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Pete Melvin -
on reverse bows
Pete Melvin of Morrelli & Melvin,
engineer of the Aeroyacht 121 writes:
"There are several reasons to consider designing a boat with
reverse rake or "wave piercing" bows. The primary reason we
consider this hull shape is to reduce pitching motion caused
by waves of certain amplitude and frequency. I think an easy
way to explain it is this: assume you are sailing along in
smooth water with no pitching motion. Along comes a single
wave. As your bow starts to penetrate into the wave, you
pick up buoyancy in the bow region. The boat reacts to this
increase in buoyancy by pitching bow-up. As the wave passes
under the boat, the bow then must fall back to equilibrium.
The more reserve buoyancy the wave sees
as the bow penetrates through the wave, the more pitching
moment is imparted onto the boat by the wave and the greater
the potential for pitching motion. So if your bow has a lot
of flare (vertical angle of the hull sides), then you
naturally have a lot of reserve buoyancy high up in the bow
region and this may result in excessive pitching motion.
Why do we care if the boat pitches?
Because pitching increases hull resistance, reduces the
efficiency of the rig and underwater foils due to unsteady
flow, and causes motion discomfort for the people on the
boat.
In order to keep the boat from pitch
poling, you need a certain amount of buoyancy forward. On a
wave-piercing type bow, this buoyancy and lift are achieved
by making the hull fuller down low. On some wave piercing
designs, the hull is wider at the waterline than at the
deck, especially near the bow. The wider, flatter underwater
shape provides lift at high velocities and dampens pitching at
all speeds.
Other benefits of wave piercing bows
are reduced weight and windage. You can really feel this
difference on light weight multihulls like the A-Class
catamarans. The older designs with tall bows got really
pushed around in higher wind and sea states whereas the
newer designs are easier to steer and maneuver in waves.
We have developed several very
successful racing catamarans with wave piercing bows
including the A3 A Class catamaran and the NACRA Infusion
F18. Variations of these shapes are now making their way
into some larger racing and cruising designs - such as the
Morrelli Melvin 62 and the Aeroyacht 121"
reprinted from an article that appeared in
Sailing Anarchy, June 2008
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