Re: fuel consumption question
FWIW the point I made about "cruise" speed being conjecture is supported by the fact that we have two very different opinions here. One is that "cruise" is where your best economy is, the other where your best compromise of speed and efficiency is. If you take a look at the test results I posted, indeed, 7 GPH is a good number. However, it can be even below 5 GPH if you run it like Boatist suggests. If you run it like Tashasdaddy or I suggested it is going to be from 10 -14 GPH which is roughly double. So ultimately this comes down to what YOU call cruise . . . How fast do you plan on running? Are you going to run the same speeds all of the time?
Also, for clarity, while the weight does absolutely come into this discussion, particularly with marine engines, the biggest difference between spark ignited, 4 cycle engine efficiency is output as compared to displacement. An 8.1 liter engine running at 100 horsepower is going to burn more fuel than a 3 liter engine at 100 horsepower, and the reason is that the throttle (air restricting butterfly thing) is more open on the smaller engine, which doesn't support the often held belief that the bigger engine will be loafing, so it will get better fuel economy . . . However, this actually may be true at some speeds and loads.
Due to all of the variables that we are throwing at the discussion, fuel efficiency is almost to the point of voodoo in the case of marine applications . . . Hull design, weight, electronic vs. mechanical fuel systems, 2 cycle vs. 4 cycle, diesel, drive systems, propellers, speed, engine displacement, 4 vs. 6 vs 8 cyl's, planing vs. displacement speeds, application (watersports, idling along, slow cruise, fast cruise, WOT), all of these factors skew the answers . . .
One thing that is clearer with marine applications than automotive, is fuel use at a given speed. There are no hills, so if the weight is the same and the propeller is turning the same speed (hence boat same speed), the power needed to turn that propeller is fixed, so the fuel consumption is somewhat fixed as well, sort of regardless of the engine, except for throttle position vs. displacement (more open is technically more efficient) and assuming modern electronic controls.
This is why I use test data from a decent source (whatever that is) to answer these questions. Everything else simply throws in too many variables. I would always subtract (or add) 10% to whatever those tests show too. 10% less top speed, 10% more fuel use. They are always lightly loaded, so owner's boats rarely run the same as those tested . . .
