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It’s All About Performance Page 3

It’s All About Performance

Piercing Props

By Capt. Patrick Sciacca — June 2004

   


 More of this Feature

• Part 1: Surface Drives
• Part 2: Surface Drives
• Piercing Props


 Related Resources
• Feature Index

 Elsewhere on the Web
• Arneson Industries
• Seafury Propulsion

A surface drive can only function as well as the propeller it’s attached to. But a propeller designed to function both in and out of the water is atypical. “A surface-piercing propeller (or surface propeller) is a propeller that is positioned so that when the vessel is underway, the waterline passes right through the propeller’s hub,” states naval architect Paul Kamen in a recent Professional Boatbuilder article. Sounds easy enough, right? Wrong. Phil Rolla, president of Rolla Propellers, recently told me of the challenges he’s faced in designing a prop to perform in such a manner. “Mechanically, most surface-drive propellers have a splined hub, whereas submerged inboard propellers use a tapered bore and keyway,” he explains. A surface propeller also deals with a constant change in stress due to its function. The propeller is out of the water for half of each revolution. This can cause fatigue and pulsation problems, says Rolla, noting that additional blades (surface-piercing props may have five or even six blades) help reduce these issues but don’t completely eliminate them.

A surface propeller also has ventilation, which is good, as it replaces the cavitation that would be found in a traditional underwater setup, but it also means the power comes from the prop’s pressure face. Rolla says there’s no low-pressure side giving lift, and to address this he uses “cambered-face geometries” to optimize the lift-to-drag ratio. Because the surface propeller runs in two densities—air and water—there is no single program that can provide a predictable propeller result. Rolla says he has performed extensive field testing to ensure an effective prop, but adds that, in the end, “If it looks right, it stands a hell of a better chance of working right.” —P.S.

Previous page > Part 2: Like any propulsion system, there are cons, too. > Page 1, 2, 3

This article originally appeared in the May 2004 issue of Power & Motoryacht magazine.

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