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Top Knots Page 3

Top Knots - Part 3
Top Knots

Part 3: Fore and Aft Balance...

By George L. Petrie — June 2002

   
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Fore and Aft Balance: Maintaining proper trim is crucial to attaining top speed. According to Blount, experts agree that about four degrees of trim is optimal for most planing hulls. Trimming the bow higher is not so bad, because it increases lift and cuts wetted surface, but it also increases induced drag. A bow high attitude can also be corrected easily with trim tabs.

Trimming the bow down cuts speed dramatically, because it reduces lift and increases wetted surface, causing more friction drag. Bow down is sometimes preferred, however, because it can soften the ride in a seaway. One big advantage of I/O drives is that they can be trimmed in or out to achieve optimum balance, as shown in the accompanying figures 1a and 1b.

Steps: At high speeds, dynamic pressure and lift are so great that only a few feet of the hull's length are in the water. This can make it difficult to maintain proper fore and aft balance. Putting a "step" in the hull moves the pivot point farther forward, making it easier to achieve fore and aft balance without increasing wetted surface.

The stepped-V hull shown rides on small patches of wetted bottom, just forward of each step and forward of the transom, but with multiple points of support it has excellent longitudinal balance. The offshore catamaran takes the concept further, with aerodynamic lift acting on the cavity between the hulls at very high speeds.

Strakes: Running fore and aft along the bottom, strakes act like chine flats to generate lift, and at high speeds they can cause flow to separate from the hull inboard of the chine, decreasing wetted surface and cutting friction drag.

Propeller Pockets: On inboard boats, prop pockets allow a flatter shaft angle, for increased propeller efficiency. And that opens up a whole new set of issues related to propulsion system efficiency, worthy of discussion in its own right.

The Last Word: Contrary to the impressions that marketing hype may foster, there is no "best" hull form. A hull that's fast on a lake may be a disaster offshore, and vice versa. Look for features designed for the type of boating you plan to do.

George L. Petrie is a professor of naval architecture at Webb Institute and provides maritime consulting services. His Web site is www.maritimeanalysis.com.

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This article originally appeared in the February 2003 issue of Power & Motoryacht magazine.

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