Displacement and Planing craft

Displacement and planing are types of craft that are most commonly found in harbors and marinas, in lakes or on the ocean. Displacement boats are usually long and thin. Most sailboats are displacement craft and most powerboats over 60' are also displacement craft. Planing craft use lift and drag to attain higher speeds and loose wetted surface. Lots of power boats about 60' and under are planing craft. Most skiff and power catamarans are planing too. Sailboats are also starting to become planing craft. Sailboats as small as hotshot dinghies and as big as a 60' open ocean racer.

Displacement craft are long and thin so they can use Froude’s law to their advantage. Longer boats make longer waves; longer waves are faster. So longer boats make waves that are faster, meaning they have less drag. Displacement boats usually require very small amounts of power; up to a certain point, after that point they require a lot of power. Displacement sailboats are currently the fastest kind.

Planing craft are much newer and a bit more complicated. Planing craft accelerate to a certain speed as displacement craft until the lift that their flat bottoms produce overcomes the weight of the boat. As soon as you hit that speed the boat lifts up and begins to accelerate again because the drag has just decreased substantially. Power is a major factor in these craft, be it motors or sails. Sailboats in particular need to be lightweight and have enough sail area to bring them onto a plane. Planing hulls need some sort of chine to ride on, otherwise they don't work

The prismatic coefficient needs to be taken into account as well. Displacement boats have an ideal prismatic coefficient of about .58, while planing boats have a prismatic coefficient around .70. Planing boats have a higher prismatic coefficient because they are fuller in the stern to gain surface area. Displacement boats have as little surface area as possible to reduce drag. Planing boats have more surface area because at speed the extra drag is negligible.

These are the two main types of hull form; of course there are other kinds, like SWATH, UNPC, ULD and a whole bunch of others. Those designs however are all based on the two basic hull forms that I’ve described.



This is just a general discription. Feel free to msg me if you want to learn more.

Sources:
Myself, Paul Waters, and Bill Anderson

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