Ellen MacArthur wrote:
"Joe" wrote
good suggestion for a planing hull, bad bad ideal for a non-planning
hull, drag would have you pitch poling right away.
I don't think so, Joe. What's planing and what's non-planing?
Planing allows the boat to go faster by using its speed and hull shape
to lift the front part of the hull out of the water. The boat travels
on top of the water, greatly reducing the hydrodynamic drag on the
vessel. The increase in aerodynamic drag is small by comparison, and
can be compensated for by the increased power from the sails due to the
faster speed of the craft, and by the crew trimming the sails.
The term 'planing' refers to a craft which is predominantly supported
by hydrodynamic lift, rather than hydrostatic lift (bouyancy). The
extent to which a boat is said to be planing is usually derived from
the dimensionless 'Froude Number'.
The earliest documented planing sailboat was a proa built in 1898 by
Commodore Ralph Munroe; it was capable of speeds of more than twice the
hull speed.
Planing a sailing dinghy was first popularised by Uffa Fox in Britain.
In 1928 Uffa Fox introduced planing to the racing world in his
International 14 dinghy, the Avenger. It had been designed with a hull
shape which permitted planing. He gained 52 first places, two seconds
and three third places out of 57 race starts that year.
Obviously this performance had an impact: other designers took on his
ideas and developed them. Over the years, most dinghies have acquired
some ability to plane, and there are now many high-performance dinghies
(usually called skiffs, see these examples, or these in [1]), which
will plane even in light winds, at all points of sail.
How planing works
Normally a non-planing, displacement, hull is restricted in its maximum
speed by a formula related to its overall length , where HSPD (in
knots) is maximum hull speed, and LWL is the hull length in feet at
waterline. This speed is maximised when the boat sits between the bow
and stern waves, with no intervening self-caused waves along its
length.
At low speeds, a hydroplaning hull acts as a displacement hull. But,
when the speed increases the hull begins acting as a planing hull.
However, when the boat begins to plane the formula becomes irrelevant
since the boat is climbing its own bow-wave. The bow rises slightly as
it starts by mounting its own bow wave. When it reaches the speed where
it overtakes the bow wave, the bow resumes its normal attitude. The
boat can often be seen to leave its stern-wave some distance behind it.
The hull is now planing.
Beginning to plane is the aquatic, and less dramatic, equivalent of an
aeroplane breaking the sound barrier. The aeroplane at Mach 1 begins to
pierce and go beyond its own 'bow wave', i.e. the compressed layers of
air on its front surfaces and ahead of it.
A hydroplaning hull travels faster and more efficiently than a
displacement hull of comparable size due to two factors:
less area of the hull is in contact with the water. This reduces the
friction on the hull caused by water. the hull is displacing less water
from its path. Water is relatively heavy and a displacing hull must
displace its own weight of water.
The characteristics of a planing hull are that it is narrow at the
prow, with a broader beam towards the rear. The shape of the underneath
of the rear of a larger, planing, powerboat is often V shaped. To
plane, the power to weight ratio must be high; sailing boats need a
good sail area and powerboats need a highly powered engine.
Note that under some high wind conditions, very light craft (such as
windsurfers and kitesurfers) can actually be pulled up onto the surface
of the water, or into the air, by the upward lift of the sail alone.
Although this certainly reduces water resistance, it is probably better
described as flying, rather than hydroplaning. It is also not a
sustainable state, as sailing (or kite flying) involves the extraction
of energy from the shear force between the wind and the water. If the
entire hull leaves the water, the craft will quickly come to rest
relative to the wind, and lose its lifting/driving force.
How to plane in a sailing boat
Planing can happen in a suitably designed boat in moderate to strong
winds if the crew do some or all of the following:
Sail on a reach or broad reach to begin
Slacken the jib
Raise the centreboard
Increase the speed
Keep the hull level, trapeze if necessary
Observe the wake until it is smooth and fast
Move the crew weight increasingly towards the rear to begin and to
sustain planing
Sheet in as speed increases, and apparent wind correspondingly moves
forward
Keep the boat flat and level
Bear away to maintain speed as necessary
Flick or pump the sails (although there are restrictions on doing this
in a race)
While planing, keep control of the waves and steer through them,
avoiding to increase speed to collide with the wave in front. Also, in
dinghies, keep good control of the sail power. A small change in wind
direction can easily cause a capsize, watch also out for gybes. Boat
control becomes easier as planing begins, but fast reactions are often
needed to get there, to keep the speed up and to keep the boat level.
Crew balance and trim are vital, as are sail trimming and minimal
centreboard
It's only so on
a horizontal surface. On tilted water it has no meaning. A sailboat hull can't plane
on flat water. There's not enough power in the engine or sails to make it go fast
enough. But it can when it's going downhill when the hill's steep enough. Gravity gives
you enough power. Your boat would be able to break out of the wave train it makes.
The reason is because the front of the bow wave is almost horizontal on a steep hill
of water instead of being too steep to get up like it is on horizontal water.
What do you think?
I think before I got 65,000 lbs on plane with a displacement hull I
would be in a vertical position racing to stuff the bow deep into the
wave leaving to a classic pitchpole like this:
http://www.bymnews.com/photos/albums...ormal_Git1.jpg
Joe
Cheers,
Ellen