What are the differences between Displacement Hulls, Semi Displacement Hulls and Planing Hulls?

Hull design directly influences a vessel’s performance and capabilities in various water conditions. In this blog, we will shed light on their strengths and weaknesses.

Displacement Hull

In basic terms, it displaces the same amount of water when stationary or moving. It is supported by buoyancy and they are typically big, round and bulbous. Generally, displacement hulls are slow, as they are not designed to naturally plane. The speed at which they move through the water is limited by its waterline length. It wouldn’t make any difference to the power of the engine you put in a displacement hull as they are not designed to plane.

Semi Displacement Hull

Semi displacement hulls are normally a cross between a planing and displacement hull. They will generate an amount of lift but the vessel’s weight will be supported by buoyancy. A semi displacement hull creates a big hole in the water which generates large bow and stern waves. Semi displacement hulls will typically have a similar hull shape to displacement hulls but will flatten off towards the stern.

Planing Hull

A planing hull is designed so that it produces positive dynamic pressure (lift). What this means is that it surfs on its own bow wave and decreases its draft with speed. When a boat is planing it reduces the amount of hull in the water (wetted surface area). The effect of this is less drag.

Planing hulls come in different forms with the more common being a V shape, usually with a chine in it. The chine actually pushes the water downwards, which in turn aids lift.

There are different types of V shaped hulls, each giving different handling characteristics. Typically a wide deep V hull will give a nice soft ride in chop although it will need a bit more power to get it going. A narrow shallow V will require less power to get it on the plane but it wouldn’t be as comfortable as a deep V hull in big seas. We are seeing more and more deep V hulls in the form of RIBs where the hull flattens off towards the back of the boat. These are great early planers but again will not be as comfortable in waves and chop. The deeper the V, the softer and dryer the ride.

Another form of planing hull is the flat bottom (cathedral hull). These hulls require less effort to get them on the plane, but as they have a larger flat surface area, they aren’t as comfortable in choppy conditions. However, they have a very stable platform at slow speed.

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