A theoretical fuel usage discussion. Please check my logic.

[ranger58sb]

Originally Posted by JohnEasley

So, someone provided me with this very interesting link where you can punch in the type/size of engine and get some idea of GPH usage at various RPMs. Boat Fuel Economy | Outboard motors and Boat motors | MerCruiser | Mercury | Evinrude | Yamaha | Suzuki | Honda | Volvo Penta Diesel Marine engines

One of the boats under consideration has the big block, gas Volvos in it so we dialed in the 375 hp 8.1 engine on the website and found that the sweet spot seemed to be around 3000 RPM where it was burning about 11.5 gph. Now, lots of things come into play so this is just a theoretical discussion but, that would be 11.5 per hour, per engine, or 23 gph total. Right? Cruising speed is said to be 22 knots or 25 mph.

Yes, as folks have said, the site is giving you "per engine" guesses.

Not so easy to pin down the "sweet spot" or even a best "cruising speed" because that depends on hull form, weight, sea states, etc. That site's info can be useful, but I don't see where it can give you speed (therefore NMPG), comfort level (relative to sea states), and so forth. 3000 RPM? Maybe sweet, maybe not. Cruising speed 22 knots? Maybe at that RPM, maybe not (and I'd guess not, if that engine revs to 4800 RPM).

FWIW, with diesels, the common rule of thumb for efficiency is often stated as 75% of WOT, 80% of WOT, 90% of WPT, "200 off the top" -- IOW, the rule of thumb is slightly all over the map -- because there are other issues involved relative to RPMs and load.

You can often find the manufacturer's nominal performance curves for a given diesel...

-Chris