Flexible Coupling Installation
"Soft" engine mounts (that are designed to isolate
engine vibration from the hull) and a flexible coupling go a long
way to minimizing vibration. This article details some things I
learned installing a Vetus Bullflex 2 flexible coupling in our 1988
Catalina 34, Hull 563 equipped with a Universal M-25XP engine. In
addition to the coupling, we replaced the stock engine mounts with
Vetus K-50 mounts. There has been quite a bit written about changing
mounts and engine alignment, so Iíll just discuss the installation
of the flexible coupling. This installation technique assumes that
youíll keep the boat in the water. If you pull the boat,
installation is somewhat simplified, since you can pull the shaft to
aid in disassembly.
The first step in installing the coupling is to remove the old
coupling. This is much easier said than done, especially if there is
any dissimilar metal corrosion (the stock shaft is keyed bronze
inside hardened steel). After several years in a corrosive salt
water environment, couplings tend to effectively fuse themselves to
the prop shaft. The good news is that if you have a stock coupling,
on a stock shaft, itís practical to cut the shaft at the aft face
of the coupling, since the Bullflex has an elongated compression
throat that attaches to the shaft.
Figure 1. Bullflex coupling.
The shaft slides into the compression sleeve on
the left portion of the coupling shown in Figure 1 (shaft diameter
equals dimension P). Four Allen screws serve to compress the sleeve
around the shaft. There is not a traditional key for securing the
shaft in the coupling.
The first step is to loosen the nuts that hold the old coupling
to aft portion of transmission. If your coupling has been neglected,
you might consider spraying a good bit of penetrating oil on it and
all fasteners and allowing it to sit for a day or so prior to
attempting removal. You can access the nuts that attach the coupling
to transmission with a box end wrench, or better yet a box end
ratchet. If you plan to cut the shaft, itís not necessary to
remove the lock nuts or safety wire.
The overall length of the Bullflex (dimension L in Figure 1) is
quite a bit longer than the stock coupling. The stock coupling is
about the dimension of the Bullflex, minus the compression sleeve.
So if you cut the stock shaft and donít slide it at all, it will
end up just about filling the compression sleeve in the Bullflex,
which is perfect.
Note that if you donít cut the shaft, you may not be able to
slide it aft far enough for installation in the Bullflex without
lifting the engine out of the way. Ideally, there should be about 1
inch of clearance between the rudder and the prop. Itís best to
take a quick trip over the side to eyeball the clearance; but before
you loosen the old coupling, make a mark on the shaft where it
enters the packing gland for reference. After you loosen the
coupling nuts, you can slide the shaft aft until it hits the rudder
to check the clearance. When I installed the coupling in our boat, I
was careful to ensure that the final assembly kept the mark I made
adjacent to the packing nut prior to disassembly, as I wanted my
prop to stay in the same position relative to the rudder.
Cutting the shaft in the boat can be done a number of ways.
Probably the quickest is to use some sort of cut-off tool. A
hand-held hacksaw blade will also work, and odds are you wonít cut
or damage anything you donít intend to. The shaft in Figure 2 was
cut by hand with a hacksaw blade. The aft face of the old coupling
will serve to square the blade to the shaft, which will make
finishing with a file that much easier. The end of the shaft can be
finished with a file, and the shaft cleaned with 220 grit paper and
machine oil. Figure 3 is the end of the stock shaft (with key) on
the front face of the couplingóthe side that bolts to the
Once the shaft is cut, remove the nuts and the old coupling.
Before installing the new coupling, you might consider replacing the
packing or even replacing the packing nut with a dripless shaft
seal, since you can slide the nut or seal over the shaft. Measure
carefully to determine if you have sufficient shaft length to add
both the flexible coupling and the dripless seal.
Since the packing nut is easily accessed with the stub shaft
exposed, might as well change the stuffing, if itís been a while.
If youíre changing the stuffing in the water, have an old cotton
rag strip handy. Once you pop off the stuffing nut, wrap the cotton
tightly around the shaft and secure it with a zip tie or hose clamp.
This will reduce leaking to a slow drip while youíre working. You
can slip the nut off the stub shaft and clean it out. If there is
any shaft wear, you might consider sliding the shaft in or out to
put "new" shaft in the nut and cutlass bearing. Best to
take a quick dip overboard to check things out before you finalize
shaft position. Another technique is to slide the shaft aft until
the prop hits the rudder. Mark it, and then slide it forward about 1
inch (more is O.K., less isnít!). Again, if a diving mask is
handy, make sure your not doing something stupid! This falls under
the purview of the time honored guidance to "measure twice, cut
onceÖ" Install the new packing and hand tighten the
nutóyouíll need to retighten as the new packing seats in the
Another technique I picked up from a local mechanic is to not
bother with wrenches when working with stuffing glands. The packing
nut itself should always be tightened by hand, and only the small
lock nut merits more forceóbut not much! To loosen a lock-nut, I
just tap the edge with a dull chisel and hammer, seating the chisel
in such a manner that only the lock nut can turn. If there is any
corrosion, a wire brush will clean things up before disassembly.
When itís time to tighten, I tighten the packing nut by hand, then
turn the lock nut and give it another tap to lock it (only about a
1/16th to an 1/8th of a turn). With traditional Teflon stuffing, a
good rule of thumb is to tighten the gland to the point where it
wonít heat up to a point that you canít hold on to the nut.
Figure 4. Bullflex 2 Flexible Coupling installed.
The Bullflex coupling comes with detailed installation drawings
including torque specifications for all of the bolts. All of the
hardware is metric. I had to go to a local auto parts store to find
the appropriate metric Allen bit for the torque wrench. The bolts
that compress the sleeve around the shaft have an Allen head. The
lock nuts that attach the coupling to the transmission are
low-profile nuts that can be seated with a metric box-end torque
wrench. To attach the coupling to the shaft, slide the shaft so that
itís in itís final position (nominally 1" desired between
prop and rudder)óthis can be the mark you made initially showing
where the shaft exited the packing nut if you didnít want to
change anything. Then, holding the shaft in place, slide the
coupling until it butts against the transmission flange. You can
then tighten the compression sleeve. After this, the coupling will
be firmly attached to the shaft, but you can still slide the shaft
aft to assist in engine alignment and installing the mounting studs.
Before attaching the mounting studs to the coupling, you should
re-align the engine. Not having the studs sticking out of the
coupling makes it easy to turn the transmission and check the
alignment. After the engine is properly aligned, you can slide the
shaft aft and insert the studs. Although the coupling is, in effect,
a type of "universal joint" and is designed to work with
up to 3 degrees of misalignment, you should still strive to align
the engine to within the standard .003 limits, since the coupling is
only designed to turn at maximum RPM with no misalignment. The
"universal" flexibility of the coupling should be used to
damp vibration, not compensate for poor alignment.
When inserting the studs, you should use red Locktite in
accordance with the installation instructions for the Bullflex
coupling. To secure the studs, you can thread two metric nuts
against each other to make a "bolt head" that can be
tightened with a box end wrench. Another option is to use a metric
cap nut. After the studs are installed, you can align them with the
holes in the transmission coupling and slide the shaft into place
and torque the lock nuts. Figure 4 shows the final installation. The
tech support folks at Vetus recommend an occasional application of
Boeing T-9 to prevent corrosion. They indicated that it wonít
affect the rubber in the coupling.
Vetus maintains a website with extensive information, including
installation drawings. When your order the coupling, be sure to
order the hardware kit for installation, it includes all the studs
and nuts required for mounting. The coupling is maintenance free
once installed, save for the occasional spray of Boeing T-9 to
minimize corrosion effects (the coupling is anodized, but the
hardware is not). The Vetus tech rep I spoke with recommended this
and stated that T-9 was safe for use on the rubber components of the
coupling. As with any rubber in the engine compartment, use caution
not to allow petroleum products to come in contact with it.
Spirit '88 Hull 563