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My previous boat
had a traditional stuffing box, and the pre-owned boat I recently bought
has a PSS dripless mechanical shaft seal system. How does this work and
what kind of maintenance is required? J.N., via e-mail
The
mechanical shaft seal, also known as the dripless shaft seal, has more
or less replaced the old large nut—a.k.a. stuffing box—on your
propeller shaft between the transmission and your engine.
As you are aware, the
stuffing box is an important piece of equipment due to its two major functions:
First, it keeps the water out of the boat, and second, it lubricates the
shaft as it spins inside the stuffing box. With the old-style gear, the
box would “sweat,” or leak water, to make this happen whether
the shaft was spinning or not. During this process water naturally accumulates
in the bilge where it mixes with oil and any other nasty stuff that may
be sloshing around down there. As the liquid level reaches and floats
the bilge pump’s switch, all that swill goes overboard. Moreover,
when the stuffing box leaks more than usual or becomes hot to the touch,
it’s time to replace the packing. Repacking the stuffing box is
not hard to do but it must be done correctly or the box will drip constantly
and, in the worst case, the shaft will be scored.
Shaft seals, like PYI’s
Packless Shaft Seal (PSS), work by forming a mechanical seal between two
surfaces. As one turns, the other stays stationary. The static surface
is a high-density, carbon-graphite flange held in contact against a stainless
steel rotor that turns with the shaft. The flange in turn is attached
to the boat by a reinforced bellows made of nitrile (a strong, flexible
synthetic rubber that’s impervious to petroleum-based products).
As shown in the illustration,
the bellows (D) is below the waterline and therefore under the influence
of water pressure. This causes it to be pushed outwardly, keeping the
flange (A) pressed against the rotor (C). The resulting seal is unaffected
by engine use or vibration. Nitrile double O-rings (B) inside the rotor
ensure alignment and seal the rotor to the prop shaft. Double hose clamps
(E) secure the bellow at both the stern and flange ends, and a pair of
Allen screws (F) secure the rotor to the prop shaft. A second set screw
prevents the Allen screw from backing out.
The company reports
that commercial vessels do not have to replace either the flange or rotor
until about 20,000 hours of use. Therefore, it is quite possible that
recreational boaters like yourself can expect many years of use.
Maintenance includes
checking the hose clamps and set screws for tightness. For more information,
you can visit www.pyiinc.com or call
the company at Phone: (800) 523-7558.
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