Negrini- I realize that I am not using all 62 hp when at 2500 rpm on an engine that will do 3500, but do the math, it still does not equate. Even if I were only using 40 hp to push Laurie Anne (40' - 24,000 disp.) by that formula I would be burning 2.2 gph which is over twice what in fact it uses. By this formula I should only be using 18 hp at cruise speed since I only use 1 gph. In fact, using that formula it is wrong for any of the scores of diesel engines I have owned and have paid for the fuel bills on, so I know how much they use. My current crab boat, for just one more example, has 200 hp, 190 at the prop, and I run it at 3400 rpm. WOT is 3800 and I average betweeen 6 -7 gph at that speed(I log every gallon that goes into it). According to this formula it should be burning close to 9 gph. The Volvo fuel consumption graph is close to my actual burn rate, a little higher. All I am saying is that this does not seem to be a reliable formula to use to estimate anticipated fuel consumption, and therefore required tankage.
SV Laurie Anne
1988 Brewer 40 Pilothouse