Passagemaker Magazine

Motor Mount Interference Steve D’Antonio

Motor mounts are the unsung heroes of the engine room. They endure terrific stress, vibration, heat, compression and tension; yet they soldier on with nary a complaint and rarely fail in a dramatic fashion.

Most mounts consist of a metallic shell that is connected to a base plate via a flexible medium, a rubber or rubber-like material. Some motor mounts are more sophisticated than others, embodying the ability to indicate the amount of load they are carrying, thereby allowing them to be balanced during installation.

The mission of most motor mount
installations is threefold. First, they must
absorb vibration and movement imparted
by the engine and running gear. Second,
they must also absorb thrust created by
the propeller. When the vessel is under
way and traveling forward, hundreds or
thousands of pounds of thrust created
by the propeller are transmitted through
the propeller shaft, into the transmission,
onto the engine block, and then into the
stringers and hull via the motor mounts.
As if that’s not enough, when the vessel
is shifted into reverse all of the loads
reverse, pulling the mounts aft rather than
pushing them forward. Third, they are
the adjustment mechanism for affecting
alignment with the propeller shaft. These
simple devices are all that connect the
propeller’s energy to the boat.

Serving these three masters— vibration mitigation, absorbing thrust and alignment—is no mean feat. At best this is a compromise for even the most sophisticated mounts.

When motor mounts do fail to
perform as expected; when they transmit
too much vibration or when they crack,
literally, under the pressure of propeller
thrust, it’s typically a result of an under-
engineered design or poor installation
practice. In the accompanying image, a
new motor mount has been installed;
however, the head of the fastener that’s
been used to secure it to the hull is too
large. When loaded, the mount shell and
the fastener make contact, bypassing the
mount’s flexible medium, which in turn
transmits vibration to the hull. Inspect
your motor mounts; look for corrosion,
loose hardware and interference of this
sort. S t e v e
D’Anton
io