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Continuing the boat building dream…

Today we offer yet another great article from our founder, Glen L. Witt. This one describes the difference in a planing boat and displacement boat how that affects performance and speed. We get a lot of questions about powering a boat and hopefully this will help…

Glen L. Witt, Founder of Glen-L Marine

Planing vs Displacement Boats

by Glen L. Witt

There are two recurring questions that we hear from our customers. “Can I put a sail on (name of planing boat)?” and “If I use a larger motor than you recommend (on a displacement boat) can I increase the speed?”

Different hull shapes have advantages for different applications. The ideal characteristics for sailboats, row boats, and eventually low speed power boats have been long understood. These boats need a hull shape that will move easily through the water with a minimum of power. When more power became available, it was found that a new hull form was needed. In order to overcome the limitations of the bow wave, it was necessary to rise out of the water, to plane. On planing hulls, the bottom shape is extremely important to performance, with minor variations causing sometimes dramatic problems. Let’s look at the two basic hull forms to understand why they work the way they do.

Displacement hulls: The overriding fact about displacement boats is that their speed is limited by their length. Speed in knots = 1.34 X square root of the waterline length. The other part of the story is that these hulls are very fuel efficient; they require little power to reach their maximum speed. Attempting to exceed hull speed is guaranteed to waste fuel and money and gain little if anything in speed. Row boats and sailboats are most often displacement hulls.

Planing hulls: In theory, a full planing hull (with infinite power) has no speed limit. Our Tornado and Thunderbolt regularly exceed 100 mph in competition, with one Thunderbolt clocked at 148 mph. Planing boats, when they achieve planing speed, ride on top of the water. They are, however, not properly shaped to be efficient at slow speeds.

When a boat is planing, the water flows off the bottom in a sheet and the wave breaks behind the boat. If there is a downturn at the transom (hook), the bow will be driven down, increasing wetted surface and decreasing speed potential. If there is a bottom rise at the transom (rocker), the hull will tend to porpoise (jump out of the water and fall back into the water). If the trailing edge of the bottom at the transom is rounded, the boat will behave as though it has a rocker. This results from water following the radiused bottom, up the transom, sucking the transom down. Speed amplifies these effects, with minor imperfections only causing performance problems at high speed. Having said all this, there are instances when you might want very minor hooks or rockers in the bottom …this is what cavitation plates and trim tabs are for.

Others: Yes Virginia, there are others. There are boats that fit between displacement and planing boats; boats that exceed the speed limitations of displacement boats, but do have higher speed potential of full planing boats. They tend to be less efficient than displacement boats at slow speeds, and less efficient than planing boats at high speed. Their exact characteristics will depend on where their hull form falls between displacement and planing hulls.

Anyway… Is it possible to put a sail on the 13′ Fisherman? Ans.: Yes, however, because the Fisherman is a planing hull it will be very inefficient (slow) under sail. If I double the horsepower on the Noyo Trawler, how much more speed can I get? Ans.: Very little, perhaps a half knot, but your fuel consumption could increase dramatically when the motor runs at full speed.