Why Does a Boat Plane?

Chris Riley by Chris Riley Updated on August 8, 2019. In

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To answer this question, let’s first look at what kinds of boats you may see.

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Displacement Boat Planing Boat Hovercraft Hydrofoil

Hovercraft and hydrofoil boats are not as common so we won’t cover them here. (A later article!) Most recreational boats are either planing or displacement. To get a better understanding, you should first read the article How Boats Float .

santamaria.gif (1380 bytes) In the olden days, before there were engines, all boats were displacement boats. A displacement  boat is designed to glide through the water smoothly with a minimum of power (like a canoe with oars or a sailboat under sail). Generally, these boats are very stable and ride smoothly. Larger displacement vessels were also designed to efficiently and safely carry lots of cargo or people.

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Planing boats, on the other hand, are designed to rise up on top of the water. They can go very fast, but need more power to get up on top of the water. The heavier the boat, the more power required to get it “on plane.”

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To understand how these boats work, try putting your hand in a sink or bucket full of water. Start slowly and smoothly, then speed up. The faster your hand hits the water, the more resistance the water presents against it. (Ever do a belly flop? The water felt like concrete, didn’t it?)

The displacement boat has smooth lines and curves that gently move water out of its way, both sideways and down. Because the water moves gradually, there is less resistance against the boat. The planing boat moves quickly and the water can’t get out of its way, it resists the boat so much that the boat stays on top of the water. If the planing boat slows down, as it sometimes must do, it sinks into the water just as a displacement boat does. Since a planing boat is not designed to glide through the water, the ride can sometimes be bumpy and unstable. Also, if you overload a planing boat, it might not have enough power to get up on top of the water and it will also be much more likely to tip over. A good reason to wear your PFD at all times!

So, even though the boat does not weigh less when planing, the water resistance at high speeds keeps it on top of the water. When the speed of the boat is slow enough to let the water move away, the planing boat no longer planes.

The design of the boat’s hull also has an effect on how it works. A flat bottom boat will get up on top of the water more easily than a boat with a round or V shaped bottom. To see the most common types of boat hulls.

Both designs have their purpose and this is why you choose a boat suited to the kind of water activities you will be doing. If you want to water ski, for example, you wouldn’t use a canoe. Nor would you attempt to cross the ocean on a jet-ski.

The more you know about boats and boating, the more fun and safe your boating experiences will be.


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