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Heavy Boats, Light Boats, and Hull Speed

Here's an attempt to explain these things in layman terms.....

A boat displaces its own weight in water. When the boat is moving, it must push that much water out of the way as it goes forward. Since a heavy boat has to push more water out of the way, it makes bigger waves. (As a boat moves faster it has to push aside more water in less time, so that makes the waves bigger too.)

Each boat creates a bow wave and a stern wave. When a boat reaches "hull speed" the bow and stern waves coincide to make one huge wave system. A heavy boat gets trapped in its own wave system. (For a 20 foot boat, hull speed is about 6 knots. For a 30 foot boat, hull speed is about 7.3 knots.)

The best example of this is a tugboat. Tugboats are very heavy, since they have huge engines for shoving ships around; and when they are not shoving a ship, they are racing as fast as they can to the next job. That's why you see them with a huge bow wave, a huge stern wave, and a deep wave trough in between. In spite of their enormous horsepower, they can't break loose from the trap of their own wave system. They dig a big hole in the water, and can't climb out of it.

A light displacement boat such as a dinghy, a ULDB, or a multihull doesn't have so much water to move out of the way - so they make smaller waves. When they reach the speed that would be hull speed for a heavy boat the wave system is not big enough to trap them. They are able to exceed the "speed limit" where bow and stern waves coincide.

A planing hull actually climbs up its own bow wave and is lifted partially out of the water. Obviously ocean waves affect a light boat more strongly, since the weight of the wave is bigger compared to the weight of the boat. Consequently light boats surf more readily; but are often slowed down more when going against the waves. The upwind loss is diminished though, because light boats tend to be narrower and more maneuverable. Therefore, they can sometimes slither through and around waves a bit better at the hand of a skilled helmsman.

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