Displacement vs. Planing Hulls

IT’S BEEN MY EXPERIENCE that newcomers to boating often find it difficult to understand the difference between a displacement hull and a planing hull. It’s also been my experience that it’s not very easy to explain the design characteristics that cause some boats to plow through the water while others fly along on the surface, hardly touching the water at all.

As anyone who has been around boats for a while knows, a displacement hull is one that pushes water down and around itself as it makes progress, as opposed to a planing hull that skims along on top with much of its body clear of the water.

A displacement hull at full speed fits tightly into a large, self-made wave that has its crests at the bow and stern. Now, the speed of a wave in water is 1.34 times the square root of its length between crests, measured in knots and feet. Thus, the maximum speed of a displacement hull is often said to be the same as that of the wave it creates.

For example, you’ll often hear that a sailboat with a 25-foot waterline will have a maximum speed of 6.7 knots (square root of 25 = 5 x 1.34 = 6.7). But that’s not quite true because a boat can sometimes exceed the speed of the wave it’s trapped in, at least for brief periods such as when it’s surfing down the face of a big swell. What is true is that the hull-speed formula tells you the maximum speed that your boat can reach reasonably easily. Any attempt to go faster — to push a displacement boat up the back of the wave she’s sitting in — requires an extraordinary extra amount of power and a very flat run aft to provide dynamic lift.

On the other hand, the speed of a planing hull is governed almost exclusively by the power-to-weight ratio. If you can keep total weight down to 40 pounds for every 1 horsepower available, you’ll do 25 knots or so. And if you can keep the weight down to 10 pounds per horsepower, you’ll do 50 knots.

Sailboats can plane, too, of course, given enough sail area, but in the usual course of events they have to make sacrifices in other areas, usually seaworthiness and accommodation.

Displacement hulls have midship sections shaped like wine glasses, and often have more deadrise, or V shape, at the bow and stern. This makes them more seakindly and allows them to recover more easily from a 180-degree capsize. Planing hulls tend to be more flat-bottomed for a greater length, which causes them to pound to windward. They carry lots of beam aft to allow them to stand up to a greater sail area, but that also makes them liable to remain upside down if they are capsized by a wave.

But I think that all a beginning sailor really needs to understand at first is that displacement hulls, in general, offer a safer, more sedate ride, roomier accommodation, and more seaworthiness. Planing hulls offer the ultimate in speed, thrills, and excitement for strong, skilled crews.

Today’s Thought

The sea is only safe and harmless so long as the ship is safe and seaworthy and ably handled.

— Felix Riesenberg

Tailpiece
The pessimist gripes about the wind. The optimist hopes it will change. The realist trims the sails.

(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)