The system, designed for top speed, was not efficient at cruise. Resizing and repitching the prop for cruise speeds made it difficult for a boat, loaded down with full fuel tanks, to get up on plane and get the prop out of the water. As with stern drives that use the familiar trim system to optimize efficiency under varying loads and speeds, surface drives also need to be trimmed, but it's not as easy because prop position is much more critical. Surface drives present another problem because of all the hardware, including hydraulic trim cylinders, large castings, shafts and the props themselves hanging 3 to 5 ft. behind the transom. The situation isn't helped much by the fact that surface drives don't back up as easily as conventional drives.
Some boaters take the simple approach to the benefits of surface drives.
They install a surface prop on a MerCruiser and mount it 10 in. higher than they're supposed to. The boat builder takes the Power-Vent plywood and Formica box, drops it into his mold and lays fiberglass over the insert. The system forms a tunnel at the aft of the hull that mounts props, struts and rudders, and can be adapted to everything from a single-prop PWC to an aircraft carrier. Power-Vent has no moving parts, there's no trim control, no nothing. Because the props are tucked up inside the tunnel as opposed to an inboard arrangement of angling down under the hull, draft is reduced considerably.