You can use any combination of the pre-
ceding intermediate bearings as required or
convenient to reduce the bearing spacing to
the maximum allowable. I try to avoid inter-
mediate strut bearings if possible, because
they add external appendage drag. Often I’m
able to arrange things so a rigid stuffing box
will do this job without the extra external
drag. The shaft runs from the engine coupling,
through the rigid stuffing box, and then to the
stern bearing in the propeller strut. There are
situations, though, when an external interme-
diate strut bearing (occasionally two of them)
is unavoidable. This is not a problem; it
simply represents a bit more appendage drag.
Shaft Couplings and Keyways
Propeller-shaft couplings are available in sev-
eral styles or types: there are tapered-bore,
split straight-bore, straight-bore, and more
(Figure 1-14). The straight-bore couplings are
easier to machine, having no taper, but they
are not my first choice for propeller shafts.
It’s not that they’re weaker than tapered-bore
couplings, but it can sometimes be quite a
challenge to fit a shaft into a straight-bore
coupling, even a split coupling, in a boat. It
can also be nearly impossible to remove a
straight-bore coupling that isn’t split. By con-
trast, a tapered-bore coupling helps guide the
shaft in during installation, and it breaks free
more cleanly for disassembly.
I recommend only keyed tapered-bore
couplings (Figure 1-15) for propeller shafting
(unless an interference fit is used, as de-
scribed later). Not only do these couplings
make it easier to install and remove the shaft,
but the nuts on the shaft ends, in the cou-
plings, ensure that the shaft cannot pull out
when the engine is reversed. Other forms of
couplings rely on pressure/friction and the re-
taining power of setscrews to resist axial
withdrawal during reverse operation. I’ve
never felt secure with this. Nevertheless,
straight-bore couplings of various types have
been and are used successfully on many
boats. There is no need to replace these with
keyed tapered-bore couplings on existing in-
stallations. Most split straight-bore couplings
have a machined hole on either side to install
a retaining pin through the coupling and the
shaft. This would effectively keep the shaft
from pulling out in reverse, but I seldom see
this installed. It is also more difficult to install
the coupling with this pin.Chapter 1: Propellers and Shafts
Figure 1-14. Shaft
coupling types
(Courtesy Marine
Hardware, Inc.)