Good questions, even for a newbie ;-)
At speeds below the square root of waterline, it just takes very little power to move any shaped hull
. At 1.2 to 1.3 times the square root of the waterline, then wetted surface and wave making start to matter. At speeds a little over 1.34 times the square root of waterline, wave making dominates, but surface friction still increased by the square of the speed.
Most people drive their powerboats at a little over hull speed
, say 1.4xsqrt(LWL). At this speed, the lowest wavemaking comes with a very high prismatic hull
-- nearly blunt. That's why you see transoms, and why you don't see deep fine knife like forefoot on nearly any powerboat.
Many trawlers have a LOT of wetted surface in the form of deep forefoot, long keel
, and then often a wide flat afterbody. Its hard to come up with a higher wetted surface shape! Therefore, the resistence goes up rapidly, and the fuel
burn also skyrockets.
Many planing boats have the transom, but a more blunt bow too, and usually a deep V instead of a long keel
for the directional stability. While nowhere near as low wetted surface as possible (a semicircular cross section), its much lower wetted surface than the typical trawler
hull form. Hence, at 1.4 or 1.5 x sqrt(LWL) a planing hull may be less resistance than a displacement
If its not a planing hull, you really want to go about 1.2xsqrt(LWL) or lower for efficient cruising, and you really never want to exceed hull-wave speed (1.34xsqrt(LWL)). That is pretty slow unless its a pretty long boat. But potentially VERY low resistance.
To get the efficiency, you also need to size the engine
properly: as small as possible. Engines that are running off their torque peak are MUCH less efficient than when they are running at torque peak. Besides people going for records, you'll never find a boat set up to run at the engine's torque peak.