Game Development Reference
In lieu of enough information to calculate the projected transverse area of the ship, you
can approximate it by:
A p = B 2 /2
where B is the breadth (width) of the ship.
Planing craft are different than displacement vessels in that when they reach their cruis‐
ing speed they are not supported by buoyancy. Unlike a super tanker, a recreational
speedboat has a much “flatter” hull form. It almost looks like a very fat foil, which it is!
When a speedboat is just sitting in the water it is fully supported by its buoyancy, just
like a tanker. However, as the boat starts moving forward, the hull is at an angle of attack
to the water. Like a super tanker, the boat is also creating a wave in front of it as it moves,
called the bow wave . However, similar to an airplane breaking the sound barrier, a
planing craft is fast enough to catch up to this wave. As it reaches its own bow wave, the
vessel will start to tilt backward. This tilting increases the resistance by virtue of in‐
creasing the angle of attack of the vessel's hull. However, if the vessel has additional
power to overcome this increased resistance, the lift force generated from the hull foil
will begin to lift the hull out of the water. At this point, the forces in Figure 16-4 begin
Figure 16-4. Forces on a planing craft
As you can see, compared to when the boat is at rest, very little of the hull is in the water
now. This, in turn, provides positive feedback because it reduces the wetted surface area
and skin friction and allows the craft to go faster, generating more lift and reducing the
skin friction yet again. Simultaneously, the drag created by the foil moving through the
fluid increases until eventually the available power is exhausted and top speed is attained.
If we plot the resistance versus speed, it might look something like Figure 16-5 .