Game Development Reference
In-Depth Information
0.075
C
=
=
2.25
e
3
(9.22)
f
(
)
2
(
)
log
5.86
e
+−
7
2
10
Form Drag
Form drag on boat hulls is similar to form drag on projectiles. As the hull pushes its way through the
water, the pressure on the front of the hull is greater than the pressure on the back of the hull.
The pressure difference sets up a drag force that acts in the opposite direction to the boat
motion. The magnitude of form drag for a given hull depends on the geometry of the hull and
is a weak function of the boat velocity, the water density, and the water velocity. The value of
form drag for a given boat is usually obtained experimentally. As we shall see in the next section,
the evaluation of form drag and wave drag are sometimes combined.
Wave Drag
When the hull of a boat travels through water, waves are created by the action of the hull
pushing water away from its surface. The waves radiate away from the body and carry with
them a certain amount of energy that is dissipated in the ocean, lake, or river. The wave drag
experienced by the boat is a consequence of the work performed by the hull in maintaining the
wave structure.
When a boat with a displacement hull travels slowly, there will be a wave created at the
bow of the boat and many other smaller waves behind it. As the velocity of the boat increases,
the bow wave increases in height and the spacing between the waves increases. If the boat
velocity continues to increase, there will be a point where there will be a wave crest at the bow,
a crest at the stern, and a trough at in the middle as shown in Figure 9-7. The distance between
the wave crests is equal to the waterline length of the hull, L w .
L w
Figure 9-7. Wave structure caused by a displacement hull
Tidbit The waves created by large ships can be quite substantial. Off the coast of the Gulf of Mexico,
surfers ride the waves caused by large container ships.
William Froude, a nineteenth century English engineer and naval architect, determined
that the velocity of the waves, v wv , produced by a displacement hull relative to a fixed coordinate
system was proportional to the square root of the length of the wave, L wv .
v
=
1.34
L
(9.23)
wv
wv
 
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