Originally Posted by twistedtree
I think that number is off by a factor of 2? I've seen a variety of references
to 0.06 gal/hr/hp. That would put 250hp burning about 15 gph. I checked the specs for my Cummins QSC (a modern common rail diesel which in theory is the state of the art) and it comes in at about 0.05 gal/hp/hr, or 12.5 gpm when producing 250 hp.
Exactly what I was going to say.
1 gph in a diesel engine makes 20 HP.
1 gph in a gas engine makes 10 HP.
Power required to move a boat at displacement speeds is primarily a function of waterline length and displacement, and its an exponential curve: a little more speed requires a LOT more power. Try rowing your dinghy
and you'll see that it takes almost no effort to get going, but it takes quite a lot more to go faster, and even a world class athlete won't be able to plane.
If you want to cruise
around using very little money
, as it sounds you do, you must adjust your expectations to go slow: assume your cruise
speed is the square root of the waterline length: a 36 footer at 6 knots.
Since the power is so non-linear, a small decrease in speed can easily halve the power requirement, so a gas engine at 5.5 knots probably will get the same MPG as a diesel at 6 knots.
Gas engines have advantages: quieter, cheaper, happy to run at very low power
Diesel engines have advantages in this use: a tiny bit faster for the same mileage, and extremely long life (decades) if its a low powered diesel (say 30 or 40HP for that hypothetical 36 footer).
Gas engines have disadvantages when not brand new from the factory: they don't run well without very good electricals, and these degrade rapidly especially in salt water
. Also, everything that not the block, crank, and pistons deteriorates quickly, including exhaust
manifolds, valve springs, valves, carbs, wiring
, ignition, oil
pan, couplers, fuel filters, ...
Diesel engine disadvantages include smell (a lot of people get sea sick smelling diesel fumes or exhaust) and noise
. They are also sensitive to aging exhaust
issues. Diesel is far more sensitive to junk in the fuel, so really good fuel filtering is REQUIRED.