Planing and Semi-planing Hulls
 

When a planing hull is either not moving or going very slowly, it is,
in effect, a displacement hull. As power and speed increase, however, a
planing hull lifts itself up on top of its own bow wave. This causes
the boat to displace much less water. As a result, there is much less
wetted surface on the hull bottom, meaning much less friction as well.
The speed of the boat will now increase at a great rate. With this
hull, the more horsepower added, the faster the boat will go.


The advantage of a planing hull is that it is capable of much greater
speed. Just keep adding horsepower and away you go! However, a
disadvantage is that the boat is no longer going through the waves, but
is now going from one wave top to the next. Consequently, as the waves
get larger, the pounding of the boat increases. This, at the least,
causes crew discomfort. At its worst, pounding can injure passengers
and cause considerable damage to the boat's hull and equipment.

The semi-displacement hull, generally seen on larger cruisers, has some
lift capability. This is shown by the fact that as speed increases, the
forward part of the hull lifts up and allows the boat to exceed its
maximum displacement speed. Some disadvantages to semi-displacement
hulled boats are that they need much bigger engines and consume much
more fuel.

There are many other combinations of hull shapes. Some are Vee shaped
forward and flat further aft. Others are Vee shaped forward and round
further aft and so on. All of these many shapes (compromises) are
attempts by the boat manufacturers to make the boat perform the way
that the buyer expects it to perform. Again, a reminder: no one boat is
going to answer everyone's needs!

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