Subway:
Not to disagree with all the some of the other replies but having been a Yanmar rep for over 15 years and sailboat
propeller business for another 15 years, I have to put my two cents in.
Your Yanmar 2GM20F in a 28' boat should push that boat along at over 4 knots at those RPM, especially with a newly painted bottom. You should have the reserve to WOT to fight into
head winds and seas at approximately the same speed.
You don't mention the
propeller at all, number of blades, diameter,
pitch or
transmission ratio. This will, like they say make a difference to your wide open throttle rpm.
In neutral at the
dock you should be able to reach 3800 - 4,000 rpm to show you have throttle settings correct and no restrictions. Scary to do but best way to check the linkage.
Underway at WOT you should as they suggested be able reach 3400 - 3600 rpm. Naturally it would be preferable to do it after the engine is warmed up. If it goes over the 3600 your pitch/diameter is probably too low and could be increased. That would also give you higher boat speed at your optimum running rpm that you are presently using. If it only reaches 3200 or less your problem is not likely to be resolved with the propeller as it may be a transmission or engine
power problem since you aren't getting the speed at low rpm with an over pitched prop.
At any rate running at the 2000-2500 rpm range is not a problem but you should run it up over 3000 rpm for at least 10 - 20 minutes each time you use it to heat up the
exhaust and blow out the carbon and it also can help to keep your
hull clean. The water mixing
exhaust elbow needs this particularly as it can close up and restrict exhaust flow and cause
low power problems in your engine.
I am assuming that this is not a new
installation and it has had a previous owner.
Another point never use your engine for
charging battery by idling it at 1200 - 1500 rpm as it is not loaded up, as others have suggested and it does need to
work. Put the engine in reverse at
anchor if that is when you need to charge or at the
dock in
gear. At the dock better yet,
plug in and use
shore power to charge. Or even better change
anchorages and motor over
charging at the same time.
More than two cents, I know but once started hard to stop.