Join Date: Oct 2009
Location: Gloucester, MA
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It does sound possible that the gearing and propeller pitch are not set up properly.
To the original question, there isn't a magic rpm, there are a few things that you want to avoid but a relatively wide range of workable rpms. If you are in fact overpropped, then you are lugging the engine. This means that you are trying to get too much power out of the engine per combustion cycle. For the same power output, lower rpm means less combustion events which means that each event must supply more power. This puts greater stress on the mechanical parts of your engine (a common problem from lugging is blown bottom end bearings) and raises EGT.
Making the assumption that the prop is correct, then the dangers of low rpm are related to engine temperatures. If the engine is not doing enough work to bring all of the components up to the working temperatures, you could have problems with wet stacking or stuck rings. Wet stacking is when not all of the fuel is burned so it starts to wash your cylinder walls, coat valves, coat the exhaust, etc. Since I doubt that you have an EGT gauge, the only gauge that you have to help you with this is your temp gauge. If it ever reads low, give the engine more throttle. Also, if you see signs of unburned fuel, then you need to throttle up. Taking a guess since I don't know the exact setup, I would think your boat with the correct prop would be safe over 1500rpm but you could always rev up a bit more to be safe.
The issues with really high rpm are related to longevity. Fatigue is due to two factors, stress and the number of cycles. More rpm means more cycles for a given period of time. Also, high rpm tends to also mean high stress. These factors combine to mean that parts fatigue more quickly.
The acceptable rpm range is relatively wide. You may find that the sweet spot is something like 1800-2400 on your engine but it really comes down to how you value your time (boat speed), engine life and fuel consumption. It does sound like you may have a more fundamental setup issue to solve first though.