That's not quite accurate. Each engine has a point of maximum efficiency, lowest specific fuel consumption. On many engines that is near 80%-90% and the fuel efficiency below 50% drops significantly. Burning perhaps 20% more fuel per hp.
Best to pull up the power fuel curves for your proposed engine and map it against the prop power curve.
I agree with your 20% - the consumption per hour per hp is pretty standard for diesels (in the anomalous pounds per HP, which is slightly easier to deal with than roman avoirdupois grain - not to be confused with the Trone or Jersey pound. I'm all for bringing in the
beard-second and
donkeypower…).
So 20% difference between diesel engines across situations is easy to see. But I think the OP isn't worried about differences of 20%, more 100-1000%! ie. the difference in 1000 gallons (that's
wet gallons, not
imperial gallons or
dry gallons...OMG) to 1200 gallons (20%) doesn't seem like the issue...
For the OP, I'd say that
if fuel is the issue, then best consumption choice is...
- if you go less than 5kn, a sailing cat with outboard or small diesel
- if you go between 5-7kn, a medium trawler, mono or cat
- if you go between 7-9kn, a light sailing cat or larger trawler cat
- if you go more than 9kn, a larger trawler cat
This is ignoring money. If you put a limit on that, these will change!
This is ignoring cruising style. That would change the choices.
This is ignoring your personal preferences/biases. That would change the choices.
Jinbi, if you want to go 15-18kn you really only have a couple of choices - fuel costly but cheaper to buy (lots of choice of planning monohulls although range would be low), or fuel cheaper but more expensive to buy (power cats).