would use only 316L stainless for struts, and
then only if I had no other choice. Any other
stainless is far too likely to suffer from pit-
ting corrosion, especially at the welds. Even
316L suffers from this to some degree. Com-
mercial bronze and manganese bronze are
very common strut materials. They are
strong, inexpensive, and cast easily. Though
they’re acceptable, both commercial and
manganese bronzes are really brasses con-
taining zinc—they’re not bronzes, in spite
of their name. (Bronze is copper alloyed
with tin; brass is copper alloyed with zinc.)
All brasses can suffer from a corrosion
called dezincification and are not recom-
mended for underwater work. If the struts
are made of manganese or commercial
bronze, you have to be careful to protect
them with anodes.
Aluminum struts that are continuous
through the bottom of the hull and are struc-
turally welded to framing inside the hull
(with welds at the hull shell only for water-
tightness) can use the full yield strength. If
the aluminum strut welds directly to the bot-
tom of the hull or to plates mounted to the
bottom of the hull, you must use the as-
welded yield strength.Additional Strut
Dimensions
The controlling dimensions for other aspects
of the struts are either the drop (the vertical
distance from hull bottom to shaft centerline)
or the prop shaft diameter. Cutless bearings
come in varying dimensions for the same
prop shaft diameter, so the bearing must be
selected before determining the bearing tube
diameter. The bearing tube wall thickness
should equal shaft diameter divided by 4, as
shown in Figure 2-1, but should never be less
than^3 / 16 inch (4.8 mm).
Note that the standard Cutless bearing
length is four times the shaft diameter. If you
can go less on high-speed boats, it will reduce
wetted-surface drag. Thordon makes bearings
that are designed at just two times the shaft
diameter in length. These are well worth in-
stalling if the last ounce of speed is critical,
and the previously shown struts can simply be
trimmed down (tapered more) at their bot-
toms to match this shorter bearing length.Strut Fastening and
Mounting Considerations
Of course, sturdy struts alone do not neces-
sarily make for strong support. The struts are
no stiffer or stronger than their attachment to
the boat. One 50-footer (15.2 m) that I was
called in to fix had severe vibration prob-
lems. Her V-struts were massive, her props
were new and carefully balanced, and her
shaft alignment had been checked and
rechecked. She was a new, twin-screw fiber-
glass boat.
When I clambered into her after bilge, I
found that her V-struts were through-bolted
to the fiberglass hull bottom with nothing
more than individual backing plates of the
same dimensions (footprint) as the V-strut
bases below them. The resultant installation
gave the illusion of being strong, but it wasn’t.
(Certainly, it fooled the boat’s builder, who’d
otherwise done a decent job on her.) Even
though the bottom was a bit over^7 / 8 -inch
(22 mm) thick solid glass, it wasn’t suffi-
ciently stiff. The hull itself was flexing at the
strut attachments—flexing enough to gener-
ate noticeable hairline cracks after only a few
dozen operating hours.
Fiberglass is middling strong for its
weight, in terms of pure tension, but it can
only be described as weak in bending. (Its
flexural modulus is low.) The stiffening for all
fiberglass-reinforced plastic (FRP) hulls
needs to be carefully thought out, with great
attention to spreading loads and avoiding
hard spots. It was relatively simple (though
hard work for the repair yard) to fix this boat.
The hull bottom was built up inside—around
the struts—with another^1 / 4 inch to^3 / 8 inch
(6.3 mm to 9.5 mm) of glass for about two
feet square (60 cm squre). Then large backing
blocks and backing plates were installed.
This made all the difference.Strut Backing Blocks
According to my experience, all FRP hulls
require large backing blocks and plates
behind the strut bolts (Figure 2-4). The
wood, ply, or G-10 backing-block thickness
should be at least 2 times the strut-bolt di-
ameter, and the block’s footprint should be
1.3 times the footprint dimensions of the
strut base below it—more doesn’t hurt. WithPART ONE:DRIVETRAIN INSTALLATIONS