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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #1  
Old 09-24-2008
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Should a prop be pitched . .

. . . . at having the ability to run at max RPM?

This is not an attempt to discuss the finer points of prop design - I would just like to get opinions on this given some sentiments expressed in another thread.

I have never had a desire to achieve max RPM on any engine and have personally never run any engine at max RPM intentionally. My present boat has (in my ownership, at least) never run at over 2600 rpm for any reason.

My props have always been pitched to run at optimum RPM with adequate grunt to move the boat at an acceptable cruising speed when the engine is in the best combination of torque and fuel economy and coincidentally, the boat I now own appears to have this attribute. When I have had to motor long distances, I have done it at 2400 RPM which is 6.5 knots with very good fuel economy. I reckon that when I reach hull speed the only difference any additional throttle will provide will be the amount of fuel I use, not a speed benefit.

Have I got it wrong? Should the prop be pitched to ensure that the engine can run at max? Surely this would mean that when you're not at max RPM, your boat is running somewhat slower than it could?
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Old 09-24-2008
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prop pitch

the reason they recommend the prop to have a pitch that allows the engine to reach max RPM is that the engine attains max HP at max RPM. if the prop has too great a pitch the engine will be overloaded trying to attain max RPM. if the pitch is too little the boat speed will be slower & fuel economy will suffer. i have a nimble 30 express with a yanmar 2GM20F . tranmission gear ratio is 2.62 so at 2750 rpm the shaft is turning 1049.6 rpm and 6.3kn. at 3600 engine rpm i get 6.9 kn , which is hull speed. i have a Perfect Pitch RH 15x11 prop. i would have liked to have had a 15x12 , but they did not make one and i had to have one in a hurry because my old prop broke its blades
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Old 09-25-2008
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Quote:
Originally Posted by captbillc View Post
if the prop has too great a pitch the engine will be overloaded trying to attain max RPM.
That much I understand (although I believe the pitch would need to absurdly coarse to cause an overload).

This is where I lose the understanding.

Why do you want to attain max RPM? What benefit do you derive from this? If you look at the specific fuel curve of any engine, max RPM and max fuel consumption are at the same revs. Why do you want your engine to operate there?
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Old 09-25-2008
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You might want acceleration thrust; when maneuvering or to get out of the way of something while underway. You also might want to open the throttle fully now and then to blow carbon deposits out of the combustion chambers. You'd be amazed at how carboned up a diesel can get if it is not goosed or run hard now and then. I'm not saying that "abuse" is good; but occasional hard driving is beneficial to any engine; espcially a diesel because they are designed for heavy loads (high torque, lower HP). Nearly all diesel engines are designed to run for short periods at high RPM without damage but check your particular engine's manual to be sure). Diesels are speed governed and the max RPM is set by the factory by adjusting the injector pump max flow. Running too long (more than 5-10 minutes) above max cruising RPM would likely cause it to overheat.
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Old 09-25-2008
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I'm with you, Andre. Max. RPM is only for emergencies. High RPM (2,800-3,200 RPM in my case with max being 3,500) would be to blow out the carbon or if I had to go seven knots (just short of hull speed) for some medical reason, say.

Or if running a day at a knot more would take me 24 NM further away from a cyclone, maybe...

If I can't use the sails, I motor, but I want to motor to a sweet spot in the fuel economy curve. That last knot of speed tends to cost 40% or more in fuel in my experience, and so I find a "flat five" knots at 1,900-2,300 RPM (depending on seas) works quite well for me. Usually, I motorsail at 6.5 knots, of which 4 knots is engine at 1,600 RPM or so and this is because it's worth 10 dollars of fuel to me to run the engine up to temperature for two hours if I'm using the big boat for light-air sailing.
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Old 09-25-2008
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Quote:
Originally Posted by KeelHaulin
You might want acceleration thrust; when maneuvering or to get out of the way of something while underway.
Exactly! When something goes wrong and you need to move NOW! you should be able to give it full throttle, achieve max RPMs, and GO!
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Old 09-25-2008
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Perhaps the ability to achieve the rated engine RPM in gear tells us that the boat is not overproped? If the engine cannot achieve rated RPM then it cannot produce max horsepower...thus we shoot ourselves in the foot.

Look at the following picture for my engine.... the more it revs the more HP it produces...usefull when I need it

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Old 09-25-2008
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There is not any boat motor of any type that is NOT required to be able to reach full RPM with its normal load

Over proping cause problems on everything ,while it may be more dramatic on a 4s outboard it is not good for a diesel over the long term
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Old 09-25-2008
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My old Volvo MD17C has peak power at 2500 rpm. The current prop will not let it get beyond 2250 rpm. The motor is not built to hold peak power for long anyway, methinks.
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Old 09-25-2008
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The biggest issue with improper over-pitch on a propeller is not engine strain but cavitation. This is the formation (and subsequent collapse) of vapor bubbles on the prop blade that causes noise, vibration, and can extensively damage the blades and drivetrain. These collapsing bubbles are literally hammering on the blades, and the mechanical force of these collapses should not be underestimated.

However, even a cavitating prop at max RPM will still give you acceleration up to hull speed. It is after hull speed is reached that RPM should be reduced to stop cavitating. Short bursts will not damage the propeller: the damage is caused by continuous cavitation over time.

As to Omatako's original query, I can tell you this much: on my old nuclear submarine, the prop was not pitched for max RPM, it was pitched and optimized for patrol cruising RPM.
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Last edited by TillerJohn; 09-26-2008 at 03:49 AM.
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