Q&A - RPM - Soundings Online

Q&A - RPM

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Q: How can you find the most economical rpm at which to run your planing boat?

Q: How can you find the most economical rpm at which to run your planing boat?

A: You can spend a lot of money on fancy gadgets and get a fairly precise answer, but you also can get a reasonable idea by experimentation. Generally I’ve found that when I want to go from point A to point B, the most economical speed is that which gets me a little beyond the point where I’m up on a plane. I want to be on a stable plane so that I won’t be coming off it if hit by a small wave or gust of wind.

The feel of the boat usually is a good indicator. If she feels like she’s having a tough time staying up, I’m probably burning more fuel than I need to go the distance. If she feels like she’s lightly flying along — but just at that point, not too far beyond — I’m probably at an economical speed. Usually it’s most economical to get on this point of plane quickly, rather than slowly increasing the throttle and dragging the stern. Soon you’ll know the acceleration rate needed to reach a stable plane without overshooting. Don’t do this in a crowded area or where it would be unsafe.

The most economical rpm will vary with such factors as sea and wind conditions, hull condition, loading and weight distribution. Even boats of the same model and power configuration may have different performance parameters. Weight and trim substantially affect performance. If I have a heavy load of passengers, I position them in the area or areas that help the boat get on a stable plane at the least rpm, assuming this positioning is consistent with safety. Other items, such as loaded coolers, also can make a difference. With experimentation, you’ll soon know how to distribute weight.

Trim tabs provide an extra tool, though they should be used to complement or fine tune the boat’s plane, not to achieve it irrespective of the other factors I’ve mentioned. The boat should be loaded properly and powered properly before you start adjusting trim tabs. The builder of your boat may have some helpful suggestions with regard to the use of trim tabs. If you have an outboard or sterndrive, the angle of the drive relative to the transom also is important. Consult the boatbuilder and engine manufacturer to get the best adjustment.

Experimentation should never be to the exclusion of full attention to traffic, conditions around the boat, the boat’s other performance issues, safety issues and prudent seamanship. It should be done only in open, safe waters and conditions with no other traffic or obstructions. Be very careful anytime anyone is moving about the boat while under way. If they must do so when the boat is up on plane (or running at any speed that could result in falling or instability), it’s best to have them crouch down so that they’re lower than the gunwales.

Often I’m out just messing around and simply want to enjoy a boat ride, as in the evening. I’m not trying to make a destination. At these times I get better economy at a very low rpm and not getting up on a plane. If I do this, I’ll get her up to fast speed at least a few minutes before I come in, to help keep the engine in good condition. (And I have to admit: That few minutes of high speed at the end of the evening sure feels good.)

A fuel-flow meter can give very precise information, but this is relatively expensive and must be installed properly to avoid performance and safety issues. Spending time instead of money, you can fill your tank at the beginning and end of each test run at different rpm over a constant distance — a GPS or knot meter is handy for this — and get very accurate information.

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