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As even the hardiest
salts will admit, uncomfortable motions in a seaway (especially roll)
can put a damper on almost anyone’s cruising pleasure. Naval architects
and yacht designers place a high priority on achieving good motion characteristics,
and for decades roll stabilizers were stalwarts in their arsenal. The
concept was simple: Movable fins on either side of the hull could be manipulated
to produce forces in opposition to the wave forces that produce roll motion.
But executing the concept was not so easy, and until recently all that
could be achieved was a modest reduction in roll motion, even with the
best systems.
There are several limitations
with the older systems: First of all, because fins require the boat to
be moving forward to produce a force (like the flaps on an airplane wing),
their effectiveness is reduced at low speeds and virtually nil when the
yacht is at rest. Ironically, at high speeds (when the fins are most efficient),
the older analog-control stabilization systems can’t respond quickly
enough to counter the roll motion. And finally, older systems rely on
gyroscopes that sense roll rate; therefore, they can react only to roll
motions that have already begun, unable to prevent those motions from
occurring in the first place. At best, older technologies can be thought
of as “roll damping” systems, capable of reducing but not eliminating
roll on yachts running at displacement or semidisplacement speeds (typically
8 to 12 knots or 12 to 18 knots, respectively).
Several recent advances
have significantly improved the situation, making nearly total elimination
of roll motion in seas of three to four feet or more possible, whether
sitting at anchor or cruising at speed. Different manufacturers have competing
views on what’s the best approach, so here’s a compilation of
the new technologies that some of the leading companies have embraced.
Control systems.
Solid-state inertial sensors respond to roll angle as well as roll
rate, allowing high-speed digital processors to react almost instantly
to very small angles of roll. This lets the stabilizing fins stop the
roll before it begins, without having to fight the momentum of a rolling
hull.
The good news here is
that, if you have an older system, you may be able to significantly improve
its performance by simply upgrading your electronic control system. Check
with the company that built yours to see if a retrofit is available.
Stabilization at
anchor. Since many owners spend more time at anchor than underway,
the desirability of a system for roll stabilization at zero speed is self-evident.
But if there’s no flow across the fin, how can it generate a force
that will stabilize the yacht? VT Naiad Marine pioneered a solution for
the 231-foot Boadicea.
The solution lies in
the fact that any change in the angle of a stabilizer fin requires that
the trailing edge of the fin move up or down (like the flap on an airplane
wing) and that the vertical movement through the water produces a vertical
reaction force. Ordinarily the fin would be configured to minimize this
force, because it increases the power needed to control the fin. But to
achieve stabilization at zero speed, it is necessary to maximize the vertical
force by making the fin longer in the fore and aft (chordwise) dimension
and moving the pivot axis a bit farther forward on the fin. The control
system then angles one fin up and the other down; as the yacht begins
a roll to either side, the controller quickly flips both fins to their
opposite orientation, producing a pair of vertical forces that act in
opposition to the roll motion.
Next page >
Part
2: Fast yachts will require smaller
fins and drive mechanisms than would slower yachts of similar size. > Page 1, 2,
3, 4
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