Most engines has test curves ( likely on a different hull) showing speed v gph. That's usually the most efficient spot. The results may not be exactly the same. But are very close on rpms.
5000 rpms you can run. But you're on a steeper less efficient part of the operating curve.
And with boat engines we are talking fractions of an MPG improvement at cruising speed.
Of course engine trim make you more efficient too. Trim tabs use drag and are NOT efficient.
Play with trim, you will be amazed.
Set the throttles at say 3500 rpm. and watch your GPS speed over bottom. Now start trimming. You want the prop thrust parallel to the water surface. Usually the engine top is parallel to water also. You'd be surprised. Without touching the throttles ( GALLONS PER HOUR IS CONSTANT) you will see the boat speed pick up close to 2 knots in some instances w/o adding another ounce of fuel flow.
Remember rpm's do not control fuel flow- the throttles do. So more rpms for same fuel while trimming the O/B USUALLY is going to show up in more mph. Higher efficiency. Of course trim changes all the time with sea conditions.
But on those long runs in or off shore conditions could the same for over an hour. So dial it in.
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Originally Posted by Gerry Zagorski
Every hull is different but 5000 RPMs should not hurt the engine. If however you are looking for best fuel consumption RPM you will need a flow scan to tell unless you have some sort of fuel management built into your guages.
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