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Old 11 June 2007, 03:09   #1
Country: UK - England
Town: Mudeford
Make: Humber Ocean Pro 5.3
Length: 5m +
Engine: 90ho Tohatsu
Join Date: Apr 2005
Posts: 137
another rpm and prop post......

Hi all,

At WOT i'm hitting the rev limit (6000rpm, Tohatsu 90 TLDI) so i think i need a new prop. I hit the rev limit pretty easily with 2 up and a full tank of fuel. Am i right hearing that for every inch increase of pitch will reduce the rpm by 100? I think it would rev (if the engine electronics would let it) up to about 6200rpm, so i guess i need a prop with an extra 3inch pitch to reduce the WOT rpm down to a safe 5900rpm? Am i assuming everything right?

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Old 11 June 2007, 03:24   #2
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Country: UK - Isle of Man
Town: Douglas
Length: no boat
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Posts: 337

Based on the information you have given I would try a prop with 2in more pitch. Usually a 1 in increase in pitch will drop your WOT by circa 200 RPM, but every engine/boat combination is different. Ideally, you want to go to your favourite supplier and see if you can get a 'return' agreement so that if the prop does not produce the right results and is returned completely undamaged you can swap it for another, more appropriate, one. Another option is to talk to the experts at Steel Developments, they have all the best answers!!

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Old 11 June 2007, 03:30   #3
Country: UK - England
Town: Dorset
Boat name: Pending
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Posts: 198
Prop Size......

Speak to STEEL DEVELOPMENTS LONDON, the one stop prop shop, what they dont know aint worth knowing.
Tel; 0208-8747059

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Old 11 June 2007, 09:36   #4
Country: USA
Town: Oakland CA
Length: 3m +
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Posts: 6,653
From an old post I had saved:

General rules for props:

switch from aluminum to SS (all dimensions are the same) should drop WOT RPM by ~100
an increase in diameter of 1/4 inch should drop WOT RPM by ~100
an increase in pitch of 1 inch should drop WOT RPM by ~150 (range is 100 to 200)

3-blade vs 4-blade
4 blade props have some advanteges over 3 blade. If you have like material, then 4 blades promote stern lift, give better mid range fuel econ, slower top speed, better holeshot, are more balanced and dont vibrate as much and allow you to mount your engine higher due to more blade surface area in the water.

Generally, the heavier the boat the more blade surface area you want, not more pitch. So, rather than going up in pitch, go up in diameter or number of blades or a combination of the 2. This promotes less slippage, but costs you top end. Make the boat more efficient in cruise rpm range.

These are REAL general. You must consider that manufacturer A's prop might have identical dimensions as manufacturer B's prop, but there will be a large difference in WOT RPM's. This is usually due to the variation of blade width, cup, hub flare... There are many things that can cause a difference.

The other thing that could cause such a difference is the current at the time of your test. Maybe if you were able to run the other direction with the props in question, you would see a speed decrease of 5 mph. This would make your average lower and affect your calculations. With out the second pass, you shouldn't count the data. unless you can compare the one way pass with these props to the same direction one way passes with all the other props. Then the comparisons would be level.

Edited by physicistkev 4/26/2005 9:33 AM

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Old 11 June 2007, 16:41   #5
Country: UK - England
Boat name: SOLD
Length: under 3m
Join Date: Jun 2004
Posts: 794
I have the same problem i can not get a prop that will hit top speed at full RPM
i have been up and down in prop size,s
But last week i spoke to the guy from RM Marine at the Camber in Portsmouth
He said that there is no such thing as the right prop.
He said get the one that dose all things well send to the prop man from Aldershot with details of speed @ WOT The RPM etc, and he will ajust the pitch to suit
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Old 11 June 2007, 17:16   #6
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Country: UK - England
Town: The wilds of Wiltshire
Boat name: WhiteNoise/Dominator
Make: Ballistic 7.8/SR5.4
Length: 7m +
Engine: Opti 225/Yam 85
MMSI: 235090687/235055163
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Originally Posted by Roy Smith View Post
I have the same problem i can not get a prop that will hit top speed at full RPM
You don't want a prop that will allow the engine to hit full rpm. Get hold of the HP/torque graph for your engine and look at the point you get maximum HP. That's roughly the RPM you want-and it'll be below the redline.
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