Game Development Reference
barrier for some fuller hulls. Note that the speed for the 40-foot sailboat is the hull speed
of a 40-foot full-formed (not slender) hull. We can calculate the hull speed with the
V hull ≈ 1.34 × LWL
Where V hull is the hull speed in knots and L WL is the length on the water line. Some
modern displacement boats, especially racing kayaks, can exceed their hull speed if they
have very fine ends, long hulls with narrow beams, or some other optimized hull form.
There are a variety of methods to propel a boat through the water. The oldest is by way
of sails using the wind. The physics of sailing could fill several chapters on its own, so
we won't go into detail here. If you are interested in that topic, The Physics of Sailing
Explained (Sheridan House), by Bryon Anderson, is a good starting point. You can also
check out the airplane chapter in this topic and note that a sail is just a vertical wing.
We will say this: if you choose to put a sailing vessel in your simulation, make sure to
remember the golden rule! You cannot generally sail within 45 degrees of the direction
the wind is coming from.
There are many different kinds of propulsion: rudders, shafted fixed-pitch propellers,
shafted CPP propellers, azimuthing thrusters, propellers in nozzles, water jets, pump
jets, contra-rotating, and the very odd Voith Schneider type. We can't get into all of them
here, but the aforementioned list should get you started if you are interesting in mod‐
eling the specifics of different propulsor types.
The major propulsion relationship for your simulation would be the thrust-to-propeller
RPM or thrust-to-throttle curve. The fly in the ointment here is that thrust is also de‐
pendent on forward speed. As the boat moves faster and faster through the water, the
inflow velocity of the water into the propeller is higher and higher. This means that the
difference between the velocity of the intake and the velocity of the output is smaller,
reducing the amount of thrust. In general, the thrust curve versus boat speed will look
like Figure 16-6 .