Better Prop Shaft Seals

Make Maintenance Easier and Safer

Early in my maritime career an old tugboat captain was walking along the face wall and saw my feet sticking up from a small access hatch. He bellowed “STUFFING BOX?” and I yelled back ‘yes’. An hour later he came back as I was wiping the black graphite grease off my hands and arms. “Damn stuffing box, what damn fool makes a watertight hull then depends on a piece of junk like that?” he said. That statement wouldn’t have made any sense to me the day before but it sure did then.

This was the traditional fiber and grease seal compressed with a large nut. The same type of prop shaft seal was used by the first engineers that cut holes in the bottoms of wooden sailing vessels to fit combustion engines and drives.

Modern vessels use a combination of technologies to achieve higher performance with less maintenance.

Fiber and Lubricant – Just like the old days but with better products. Plant fibers are replaced with synthetics in these systems and lubricants are designed to remain in a solid state rather than liquefy when heated by friction. This type of product can be used in any regular stuffing box. In the past few years real hemp has become available again so I would say next time you clean out that stuffing box get some hemp fiber and mix up some bees wax and linseed oil and you will have better performance than the modern stuffing box repacking kits will give you.

Synthetic Putty – This is a clay type of product which resembles modeling clay.

It acts as an extra layer of defense in a regular stuffing box. Clay seals still require a proper lubricant on the shaft, and reinforcement with square braid fiber. This is a lower maintenance solution but still requires regular service.

The return of real hemp to the United States market means that stuffing box materials won't break down or melt which is what clay and putty protects against.

Loose hemp fiber is soaked with warm bees wax and linseed oil and that is held in place with a woven hemp grommet like those shown in Ashley's Book of Knots.

Mechanical Pack-less Seals – This is a product used on many vessels in a variety of conditions. The unit consists of a high-tolerance, low friction bushing in a stainless steel retainer. PYI is the most well known manufacturer for retrofit and new build applications. It requires almost no maintenance and is watertight. If you consider vessel downtime and the labor of a traditional stuffing box, the extra cost is worthwhile.

The pack-less seals also have the advantage of retaining the prop shaft itself in case it separates from the drive unit. Losing a shaft will result in a vigorous jet of water entering your bilge compartment.

One minor issue is the need for a positive pressure line to feed the surface of the graphite bushing and prevent wear. Some manufacturers do this more elegantly than others and include the positive pressure source in the shaft log housing. Some still have a need for a separate thru-hull fitting.which just adds complexity.

Including a pre-tapped port for a pressure sensor would be nice so that pressure around the bushing could be monitored automatically although this can easily be fitted to the positive pressure line if one exists on your model.

Now that I have encountered a few of these packless units when they were run dry it shows just how fast they can wear the replaceable components. The cost is far less than it would be for a new complete unit but it still requires a shaft-out repair which means alignment and tuning for each individual component from the engine to the prop.

Dry runs on old style packing is fixed with a partial turn of a man-sized wrench and cost little more than a pair of disposable gloves and some hand cleaner. With practice the synthetic putty can give you a bilge that is nearly as dry as one fitted with packless logs.

Check with your insurance provider, you may receive a discount by changing to a pack-less seal. You will certainly make the crew member who acts as your Oilier happy.

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Your Citation
Bruno, Paul. "Better Prop Shaft Seals." ThoughtCo, Nov. 1, 2015, Bruno, Paul. (2015, November 1). Better Prop Shaft Seals. Retrieved from Bruno, Paul. "Better Prop Shaft Seals." ThoughtCo. (accessed May 27, 2018).