attitude, fin position, engine shaft rotation, gear position
(fins must be centered when reversing), fluid temperature,
pressure, and so on.
Although the recommended location varies
somewhat from manufacturer to manufacturer, fin
placement is critical if the system is to perform
properly. In short, the closer to the middle of the
vessel’s waterline and the higher on the hull bottom
the fins are placed, the better the performance.
Measuring along the waterline from midships, the
area one-quarter to one-third of the way fore or aft is
well suited to fin placement, with the region halfway
between stem and stern being considered the sweet
spot (although cruising speed drives some of the
placement options, as well). Fins that are located
too far forward experience excessive pitching motion,
which limits their effectiveness, while fins placed too
far aft may cause steering problems. It’s important to
follow advice provided by the fin manufacturer, who
knows the system and knows where fins will operate
most effectively for a given vessel.
Yet another detail must be taken into account when
choosing fin location. If the fins are located too far
aft or directly forward of the propellers, it is possible
that turbulence created by fin movement can affect
propeller performance. Often the result is unpleasant
and possibly severe vibration. This is best avoided by
ensuring that the fins are placed well forward or well
outboard of the props, or both.
Of course, there are exceptions to every rule.
I’ve cruised aboard boats that could not meet the
recommended guidelines for fin placement because
access inside the boat was difficult as a result
of tankage, bulkheads, or the positioning of the
accommodations space. Thus, fins were installed well
forward or well aft of the ideal spot, but they worked
extremely well. The best approach is to consult with
the fin manufacturer (in addition to the dealer or
installer) about what’s right for your vessel. I’ve also
been aboard vessels whose fin stabilizers worked
poorly because of improper placement.
While it’s desirable for fins to be located as far
outboard and as high on the hull as possible because
they generate the greatest lift in this location, they
must remain within the “beam envelope.” No part of
the fin, in any attitude, should protrude past the beam
of the vessel. The beam envelope is designated by two
lines: one running vertically and defining the maximum
beam where the fin is to be located, and one running
horizontally along the lowest point of the keel in the
same location. This forms a triangle of sorts, and
the fin must remain within this triangle at all angles
Finally, give thought to service access. The fin
actuators, as well as all major components in the
stabilization system, should be reasonably accessible
for inspection, service, lubrication, and repair.
Of all the components in a fin stabilizer system,
the one that requires the most care and attention to
detail when it comes to installation is the fin actuator
assembly. Depending on the size of the vessel and the
stabilizer system, actuator cutout holes can be more
than a foot in diameter. Thus, great care must be taken
in selecting the fin location as well as in cutting the
holes and installing the hardware.
The area adjacent to the fins is subject to
considerable stress under normal stabilizer operation.
Remember that the fins influence the motion of
a vessel that displaces many tons, and all of the
manipulation occurs in two locations: at the actuator
attachment points. Additionally, if a fin strikes a
submerged object or if the boat runs aground, the fin,
actuator, and hull will all be exposed to substantial
stress. (Many fin manufacturers design their fins to
“break away” if they encounter an immovable object,
this being preferable to damaging or holing the hull.)
The utmost care must be taken when carrying out
this installation. The folks performing this work must
be experienced and skilled with large, below-the-waterline hardware installations, and they must also be
familiar with the hull material, whether it’s fiberglass-reinforced plastic (FRP) or metal. Depending on the
manufacturer, actuator installations in FRP may call for
achieving reinforcement via multiple layups of fabric
and resin or through installation of a backing block
made from solid, prefabricated FRP or from wood.
(The prefabricated FRP material that’s used is often
referred to within the industry as “GPO- 3.”) Of these
three options, my preference is to use a prefabricated
FRP backing block that’s properly set in epoxy.
However, you should discuss the use of this material
with your fin manufacturer before proceeding.
Although there are a host of other details that must
be followed during actuator installation, nearly all of
which I’ve seen delineated in excruciating detail in
various manufacturers’ installation manuals, a few are
particularly worthy of mention here. It’s extremely
important, during the process of determining where to
place fins and then in the actual cutting and installation
process, that the vessel’s inherent structure not be
compromised. Cutting through a stringer, frame, or
structural bulkhead to install an actuator without then