Q & A — February 2003
By Capt. Ken Kreisler
| Maintaining a dripless shaft seal, flushing an outboard, and how water gets into diesel.
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
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.
This article originally appeared in the January 2003 issue of Power & Motoryacht magazine.