Turning the Screws
|Turning the Screws|
| If hard-core
racers choose surface drives, maybe you should consider them, too.
By George L. Petrie — June 2001
Builders of high-performance, inboard-powered boats have several choices when it comes to deciding how to best convert the power of a brawny, high-torque diesel or a high-revving, big-block gasoline engine into the maximum effective thrust. Options include conventional inboard drives (either straight shaft or V-drive), stern drives, surface drives, or waterjets. All are well-proven technologies, and each has its advocates, niche, and drawbacks. But at the high-speed, high-end of the performance spectrum, surface drive units most often get the nod. How come? And if racing pros choose them, should you consider them?
Let's begin by dispelling the notion that a propeller acts like a screw, worming its way through the water. A propeller acts just like a pump, imparting to the water an increase in its velocity, thereby increasing its momentum. That change in the water's momentum creates an equal and opposite reaction force that we call thrust, directed along the axis of the shaft. It's easy to visualize that concept in a waterjet, which is actually a pump that has an impeller (think of it as an internal propeller) that accelerates the water entering its inlet and shoots it out a nozzle in the stern at a much higher speed.
Propellers work on the same principle, except the "impeller" (the propeller) is outside the hull. A well-designed waterjet can pump water much more efficiently than a propeller, because its impeller is inside a housing that controls the water's flow. And there's no rudder, propeller, or shafting to create drag. But a lot of that improved efficiency is eaten up by friction losses that result from the water passing through the casing. At speeds of 30 knots or more, a waterjet can deliver better efficiency than a propeller drive (within a narrow range of speeds for any given system), but at speeds much more than 60 knots, a waterjet's internal friction losses can become very large.
In some ways, a surface drive is like a waterjet, but without the casing. In other respects, the surface drive is more like a stern drive, but with a straight shaft instead of the latter's Z-configuration that requires two sets of bevel gears. The surface drive unit is mounted outboard of the hull, near the base of the transom, with its housing extending several feet aft of the stern. Looking like a giant eggbeater on its side, the propeller mounts on the aft end of the drive unit.
On high-performance craft, surface drives are usually fitted with surface-piercing propellers designed especially for surface-piercing drives. Having larger diameter, more blade area, and greater camber than a normal propeller, surface-piercing blades touch the water only on the lower half of the prop's rotation, thereby causing less drag. When a hull fitted with surface drives and surface-piercing propellers is up on plane, the entire drive unit is out of the water, with only the lower half of the propeller blades and a small protective skeg extending into the water. This arrangement can cut resistance up to 30 percent compared to conventional inboard drives, according to Dana Birkland of Twin Disc, the company that manufactures Arneson Surface Drives. That can mean 30 percent less fuel use at a given speed or higher speed from a given set of engines.
This article originally appeared in the May 2003 issue of Power & Motoryacht magazine.