Question about Fuel Economy

[USAFRet]

The engine cruise RPM is 3100-3200. Max RPM is 4600, it reaches 4100 with the current setup. In general, is the economy at 1500 rpm (around 12 mph by my gps) similar to the higher, cruise setting? I'm sure the fuel usage rate is down but I'm also travelling 10-12 mph slower than at 3100 rpm.

[CYO Admin]

I would expect some where around 30 GPH (give or take) at cruise for twin 5.7's

[Seif911]

From everything I have read, you hit it on the head. I have a performance chart for my boat and it is right on. The 1500 RPM's will put my boat at 7 MPH and approximately 2.0 MPG, at 3600 RPM's I am doing about 19 MPH but with the trim tabs down I am suppose to be getting 1.4 MPG. Pretty much all the rest of the RPM's will give me .9 MPG.

This is a according to the chart. Real life application, got me? I am banking that the charts are correct. I know some of these guys have the electronics tied to the fuel flow so they have real world numbers. At the cost of gas and depending on how much you use, it may just pay for itself.

[USAFRet]

Thanks for your responses - this is a single engine power plant. I used ~34 gallons over the 9 hour cruise, and my GPS app estimated the trip length to be 58 miles. These figures are rough, but that equates to 1.7mpg and 3.7 gph across a huge variety of throttle and RPM settings. I can try the 1500 rpm on my next fill up and base my fuel rate on the engine meter and gps miles travelled to get a little closer to a realistic figure.

I have next to no experience in vessels this size, but I based my driving style on the owner's manual and experience on smaller, open bow boats- "bow up" on the hull trim tabs, throttle up to planning speed, throttle back to cruise rpm, adjust outdrive to obtain best speed and small hull trim adjustments when needed. Most of my cruising will be at slower speeds (less than on plane) so I hope the 1500 rpm setting will provide better economy. If not, it's "ONLY MONEY" and I'll have to deal with it.

Please correct or advise if there is a better approach to this or my throttle strategy.