Originally Posted by blinkerfluid
My thoughts are that if we are over propped then we'd have other symptoms along with the low rpm. Does this seem like the right line of thinking?
Unfortunately I don't know the diameter or pitch of the prop as, other than for the survey, we haven't had the boat out of the water
yet. Any other symptoms we should be looking for to indicate we're just over propped?
The prop looks clean enough in the picture. To know if you're overpropped....
FIRST, you must know what the manufacturer says is the maximum rated rpm of the engine.
SECOND, warm up the engine and wind
it up to max rpm in neutral. It should reach maximum rated rpm + about 100. (Never mind laser tach, just read the analogue tach).
THIRD, take the boat for a spin under power on a calm day. Put the hammer down and, for a few minutes, wind
up the engine to max rpm with the boat at hull speed
. If the engine reaches the manufacturer's maximum rated rpm there, the prop is sized correctly. If it only reaches a few hundred rpm less
than the maximum rated rpm there
, it is overpropped.
From what you've said (It's a Campbell Sailer so it's been sized fairly recently, there's no black smoke and the mixer elbow
on the exhaust
is not plugging with black deposit) I'd bet a coffee and donut (not more!) that the prop is correctly sized.
Here, there's a Yanmar
3GM30F with a 17 X 10" prop. The manufacturer's maximum rated rpm is 3600 rpm and it winds up to 3700 rpm in neutral.
(Now note carefully==>) At the dock
, in still water, in gear the engine gets up to 3100 rpm, with smoke. (<==Did you see that?)
Cruising under power, at hull speed
, the Yanmar
happily winds up to 3610 rpm with a clean exhaust.
Now that is a perfectly sized prop. Lunenburg Foundries in Luneburg, Nova Scotia
sized it and cast it. The engine pours all of its energy into the prop at hull speed, where it counts. If it reached max rated rpm at the dock
it would be coasting a bit at hull speed, because of the slip of the water sliding past, with some energy to spare.