Muff Couplings and
Intermediate and Tail Shafts
On large boats, dividing the propeller shaft
into two sections can ease installation and
transport considerations. The forward section
(fastened to the transmission coupling) is the
intermediate shaft, and the after section is the
tail shaft. It is also less expensive to repair or
replace only one portion of the shaft (usually
the tail shaft) if it should be damaged.
When two short lengths of shaft are
joined together inside the boat, they are best
joined with keyed tapered-bore couplings. If
the joint is outside the hull, a muff coupling
should be used (also termed a sleeve cou-
pling). Marine muff couplings (Figure 1-16)
have rounded edges and are available in
bronze, aluminum, or stainless. There are
two drawbacks:1 .Even if the edges of the muff coupling
are rounded, they create more
resistance.
2 .Withdrawal resistance in reverse opera-
tion isn’t as great as a flanged coupling
with a nut on the shaft end.Nevertheless, muff couplings may be
used if they solve installation problems.Keyways and Stress
Most propeller shaft failures occur all the
way aft, at the forward face of the propeller
hub. One reason for this is that the sharp
corners resulting from the keyway create
stress risers in this area. You should spec-
ify that the keyway be fabricated to SAE
standards, which include a rounded for-
ward edge. Then go the next step and spec-
ify that the corners of the slot (both the
outer and inner corners) be radiused as
shown in Figure 1-17.
A further precaution is to hollow out the
keyway’s forward end almost like a spoon.
This removes the sharp corners there, elimi-
nating stress risers.Short-Hub Stress
Another source of stress at the end of the
shaft is short propeller hubs (Figure 1-18).
Getting propellers that have hubs shorter
than the length of a standard shaft taper is
more common than you would think. When
this is the case—if the hub is bored and ta-
pered to fit the aft end rather than the for-
ward end of the taper—the propeller will
snug up onto the taper on the shaft just fine.
You can insert the key and tighten down the
propeller nuts as usual. If you look closely at
the forward face of the hub, however, you’ll
find that it stops short of the forward end of
the taper. This means that the effective shaft
diameter is reduced to the diameter at the
point on the shaft taper where the hub ends.
It also means that there’s more overhang aft
of the shaft bearing, which increases bend-
ing stress. And the smaller effective diame-
ter itself increases local stress.
Try to avoid using short-hub propellers. If
you do have to use one, have it machined so
that it will seat up to the forward end of the
taper. You then have to have a spacer ma-
chined to fit over the aft end of the shaft
taper between the short hub and the pro-
peller nuts; otherwise, the nuts won’t tighten
down on the propeller.Interference-Fit Hubs
Though it isn’t common on small craft and
yachts (even large yachts), commercial ves-
sels often use an interference fit betweenPART ONE: DRIVETRAIN INSTALLATIONS
Figure 1-16. Muff
coupling
Figure 1-15.
Tapered-bore
coupling
(Courtesy Marine
Hardware, Inc.)