cracked_ribs
Well-Known Member
I know it's been said but 2-3X is totally unrealistic.
The only reason I would expect a really substantial change would be if you can't prop it as efficiently.
The fuel you burn is really a function of how much power you need to access - the difference in burn from a 5.7 that is doing a task requiring 150 HP, say, won't be all that different from a 3.5 that is using 150 HP, unless the gearing is such that the only way to get that 150 HP task done (like say getting on plane) is to put the prop at a speed where the engine is making a lot more power than necessary. Does that make sense? Like if you have to redline it to get on plane because the prop won't grip the water right, but the other setup had a duoprop and would plane at 2000 RPM because the props were so grippy, sure, at that point you'll see big differences, because you're running the engines really differently. But if you can tune the gearing (including the prop-water gear, if you follow me - it's more analogous to tire size etc but I'm sure you get what I'm saying) to be basically equivalent, the change shouldn't be huge.
There's a bit of variation but in general the engine is going to burn roughly the amount of fuel needed to convert the chemical energy into the correct amount of force. You'd see more of a difference if you went from some giant theoretical engine with 20l of displacement to a supercharged 1.6 to get 150 HP. If you take it to extremes you'd get these various losses and effects compounding and okay, yeah, big changes are possible.
But at its core, a motor converts the chemical energy of the fuel into motion. The amount of fuel it burns is largely a function of the energy necessary to do its thing. A 300 HP 4 stroke OB on a given boat should be not TOO far off a 300HP 5.7 on the same boat, unless the torque curves are radically different (and they're probably a bit different but without seeing them I'm not sure how much).
I'd expect some difference, but it shouldn't be crazy.
The only reason I would expect a really substantial change would be if you can't prop it as efficiently.
The fuel you burn is really a function of how much power you need to access - the difference in burn from a 5.7 that is doing a task requiring 150 HP, say, won't be all that different from a 3.5 that is using 150 HP, unless the gearing is such that the only way to get that 150 HP task done (like say getting on plane) is to put the prop at a speed where the engine is making a lot more power than necessary. Does that make sense? Like if you have to redline it to get on plane because the prop won't grip the water right, but the other setup had a duoprop and would plane at 2000 RPM because the props were so grippy, sure, at that point you'll see big differences, because you're running the engines really differently. But if you can tune the gearing (including the prop-water gear, if you follow me - it's more analogous to tire size etc but I'm sure you get what I'm saying) to be basically equivalent, the change shouldn't be huge.
There's a bit of variation but in general the engine is going to burn roughly the amount of fuel needed to convert the chemical energy into the correct amount of force. You'd see more of a difference if you went from some giant theoretical engine with 20l of displacement to a supercharged 1.6 to get 150 HP. If you take it to extremes you'd get these various losses and effects compounding and okay, yeah, big changes are possible.
But at its core, a motor converts the chemical energy of the fuel into motion. The amount of fuel it burns is largely a function of the energy necessary to do its thing. A 300 HP 4 stroke OB on a given boat should be not TOO far off a 300HP 5.7 on the same boat, unless the torque curves are radically different (and they're probably a bit different but without seeing them I'm not sure how much).
I'd expect some difference, but it shouldn't be crazy.
