Engine hours and fuel use

We have 2,900 hours on our twin 7.4s. We normally cruise at 1,300 to 1,400RPM. At that RPM we go 6+ knots and burn 6GPH. A few times were we had to go faster. At 2,200RPM we burn 16 GPH and go 9 knots. About once a year we go 18 knots and burn 55GPH.

We have burnt around 18,000 gallons in the 2,900 hours. If we had gone 2,200RPM and burnt 18,000 gallons we would have 1,100 hours on the engines. If we had gone 18 knots we would have 325 hours on the engines.

Question: Do people think one should look at gallons burnt rather than engine hours?

northern said:

We have 2,900 hours on our twin 7.4s. We normally cruise at 1,300 to 1,400RPM. At that RPM we go 6+ knots and burn 6GPH. A few times were we had to go faster. At 2,200RPM we burn 16 GPH and go 9 knots. About once a year we go 18 knots and burn 55GPH.

We have burnt around 18,000 gallons in the 2,900 hours. If we had gone 2,200RPM and burnt 18,000 gallons we would have 1,100 hours on the engines. If we had gone 18 knots we would have 325 hours on the engines.

Question: Do people think one should look at gallons burnt rather than engine hours?

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Where's that spread sheet guy at?

That's a tough question. If I were boat shopping, I'd prefer to buy a boat with an engine that was run in the 3300-3500 RPM range for less hours than one run at lower RPMs for more hours. Engines accumulate lots of carbon under light loads and engines run at higher temperatures physically wear less. I used to hang out with professional race engine builders and they always used higher temperature thermostats, usually 195s, to keep the operating temperatures up so the cylinder walls wore less. My boat runs closer to 160-165 at lower RPM and around 170-175 at 3300 RPM and above, so I tend to run at the higher RPM to keep the temperature up as high as possible. Above 160, the differences are minor, but still less wear. Here's a graph.

I'm 100% in the fuel-burned camp! I think hour meters are a ridiculous way to measure anything, it's just the easiest way.

Maybe it's just my crappy gauges, but when my engines reach temp, the needles stay put.

1,300-1,400 RPM isn't necessarily a "light load", it's just not a heavy load.

I can tell you that I won't be driving to the marina tonight in 3rd gear.