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post #7 of Old 09-20-2012
Maine Sail
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Re: Universal M25 losing power

Originally Posted by MarkSF View Post
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.

For Universal and Westerbeke engines, as well as Beta the manufacturers want to see max rated RPM or as close to it as you can get. we don't make this stuff up the MANUFACTURERS DO.... This is not where you need to run the engine all the time but where it needs to be able to get to so that the HP curve is appropriate across the curve.

This is a direct quote from the Universal M-25 XPB spec sheet. The engine is 3000 RPM rated..

"Universal recommends a propeller that will allow the engine to turn 3000 RPM underway at full throttle."

Originally Posted by MarkSF View Post
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.
Really? Is it all is good? Many of these manufacturers, Universal/Westerbeke included will void a warranty if the engine can't attain the specs they say needs to be hit. Joe Joyce of Westerbeke called it "lugging" the engine with a prop too big or over pitched so the engien can't attain its max rated RPM. The farthest off Westerbeke likes to see a prop is 100 RPM but they prefer closer to max if possible.

Originally Posted by MarkSF View Post
PS, when my max rpm starts dropping I know it's time to get the boat dived. A dirty prop makes a BIG difference.
Exactly all this is predicated on a clean bottom and prop. Even a small amount of slime can hinder performance.

Originally Posted by MarkSF View Post
PPS, You'll get less rpm when the boat is stationary that when it's moving.
Again very true. You can't test for max RPM attainment with the boat tied to the dock. Also many gear boxes have different ratios for forward and reverse.

Originally Posted by MarkSF View Post

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.
AMEN! This was a recent deviation card I left for a customer after measuring it with a digital photo tach: Engine manufacturers often show up to warranty "issue" claims with a digital tach and they do test for max RPM.

Tach Actual
1000 - 1025
1500 - 1400
2000 - 1900
2200 - 2100
2400 - 2275
2500 - 2375
2600 - 2450
2700 - 2525
3200 - 3050
3600 - 3350

Even with our engine looooong out of warranty Joe Joyce of Westerbeke flat out told me I was damaging my engine by running it 275-300 RPM off max rated with the prop we had. In the short time we ran the boat that way the soot on the transom was considerably more than with the right prop. I hauled the boat, sent the prop back and had it re-pitched. It now sings within 25-50 RPM of max rated just as it had for the previous 2700 hours..

PLEASE consult your engine manufacturer on this issue if you have doubts. They have these guidelines for a reason.

-Maine Sail / CS-36T

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Last edited by Maine Sail; 09-20-2012 at 04:45 PM.
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