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post #4 of Old 09-20-2012
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Re: Universal M25 losing power

I have the Universal 5424, which has a max RPM of 2800 but on my boat only reaches 2400. This is due to the large pitch 3 bladed propeller.

There is a misconception that you have the correct propeller if the engine just about reaches max rpm. This is based on a misunderstanding of how an engine works. Peak power is not necessarily developed at max rpm. This varies from engine to engine, as the plot of torque vs. rpm varies and power is a product of torque and rpm.

Optimum performance will be reached when the propellor is selected such that the engine just reaches it's maximum POWER, not rpm. I've seen numerous posts on fora where people say that if you can't reach max rpm, you are overpropped or the engine is faulty. Not necessarily.

Should you be selecting the prop based on performance of the boat through the water, or on reaching the rpm spec? There was a recent post where someone followed the forum advice concerning the boat being overpropped, put a lower pitch one on, and now achieves max. rpm - at the cost of 1 knot maximum speed and increased fuel consumption due to cruising at higher rpm.

Stop obsessing over getting it to 3000rpm and instead concern yourself with the boat's performance. If you're getting to hull speed at reasonable rpm with good fuel consumption, all is good.

PS, when my max rpm starts dropping I know it's time to get the boat dived. A dirty prop makes a BIG difference.

PPS, You'll get less rpm when the boat is stationary that when it's moving.

PPPS, sounds like the one thing we're sure of is that your dock cleats, docking lines, and boat cleats are all in good shape.

PPPPS (gosh), diesel tachometers are only as accurate as they were adjusted to be. Don't take any rpm reading as gospel without confirming it with a strobe.

Bristol 31.1, San Francisco Bay

Last edited by MarkSF; 09-20-2012 at 02:28 PM.
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