07 June 2016, 11:28   #6
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Originally Posted by MatFromPoland View Post
Something seems to be wrong.
It is - you're using knots and the OP quoted his WOT speed as 42MPH = 36.5kts approx.

OP - I "think" that 5500rpm is the upper limit for this motor? You can afford to drop back a bit on the revs and not stress the engine - 5200/5300 should be fine. If you go stainless and keep the same 17" pitch you should get some improvement in overall performance as the stainless flexes less. Go up to a 19" pitch and you'll see a drop in WOT revs but you may not get much extra top end - it will depend to some extent where the motor generates it's max power? A torque curve might help you decide...
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Old 07 June 2016, 11:48   #7
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Yes I was reading KNOTS not MPH In such case theory looks like this one mentioned below:

Current 17":

RPM 5500
pitch prop 17
gear r. 2
slip % 5
knots 37,60258333
km/h 67,68465
mph 42,30290625

Maybe it be like this after jump to 19" (RPM will drop about 300):
RPM 5200
pitch prop 19
gear r. 2
slip % 5
knots 39,73406667
km/h 71,52132
mph 44,700825

Still theory
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Old 07 June 2016, 13:20   #8
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Quote:
Originally Posted by Ribquest5.3 View Post
Hi,

I currently have a 5.3m RibQuest with a 2010 90hp 2 Stroke Yamaha.
Prop is 17'' aluminium standard Yamaha.

It is running at 5500rpm at 42mph top end. Acceleration is great but some cavitation when cornering hard.
Any thoughts on improving the top end? I've set the engine fairly high on the transom.
Best I got 14 x 18 Ali prop WOT 45 mph ribquest 5.85 90 hp Suzuki 4 st engine was sat on the transom no shims
No cavitation
Cheers
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Old 07 June 2016, 14:45   #9
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Quote:
Originally Posted by Ribquest5.3 View Post
Personal thoughts are trying a 19'' Stainless
Did you test having nominal load ?

If you test alone but usually will go with more people maybe 17" is optimum.
If you plan to tow a toy 17" be better also.
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Old 07 June 2016, 15:23   #10
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Did you test having nominal load ?

If you test alone but usually will go with more people maybe 17" is optimum.
If you plan to tow a toy 17" be better also.
My thoughts exactly
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