What are differences between a planning, displacement and semi displacement hull?

What are the differences between a planning, displacement and semi displacement hull?

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 plan, the speed at which they move through the water is limited by its waterline length. It wouldn’t make any difference of the power of the engine you put in a displacement hull they are not designed to plan.

Displacement hull shape

Displacement hull shape

Semi displacement

Semi displacement hulls are normally a cross between a planning and displacement hull, they will generate an amount of lift but the vessels weight will be supported by buoyancy. A semi displacement hull creates a big whole in the water which generate large bow and stern waves. Semi displacement hulls will typically have a similar hull shape to displacement hull but will flatten off towards the stern.

Semi Displacement hull

Semi Displacement hull

Planning

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

Planning hulls come in different forms with the more common being a V shape usually with a chine in it. The chine actually pushed 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, where as a narrow shallow V will require less power to get it on the plan 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 planners but again will not be as comfortable in waves and chop. The deeper the V the softer and dryer the ride

Other forms of planning hulls will be flat bottom (cathedral hulls) these hulls require less effort to get them on the plane, but because they have a lager flat surface area they wont be as comfortable in choppy conditions, they are however a very stable platform at slow speed.

Planning hull

Planning hull shape

We cannot only think of planning hulls in terms of powerboats as some sailboats plane.

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