Nomad Willy
Guru
This thread is in response to Marin's comment about the rule of thumb that it takes 1 gallon of fuel to make 20hp for an hour.
I may have stated the function in the thread title wrong. Please correct that and any of the following that is wrong or stated incorrectly.
Marin,
I think your friend Dick DOES know what he's talking about but everyone loosely refers to the hp per gallon in an approximate rounded off number for rough "in the ballpark" calculations. But if you and he were bent over his engineering desk w graphs, calculators and deep statistics at hand I'm sure your friend would say the Lehman would produce more like 16hp per gal. And that would probably be achieved at the best rpm for fuel efficiency. And when you look at a graph that shows an engine makes 60hp at a certain rpm that's at WOT and full load. That's way different than putzing around in an old trawler at 25% load. Mark has a very modern engine and he dosn't even make 20 ... maximum. When you look at a sales brochure and it says the John Deere 4045 makes 17.8hp per gallon that's at one engine speed only ... usually about 1800rpm and if so at 1500rpm the number will be different. And at WOT and full load it will be different again. And when you look at a sales brochure and it shows how much power an engine makes it's at WOT at the rpm shown. If it says 60hp at 1600rpm you can't say at 1600rpm cruising along in your boat your engine is making 60hp at 1600rpm. So when you look at a power/rpm chart all the hp numbers are at the rpm specified and at WOT. But 20hp per gallon is a rule of thumb and perhaps not a good one as I think most trawler engines are closer to 15, especially at 25% load.
I'm not sure this is all correct and if it's a bit foggy I'd like to have someone clear it up for me. Rick, Tom, FF ... what do you guys think?
I may have stated the function in the thread title wrong. Please correct that and any of the following that is wrong or stated incorrectly.
Marin,
I think your friend Dick DOES know what he's talking about but everyone loosely refers to the hp per gallon in an approximate rounded off number for rough "in the ballpark" calculations. But if you and he were bent over his engineering desk w graphs, calculators and deep statistics at hand I'm sure your friend would say the Lehman would produce more like 16hp per gal. And that would probably be achieved at the best rpm for fuel efficiency. And when you look at a graph that shows an engine makes 60hp at a certain rpm that's at WOT and full load. That's way different than putzing around in an old trawler at 25% load. Mark has a very modern engine and he dosn't even make 20 ... maximum. When you look at a sales brochure and it says the John Deere 4045 makes 17.8hp per gallon that's at one engine speed only ... usually about 1800rpm and if so at 1500rpm the number will be different. And at WOT and full load it will be different again. And when you look at a sales brochure and it shows how much power an engine makes it's at WOT at the rpm shown. If it says 60hp at 1600rpm you can't say at 1600rpm cruising along in your boat your engine is making 60hp at 1600rpm. So when you look at a power/rpm chart all the hp numbers are at the rpm specified and at WOT. But 20hp per gallon is a rule of thumb and perhaps not a good one as I think most trawler engines are closer to 15, especially at 25% load.
I'm not sure this is all correct and if it's a bit foggy I'd like to have someone clear it up for me. Rick, Tom, FF ... what do you guys think?