This rules out the following angles between
legs:
3-bladed prop—120 degrees
4-bladed prop—90 degrees
5-bladed prop—72 degrees
6-bladed prop—60 degreesThe standard V-strut angle is 55 degrees,
which avoids all of the preceding angles.
Obviously, 120 degrees and 90 degrees
aren’t likely, but 72 degrees and 60 degrees
can happen if you’re not careful. I even saw
90 degrees once on an old wooden twin-
screw boat. Someone had repowered her
and installed a new I-strut, but—to make it
“really strong”—they added a transverse
strut leg that ran sideways to the keel at ex-
actly 90 degrees. You guessed it, the boat had
a 4-bladed prop.
Some V-struts have their legs so close to-
gether that the width between them is about
the same as one prop-blade width. In this
case, they form a perfect shadow for the up-
per blade every time it swings by. You can see
that this was the case on the aluminum ferry
with the short pocket (see Figure 1-8A),
which obstructed water flow still more. No
wonder this vessel vibrated so badly.
Strut Grounding Skegs
Behind the strut, the lower portion of the
propeller is unprotected from the debris of
grounding. Many cruising or workboats in-
stall V-struts with a grounding skeg project-
ing down in front of the propeller. The
grounding strut isn’t nearly as strong as a
full keel, but it does protect the propeller
better than having nothing in front of and
below its lower half. Note in Figure 2-7 that
it’s important to sweep and curve back the
leading edge of the grounding skeg as much
as possible to avoid catching things and to
clear the trailing edge of the grounding skeg
as far as practical from the prop. The
grounding skeg is usually made with the
same thickness and section as the standard
V-strut legs, with a similar section modified
as necessary to fair it properly as the
grounding skeg tapers toward the bottom.
The drawback of a grounding skeg is more
appendage drag.
Propeller Apertures and Shaft Angle
Strut in an Aperture
In Figure 2-8, you can see a typical strut in a
full aperture. This provides good protection
for the propeller. Such struts are usually
standard I-struts with a rough mirror image
of the upper strut leg below, fastened to the
top of the skeg keel. Occasionally—when
great strength to withstand frequent ground-
ings is required—the strut is a V-strut with
the extra leg on the bottom to form a Y, fas-
tening to the top of the skeg. This is strong
indeed, but again, it creates additional
appendage drag.Open Propeller Apertures in
High-Speed Single-Screw
Vessels
The faster the boat, the more important it is
to concentrate on obtaining unobstructed in-
flow to the propeller. An example of a boat
type that can have problems with inflow to
the prop is a traditional Novi-type lobsterboat
that has been powered to run at high speed—
20 knots plus. Standard lobsterboats were
originally displacement hulls. They’re con-
structed with long, straight keels and a
vertical deadwood just ahead of the prop.
The stern bearing projects through the dead-
wood, and often this deadwood is not cut
back or faired away ahead of the propeller
at all. Frequently, the trailing edge of theChapter 2:Struts, Propeller Apertures, and Shaft Angle
Figure 2-7. Strut
grounding skeg