The essential fact to understand about hull speed is that there is an
exact relationship between the length of a wave and how fast it moves
through the water. If you time the crests as they go by a fixed point
like a buoy, you can calculate the exact distance between the crests.
Longer waves move faster.

The hull makes wave as it disturbs the water. At low speeds, there is
room for several crests and troughs along the hull. You can see by the
large wave system even a small pebble sets up that it doesn't take a
lot of energy to create a wave train. Hull resistance at low speeds is
primarily skin friction.

As speed increases, the waves the boat makes must become longer in
order to maintain the speed / length relationship. Eventually there is
room for just one wave at the bow and one quarter wave at the stern.
When the speed length ratio is 1.0, there will be a crest at the bow
and another at the stern. The boat will be sitting fairly
symmetrically without trimming down by the stern and the wave
rebounding up under the stern will actually be pushing the vessel
ahead recovering some, but far from all, of the energy required to
produce the wave train. Vessels can thus get up to this speed with
fairly modest power.

To go faster however, the crest of the wave at the stern has to start
moving behind the boat. Two things happen. First, wave behind the hull
can not return energy to it. This pushes power requirements up.
Second, the hull now starts to squat by the stern which is moving into
the trough. The bow wave always remains about in the same place so the
boat has to start climbing up a hill that it is also making. The graph
of power required starts to go straight up as the stern wave moves aft
of the transom.

The basic relationship is that it takes four times as much power to go
twice as fast. If you graph this out, you'll see that hull speed is
not a precise point but is a fairly narrow band. You quickly reach a
point where doubling the size of the engine only gains you a quarter
knot.

If the boat is shaped so that water flow over the bottom creates
dynamic lift instead of suction, the hull will start to lift up. With
sufficient power, the vessel can be pushed up the hill of the bow wave
on to the top where it can again ride level. It will still be
producing a wave train but all the crests will be well behind it. A
deep hull like a sailboat or a tug boat won't do this. The suction of
steep flow lines in the stern will pull the stern down. Some hulls
will actually pull themselves below the surface if enough power is
applied. The waves created by hull will keep the water off the deck
but, if something suddenly stops the hull, it can be swamped by its
own wake.


--

Roger Long



Does it apply to non-rigid hulls (hulls that might flex in the
middle)
Very complex question. Can't be answered in general.

Does it apply to totally submerged objects?
No.

Does it apply to towed objects, like dinghies?
Yes.

What happens when an object exceeds hull speed?
See above.

Is there any way to "fool the water" into acting as if the boat is
longer than it is?
If anybody has figured out how to fool the universe yet, I'd like to
hear about it.