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Top Knots

Top Knots - Part 1
Top Knots

How do designers get the most speed out of a hull?

By George L. Petrie — June 2002

   

Illustration: Travis Young
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Ever wonder what makes a boat go fast? Lots of horsepower, you might guess. Certainly that's part of it, but weight, balance, and hull form are also significant factors. Refinements in form, such as strakes, steps, and propeller pockets, also play an important role. And so does the propulsion system, which transforms horsepower into thrust.

To go fast, the hull has to plane. What's planing? When a boat is at rest or moving slowly, its weight is entirely supported by buoyancy; it must displace a certain volume of water to stay afloat. But as a planing hull gains speed, dynamic pressures on the bottom create forces that lift the hull partly out of the water. Wave-making resistance (which limits the speed of nonplaning displacement hulls) decreases dramatically, allowing the hull to reach much higher speeds.

Up on plane, the hull is supported by dynamic lift, much like an airplane is supported by lift forces on its wings. To develop lift, the hull must move through the water at an angle; in a wing this would be called angle-of-attack, but for a boat hull it is the trim angle. Because of the trim angle, the dynamic forces that produce lift also induce drag. At any given speed, a higher trim angle will produce more lift but also more induced drag. Skim your hand across the water, first flat and then at an angle, to see the difference.

The other major source of drag is friction between the water and the hull, just as you would feel sliding your hand across a tabletop. As boat speed increases, more of the hull may lift out of the water, decreasing the surface area over which the friction drag acts. With less wetted surface, friction drag is reduced and the boat is able to go faster, other factors being equal.

Coaxing the maximum performance out of a hull is a bit like conducting an orchestra. All the elements of the design must be in harmony. To hear how it plays out, we talked with two of the top high-speed hull designers around. The founder of Donald L. Blount and Associates, Donald Blount has devoted his career to developing high performance hulls for an impressive roster of yacht builders, commercial ventures, and military clients. And as the founder of Michael Peters Yacht Design, Michael Peters has an equally imposing portfolio of offshore race boat and high-performance yacht designs. Here's what they said about each of the factors that make a boat go fast.

Next page > Top Knots, Part 2 > Page 1, 2, 3

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

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