These early boats were constructed with "displacement" hulls. This means that the hull would push through or cruise through the water. The proper propeller for a displacement hull is one with a lower pitch when powered by an outboard or stern drive. With the advent of mechanical power came boats with "planing" hulls, which lift the boat partially out of the water to skim on the surface. Today, it's sometimes difficult to tell the difference between the two, but there are slight differences. Propellers on boats with planing hulls often are not fully submerged, so they need to provide holding ability as well as higher pitch and rake, because of higher top-end speeds.