Stabilizing fins
The fins themselves are located at convenient positions
within the middle third of the waterline length.
In multiple fin arrangements of two or more on each
side fins are placed at least eight feet apart, but still
within the middle third of the waterline. Although
some stabilizing occurs with the craft at rest, when
the fins act like bilge keels, they are truly effective
only when the vessel is moving forward. Unlike an airplane's
wings. the stabilizing fins point down, with the
post of the fin mounted perpendicular to the hull at
the turn of the bilge in a round bilged hull or under
the chine. The tips of the fins are protected to some
extent, always being within the hull's maximum beam
and the keel's maximum draft.
In shape, the fins are symmetrical hydrofoils. to
minimize drag in a trailing position and to maximize
lift when in operation. It should be noted that speed
loss caused by drag created by the angled fin is offset
by the reduction in roll inertia and in yawing
movement, and in its associated drag along the entire
length of the vessel: In effect, the stablilized vessel has
better speed performance than the non-stabilized vessel
when they ar operating under similar rough sea
conditions.
Dictated by the gyro, the fins are deflected into
position by the driving nnechanism, which provides the
necessary torque to oppose the sea forces tending to
roll the vessel. This driving mechanism consists of
pneumatic or hydraulic cylinders attached to the fin
and capable of moving it around its post, so that the
tail of the hydrofoil moves up and down from its neutral
trailing position.
As the gyro is defected in proportion to the vessel's
rate of roll, the control unit computes the amount
of pressure required to produce the necessary stabilizing
torque. A fully proportional stabilizing system offers
a gradual increase of pressure as the rate of roll
increases, which in turn yields a precise angle of fin
deflection, at all intensities of roll. In contrast, a par
tially proportional system provides two or three preset
angles of defection (neutral half stroke, maximum stroke) .
Adjustment of an angle
The pressure dictated by the control unit is exerted
on the driving mechanism's cylinders. which defect
the fin tail on one side upward. to produce down-
ward lift, and downward on the other. The amount
of lift force produced by a given hydrofoil depends
upon the angle of deflection of the fin and on the square
of the vessel's speed. Thus, to obtain maximum sta-
bilization from a vessel's system, the boat speed and
fin deflection angle must be coordinated. In this way
the lifting force can remain constant over a wide range
of speeds.
In other words. as the vessele's speed increases, the
fin angle should decrease proportionally, and vice
versa. regardless of the intensity of the rolling
motion. Without continuous speed/deflection-angle
coordination, a stabilizer will be fully effective at one
speed only. For example, if a stabilizer system were
designed for a 20 knot craft, it would be only one
quarter as effective when boat speed drops to 10
knots which is normally the time when maximum
stabilizer effectiveness is required. On the other hand,
if a boat's speed through the water exceeds the designed
value, there is danger of overspreading the fin
post.
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