cruises at 9 knots and the other at 29 knots, they will use
fins with dramatically different surface areas. Larger fins
will be used on the slower vessel.
Vessels that roll quickly, often referred to as having a
“stiff” or “snap” roll, typically require a greater fin size
than “soft” or slow-rolling vessels. A high GM equals
a stiff roll. Perhaps more important than any of the
aforementioned information is experience gained
from fin installations on identical or similar vessels.
If a particular fin size works effectively, or doesn’t,
on a similar boat, then that information should be
factored into the fin selection process.
Fin sizes range from 2 sq. ft. to more than 30 sq.
ft., and thus the selection is wide indeed. When in
doubt, or if the calculation lands between two sizes,
rubber link or a series of belts. Unless the alignment was
perfect, the rubber in the coupling or belts would wear
itself to dust in a season of cruising or less. Therefore,
conscientious installers included adjustment mechanisms
in the mounting shelves. Even then, they often required
shims to ensure proper alignment. In such instances,
setting up the hydraulic pump, shelf, and drive
it’s often preferable to err on the side of a slightly
larger fin. More surface area, to a point, will render
more effective stabilization with less movement
of the fin actuation mechanism. Additionally, it’s
possible to modify a fin installation to suit a different
desired cruising speed. If you have been slowing
down lately to save fuel, your fins may no longer
be optimized for your cruising speed.
The heart of a hydraulic fin stabilization system is the
hydraulic pump or pumps. When I first began working
in the marine industry, nearly every stabilizer hydraulic
pump installation I encountered consisted of an elaborate
shelf arrangement located forward of and attached
directly to the propulsion engine. The shelf, made of steel
or aluminum, had to be extremely rigid to support the
considerable weight of the pump and the torque induced
by the engine’s crankshaft, to which it was connected.
The engine’s crankshaft rotated the pump via a flexible
Top right: The reservoir tank and heat exchanger assembly are
where the supply of stabilizer hydraulic fluid is maintained,
checked, and filled. The sight glass and thermometer allow
the user to check the fluid level, temperature, and clarity.
Above right: Because the actuator moves and often does so
quickly, it’s important to ensure that all wiring, hoses, and
other gear remain clear of the assembly. Left: Referred to as
line, weed, or kelp deflectors, fins located forward of (or on)
the leading edge of stabilizer fins will help prevent debris
from becoming lodged between the stabilizer fin and the hull.