|Turning the Screws|
2: Surface-piercing drives continued
By George L. Petrie — June 2001
Twin Disc has tested a pair of identical 53-foot Magnums powered by twin 1,080-hp diesel engines, one boat fitted with conventional inboards and the other with Arnesons. According to the company, the inboard-powered boat (weighing 57,500 pounds) reached a top speed of 45 mph, while the boat with surface drives (weighing slightly more at 59,500 pounds) was clocked at 56 mph. A similar test with identical 18-foot runabouts, each powered by a single 400-hp gasoline engine, compared a waterjet and an Arneson surface drive. The company measured a top speed of 67 mph for the waterjet boat compared with 88 mph for the surface drive boat. Twin Disc has no bias in such a test since it manufactures both surface drives and waterjet drives.
In addition to cutting appendage drag, the main performance advantage of a surface drive is its ability to change the angle of thrust. Like a stern drive, a surface drive unit can be tilted up or down to vary the angle of the prop-to-water interface, allowing it to achieve the optimum setting for a particular combination of speed and trim. And again, just as with stern drives, surface drive units can be trimmed individually. This comes in handy in high-speed turns especially, where a boat banks inward and its outboard prop must be slightly lowered to efficiently interact with the water.
Just as important as speed and efficiency is maneuverability. In this respect, a surface drive is comparable to an outboard or a stern drive in that the propeller turns with the drive unit, redirecting thrust to steer the boat. Thus surface drives can offer potentially better maneuverability than either inboards or waterjet drives, especially at low speeds.
Another factor to consider is cost. Looking only at initial cost, a conventional inboard drive is the least expensive route to take, but for boats that typically run at speeds much more than 30 knots, that means higher costs for fuel and big repair bills if the props, shafts, or rudders run afoul in shallow waters. For engines up to about 500 hp, the purchase price of a stern drive is significantly lower than that of a surface drive, due mainly to the economy of large production volume. On the downside, however, servicing a stern drive may require special tools and factory parts, whereas Birkland asserts that an Arneson drive unit can be completely torn down and rebuilt in two hours, using standard bearings and U-joints available in any automotive store.
Maybe the biggest drawback with surface drives is that they protrude so far aft of the transom, leaving the propeller totally exposed. To mitigate this, the usual tactic is to mount a swim platform above the drive, but the imposition of a five-foot extension to the transom may be an undesirable constraint on some designs.
For a high-performance edge, it's easy to see why surface drives are such a turn-on for serious, hard-core offshore racers. But considering all the advantages, maybe we nonracers should take another look at what's turning our screws.
George L. Petrie is a professor of naval architecture at the University of New Orleans and provides maritime consulting services. His Web site is www.maritimeanalysis.com.
This article originally appeared in the May 2003 issue of Power & Motoryacht magazine.