It’s All About Performance Page 2
It’s All About Performance
Part 2: Like any propulsion system, there are cons, too.
By Capt. Patrick Sciacca — June 2004
However a builder maximizes the system’s installation, one thing is guaranteed: This technology is all about speed. Some racing catamarans with surface drives have been clocked at speeds in excess of 150 plus. Now that’s fast. Naval architect Donald Blount says that nonracers looking to use surface drives on their boats should consider that they’re most effective for hulls up to 80 feet and boats that travel at speeds of not less than 25 knots.
Now, it’s been proven that surface drives can provide you with extreme speed, but there are several other advantages for boats equipped with this technology.
First, Theodoli says that surface drives create straight-line propulsion, and if a surface-drive-equipped boat leaves the water, it develops “positive thrust” on re-entry, Blount adds. A straight-inboard or stern-drive boat would likely bog down before taking a bite. There’s also reduced draft and drag with a surface drive, since there’s less underwater gear. Twin Disc, which has produced the ubiquitous ASD since 1992, says its drive reduces underwater drag by as much as 50 percent. Magnum says that drag reduction has resulted in 18 percent more speed compared to straight inboards, and Theodoli adds there have been fuel savings, too. Although Magnum didn’t have fuel burn numbers available, the consensus among surface-drive manufacturers is to expect fuel savings of anywhere from 15 to 30 percent.
But like any propulsion system, there are cons, too. With the aforementioned fixed-shaft drive, you can’t trim the boat’s running attitude for weight. But there are other difficulties, too. Those big surface-piercing props that are great at speed don’t bite as well at slow speeds, and you may have to deal with reduced control in close quarters and while backing down. Theodoli says that the lack of drag that’s an advantage at high speeds reduces maneuverability in big winds. This is a result of so much of the drive and hull being out of the water. In addition, Blount explains that surface-drive-powered vessels have “low thrust margins” to accelerate a boat through the hump from displacement to planing speed. In other words, getting from a standing position to planing speed puts the most demand on the drives and props. However, the surface drives and props are limited in their ability to generate thrust during this transition. As such, there are significant lifting and side forces that vary with boat speed at the stern and must be carefully considered at the point of the boat’s design to ensure stability. Last, there are big costs associated with these systems. I recently saw a 50-plus-footer with surface drives, and the price tag on the drives was six figures.
What does this all mean for you? If you enjoy speed and technology, you can get a surface-drive-powered boat and skip across the water like the aforementioned flyingfish. And while you’re having your high-speed high jinks, your fuel gauge won’t be racing like hungry little lemmings across a tasty grass field. I can’t say the same for your heart, though.
Editor’s note: If you’re considering surface-drive propulsion, be sure to get instruction on how to use it, since it differs from most other propulsion systems you’re probably used to.
This article originally appeared in the May 2004 issue of Power & Motoryacht magazine.