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.
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
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
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
. At the dock better yet, plug
in and use shore power
to charge. Or even better change anchorages
at the same time.
More than two cents, I know but once started hard to stop.