gasoline engine fuel efficiency

[david quillin]

I have a 39' Hooper Island Draketail. It has an 8' beam, and is supposed to be especially efficient moving through the water due to its long narrow shape. I am a little disappointed in my fuel consumption, however. I have a 320hp gasoline engine, new, in good shape. Running at around 2800rpm, or about 10-12 knots, it drinks 5-6 gallons per hour. Running around 3200rpm, or about 14-16 knots, it takes 8-10 gph. The recommended cruising rpm from the engine manufacturer is the 3200rpm. The boat displaces 7500 pounds. This is my first inboard engine, so my question is, do these numbers sound typical? Good, bad? It seems to me she should do better, but I don't have much to base that on. Also, does it hurt the engine to be typically running well below the recommended cruising rpm? Any thoughts? p.s. I would love to add a pic of the boat but can't figure out how to do it...

[Gresham CA]

I'm not familiar with that much hp but from what I'm seeing on another forum that sounds about right.

[John Bell]

That sounds like average to pretty good fuel economy for a gas-powered four ton boat operated at greater than displacement speeds.

[ssor]

These are displacement hulls and about 8 knots is hull speed. After that you are just making waves. My guess is that you are overpowered. Your engine will last much better @2600rpm than @ 3200rpm.

[david quillin]

Thanks for the feedback, it gives me some reassurance. This is a semi-diplacement hull, not a displacement hull, and requires a bit of extra throttle to get up out of the water at around the hull displacement speed (seems to be closer to 10 knots in the water) but then drives along fairly easily over that.

[WoodenBNut]

I have a 32' x 11' beam semi-displacement hull powered with a similar type engine and that is the same fuel consumption range that I am experiencing.
I would condider that gph to be good to average for that size boat/motor.

[Dan McCosh]

The fuel economy sounds pretty good to me. As for the wear-and-tear on the engine, it would depend on how the rpm is achieved. Assuming the prop pitch is set so that you can reach the maximum rated horsepower, lowering the rpm should simply mean less wear and tear. If the lower rpm is achieved by an overload on the prop, it would actually be harmful to the engine. In your case, all things seem correct, and the rpm is mainly a choice as to fuel consumption.