Types of Prop Shaft Seals, Old and Modern

Types of Prop Shaft Seals, Old and Modern Modern vessels use a combination of technologies for sealing prop shafts to achieve higher performance with less maintenance. Fiber and Lubricant These are 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 next time you clean out that stuffing box, get some hemp fiber and mix up some beeswax 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 wont break down or melt, which is what clay and putty protect against. Loose hemp fiber is soaked with warm beeswax and linseed oil and that is held in place with a woven hemp grommet like those shown in Ashleys 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. Weve 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 are 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.