[QUOTE=FlyMeAway;14710]I have the M35.

Operating manual says max throttle should give me around 3000 RPM, but I can't get it to top out over 2500. I've had the boat since April but just discovered this over the weekend (I knew top RPM was 2500, but I didn't realize that the manual suggested it should be higher than that).

Anybody else have this happen? What are the typical causes of this? Anything that should be particularly worrisome?[/QUOTE]

If you refer to the speed/power chart on page 2 of the document at [url]http://www.westerbeke.com/productBrochures/UNIVENG,%20M-35B.pdf[/url], you can see that 3000 RPM is where you get 35 hp. If you remove your propeller there's little doubt you can reach 3000 RPM. If your engine cannot exceed 2500 RPM that means that, given the prop you have, your boat requires about 28 hp at 2500 RPM. Since the power requirement curve is steeper than the power delivery curve, the motor cannot go faster.

To describe the situation in another way, if you had an infinite hp motor and made it spin 3000 RPM, you would find that the shaft hp at 3000 RPM exceeds 36 hp. You don't have an infinite power motor and your motor cannot deliver more than 36 hp, so you will be limited at a lower speed. Essentially, the max speed you are experiencing is where the shaft power requirement of the boat & propeller combination intersects with the shaft power delivered by the motor.

The manual recommends putting a prop on the boat that will allow the motor to spin at 3000 RPM. While that is a fine recommendation toward utilizing the maximum motor power, the fact of the matter is that the power required to move the boat grows logarithmically after something like 2000 RPM. The result is that higher engine speeds will consume a great deal more fuel yet provide only incrementally more boat speed. Most people don't feel that the last 0.5 knots of speed is worth doubling the fuel cost.

[QUOTE=FlyMeAway;14712]Thanks Greg! So this is a good thing?[/QUOTE]

David,

Not necessarily. First you have to calibrate the tachometer. Our tachs are notoriously inaccurate. The association has a free loaner portable tach to calibrate your tach. For a quick eyeball check, you can take your warmed up engine and while in neutral, floor the throttle and see what RPM the engine will turn. My mechanic says that the engine is governor limited and a 5 second WOT will not harm it. It should turn close to 3000 RPM.

If the tach is NOT out of calibration then there could be a number of problems which could keep you from turning maximum RPM (getting maximum hp) in gear. You could be "overproped", the bottom could be dirty, or your engine could have a problem such as a clogged fuel filter or injector. Only being able to turn an honest 2500RPM would not be a good thing.

I had questions about the accuracy of my tach. Picked up a decent laser tack at Harbor Frieght tools for about $30.00. Then used it to recalibrate the tach. Its really easy to do. And the cost of the one at HFT is not much more than the shipping cost of the Association loaner.
Sam