Game Development Reference
In-Depth Information
Thrust and Drag
The basic relations between thrust and drag are the same for a jet ski as they are for a planing
speedboat. The engine generates thrust that propels the jet ski forward. Various aerodynamic
and hydrodynamic drag forces including skin friction, wave drag, form drag, and aerodynamic
drag resist the forward motion. To develop a general model for the motion of a jet ski, as was
the case with planing speedboats, requires a lot of detailed information including drag coeffi-
cients, wetted area time histories, propulsive and hull efficiencies, and so on.
Unless you have access to all of this technical information, you may be better off modeling
the jet ski motion by generating an acceleration profile from existing performance test data.
Table 9-4 contains acceleration data taken from a performance test of a Kawasaki 800 SX-R
(superstock model) jet ski that was performed by Personal Watercraft Illustrated magazine.
Table 9-4. Acceleration Data for the Kawasaki 800 SX-R
Time ( s )
Velocity ( m/s )
0.0
0.0
0.78
8.9
1.9
15.6
12.4
64.5
Comparing the jet ski acceleration data in Table 9-4 with the Fountain Lightning speedboat
data from Table 9-3, we can see that the jet ski initially has significantly greater acceleration
than the speedboat. It takes the jet ski less than 2 seconds to reach 15.6 m/s , a velocity that takes
the speedboat almost 8 seconds to achieve. However, the speedboat eventually catches up to
the jet ski in that the two watercraft are at about the same velocity after 12 seconds.
To model the performance of a jet ski, we can use the same process as that for modeling
the powerboat. An acceleration curve can be developed based on the measured velocity as a
function of time. The more data points that are available, the more accurately the curve fit relation
will approximate the acceleration performance of the jet ski.
The Physics of Sailing
Up to this point in the chapter, the discussion has been on modeling powerboats. As you have
learned, with a powerboat, a propeller or jet drive provides the thrust. With a sailboat, the thrust
for the boat is provided by the wind blowing into or over the sails. Figure 9-12 shows a schematic
of a typical sailboat. Some of the terms that apply to a sailboat (bow, stern, beam, and so on)
are the same as for a powerboat and are not shown in the figure. The mast is a vertical pole on
which the sails are raised. Most mid- to large-sized sailboats have more than one sail. The boat
in Figure 9-12 has two sails, a mainsail behind the mast and a smaller jib in front of the mast.
Sailboats also have an extension of the hull known as the keel , which is used to help stabilize
the boat.
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