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.