In contrast, the most efficient way to move things on the water is with a displacement hull, which needs only 0.5 hp to 2.0 hp per ton of boat weight. The only problem is that you have to go slowly. There is a compromise hull form, though. If you’re willing to own a boat that’s long, relatively narrow and light in weight, and that has modest power, rounded chines and a flat deadrise aft, you’ll travel at a reasonable speed while going farther on less fuel. The semi-displacement hull can get into the 25 mph range even when the water’s rough, and it tops 3 mpg. The other options are some of the new catamarans that are claiming 6 mpg. Remember, for better miles per gallon, it’s all in the hull.