DSK wrote:
I get the impression you're looking for other people's comments, but can
I answer too?

Short Wave Sportfishing wrote:

So the question occurred to me after I closed a reply to Doug King
about sailing hulls.

Are sailboats considered planing hulls?

Sure... wait, not all of them! Some sailboats were planing back in the
1890s, long before any motorboats had reached the necessary power/weight
ratio.

For example, a Hobie Cat - is that a planing hull?

No, because the hull does not provide any lift from the force of the
water going past/under. This is the key to planing. Catamarans can
exceed what's thought of as "hull speed" limits because they are not
bound as strongly by their wave train. A hull narrower than about 8:1
length-beam ratio does the trick.

DSK


A hobie cat with a huge side load on it -caused by the sails- does
plane. The only thing some can't quite grasp is that the boat is
pushing not quite sideways against the water, and is sort of
squeezed between the sail and the water, so it goes fast, pushing
water aside at such a speed and in such quantity as to
counterbalance the weight and forces of the boat and sail, and it is
planing on it's side, almost, with enough of an angle so that the
boat rises in the water just like any "conventional" planing boat.
Planing Hobie Cats leave a vee shaped groove behind them in the
water, which fills up quickly. Only one, note, as they plane with
one hull flying.

Only thing, it's not as stable as a regular boat, it's a balancing
act, kinda like carving a turn in snow, or on water skis.

You just need to turn your head to the side to see it, right?

What the action of planing is, is pushing water out of the way so
vigorously that the reaction to the force of that action is so great
that it exceeds the weight of the vessel, so that it no longer has
to displace the water purely as weight to stay afloat, but the
reaction to moving so much water aside or down so fast is sufficient
to lift the boat above it's displacement lines, which results in
less surface wetted, less friction, more speed. The geometery of the
shapes determines how the vector sums work out, and at what speed
you might decide that you are planing.

The action of planing begins as soon as the vessel moves, increasing
with speed to the point where it becomes the predominant force
supporting the boat.

*/ optional puzzeling humour segment begins

There is a gray area between sitting still and flying so fast as to
never fall back down to the surface of the planet. This zone is
called the intra-planetary zone. It is a dense, gray zone, thick
with logic, numbers, sea foam, propulsion technology, and opinion.

optional puzzeling humour segment ends /*

Any thing that moves in the water develops opposing forces. If
those forces contribute sufficiently to supporting a boat to reduce
the wetted area, it is to an extent, planing.

Some shapes plane at different speeds, and depending on conditions,
to different extents. The bombs used by the dambusters planed for a
while before sinking to the face of the dam, just like skipping
stones, or red hot cannon balls skipping above the waves. They were
not flat bottomed, but round.

Planing hulls are hulls that go fast enough, period. Everything can
be made to plane. Some plane easier than others. Flat bottoms are
more fun in this regard, doing it slower and better, too.

Terry K