Planing (sailing) - How Planing Works

How Planing Works

When it is at rest, a vessel's weight is borne entirely by the buoyant force. At low speeds every hull acts as a displacement hull, meaning that the buoyant force is mainly responsible for supporting the craft. As speed increases, hydrodynamic lift increases as well. In contrast, the buoyant force decreases as the hull lifts out of the water, decreasing the displaced volume. At some speed, lift becomes the predominant upward force on the hull and the vessel is planing.

A simple model of this effect is a solid slab of material which is heavier than water (like a steel plate) but is shaped and oriented to have a positive angle of attack. At rest, the slab will sink because it is heavier than water; the buoyant forces are overwhelmed by the force of gravity. However, if the slab is kept in the same orientation and pulled horizontally through the water, it will force the incoming water downward. This results in a reactionary force upward on the slab. At a high enough speed, this reactionary force (plus any small buoyant force) are larger than the force of gravity and the slab will stay afloat. In this way, the horizontal force (which may be supplied by a motor or a sail) is converted into a vertical force upwards. This is highly analogous to an airplane wing in the air and, like for airplanes, there are many complications to this simplified explanation of planing in boats. See lift on an airfoil.

Although any hull will plane if enough power is provided and enough speed is attained, a hull designed for operation in the planing realm is sometimes distinguished by a flat run aft. In other words, in side view, the bottom is more or less a straight line towards the stern. (Exceptions to this include surfboards and other recreational planing hulls, which utilize rocker throughout for enhanced maneuverability when banking through turns.) In contrast, in a displacement, or non-planing hull, the bottom is curved in side view (the curvature is called "rocker") all the way from bow to stern, in order to minimize wave drag. In front view, the sections in the aft area may be straight, as in a racing hydroplane, to maximize planing forces and speed, but for practical reasons of stability and comfortable ride are often V-shaped, especially in boats intended for offshore use.

To plane, the power-to-weight ratio must be high, since the planing mode of operation is quite inefficient; sailing boats need a good sail area and powerboats need a high-power engine. Most surfboards are planing or semi-planing hulls that utilize the push of the waveform more or less in combination with gravity to achieve planing lift.