My former vessel had a single 120 Ford Lehman that only burned 1,6-GPH @ 1400-RPM, water speed of 5.5-knots, but burned over 10-GPH at WOT of 2400-RPM, mustering a whopping 8.4-knots at full hull speed... Do the math... Over 3 knots per gallon if one is content to cruise at a brisk walk' space, but less than 1 at a jogging pace... And that means one has to plan trips very accurately in conjunction with wind and tidal flows... Here in SE Alaska, we have peak tidal flows that can exceed 5 knots in channels between islands! If you travel WITH the tide, you can nearly double your speed and fuel economy, but running against it, well, GOOD LUCK GETTING HOME!!!
My Sea Ray cruising up on minimal step-speed of around 21 knots gulps about 14 GPH (7 per engine @ 2500-RPM), or about 1.5 knots per gallon, about the same as the CHB did @ 6 knots... But takes less than a third the travel time! And if I'm content to slow cruise at the same speed as the typical Trawler under-8-knots range, with both props spinning at barely above idle of 1000-RPM, she floats along at over 7 knots, burning less than 3-GPH, granting actually more speed for less fuel! With over 700 HP "at my command," I can easily outrun all but the most sudden of storms, AND,I don't have to leave 3 hours earlier to GET to my favorite fishing spots, nor LEAVE them just as the "bite" is on to ensure getting home, since tidal flows have much less impact on a planing hull, skipping above it all!