Cars and trucks operate at a completely different speed from displacement boats, in hydro and aerodynamics tiny changes make a huge difference in drag.
But the trailing edge of the keel on a power boat is not tapered to reduce drag (though that is a minor benefit), it's to increase thrust. Getting clean undisturbed water to the prop is an important issue, a big fat keel in front of the prop blocks the flow, reducing thrust for the HP expended. The keel should be thick enough to house the shaft but tapered to 1" or less above and below the shaft line and the taper should extend forward a decent distance. Water doesn't like sharp corners, gentle bends are better......