slow speeds, but lift toward the surface of
the water as they move faster. At full plane,
they ride nearly on top of the water with only
a portion of the hull in the water. They can
skim along at high speed as though riding on
top of the water rather than pushing the water
aside. Runabouts, smaller cruisers, sportfi
shermen, personal watercrafts, and small
sailboats are examples. It takes considerable
power to get the boat up and hold it on top of
the water, so these boats tend to be light, have
large engines, and use considerably more fuel
than displacement hull boats. They tend to
pound at high speed and in rough seas.
Semi-Displacement Hulls
Some boats combine some features of the
two basic types of hulls. They are almost
as fuel effi cient at low speeds as displacement
hulls. They can attain higher speeds
than displacement hulls but do not go as
fast as planing hulls. These are referred to
as semi-displacement hulls. These boats
Section 1: Getting Started 7
Figure 1-8. Displacement hull
Figure 1-9. Planing hull
have suffi cient power and a hull design that
allows them to rise partially out of the water.
Bottom Shapes
The bottom of a boat may be round, fl at, or
V-shaped.
Round bottom boats offer a
smoother ride through the water because
they have no hard surfaces for waves to
pound against (Figure 1-10, next page).
However, they roll in seas and may tend
to tip when people or loads shift on
board. The rolling may lead to some discomfort
in seas, but this hull type can
be quite resistant to capsizing. Usually,
larger ships such as freighters and cruise
liners have round bottoms and often
have underwater appendages to help displace
water and improve fuel efficiency.
Flat bottom boats represent the basic
planing hull and they can go rather fast.
They offer a rough ride in choppy seas
because the fl at bottom surface is exposed to
74
75
76
77
Quarter Wave
Bow Wave
Bow Wave
Wetted Surface