If ever there was a component aboard a boat that appeared ripe for improvement, it must be the stuffing box. In its traditional form, discussed last month, it carries out a vital role, allowing a spinning propeller shaft to pass from the inside to the outside of the hull, while keeping the water out, for the most part at least. It has been used and worked well and reliably aboard thousands of recreational, commercial and military vessels for decades. It’s simple, rugged, and, for the most part, reliable, although it requires routine maintenance.
So what’s not to like? Not much. However, its one failing annoys many users to no end, so much so that many manufacturers have attempted to improve on the design. It’s flaw, albeit necessary, and one many perceive as a fatal one, is its propensity to leak or drip. That drip, while irksome, serves a vital purpose. It ensures that the packing material within it remains wet, lubricated, and cool. Without this vital flow of cooling water, the packing will quickly overheat, which in turn leads to more profuse leakage and after some time, potential accelerated shaft wear.
A range of alternative dripless stuffing boxes are available—with a few exceptions, most fall into one of two categories, face seals or lip seals.
Face seals rely on an interface between a stationary component, most often carbon graphite, and a rotating stainless steel ring. Slight pressure is applied between the two using spring tension built into the bellows-type stuffing box hose. Carbon graphite is tough, durable, and abrasion resistant, and with just a very slight film of water between it and the stainless steel ring, the system works quite well. Ensuring that water film is always present is critically important to this seal’s reliability and longevity. Even though the film between the seal surfaces is ever so slight, water moving through the seal and bellows ensures it remains present and, in the process, absorbs and removes heat. Without it, these seals will quickly overheat and ultimately be ruined. Depending on the vessel’s speed, water injection can be obtained from a pressurized source (i.e., the engine’s raw water cooling circuit; from a forward facing ram or scoop mounted to the hull; or it can simply be a riser that allows trapped air to escape, thereby ensuring the seal remains submersed).
As an illustration of its longevity, I installed a face seal stuffing box on a yacht club launch, as part of a repower package. Years later I returned and decided to check on the engine installation as well as the seal. Remarkably, the seal had been neither serviced nor adjusted since it was first installed; the engine’s hour meter had just clicked over 3,000. Such long-lived performance is not unusual for face seals.
This sort of reliability can only be achieved, however, if the installation follows the manufacturer’s instructions without deviation. A few of the more frequently violated installation guidelines involve incorrect compression of the bellows, resulting in too much or too little tension between the seal faces, and either leakage or overheating. Failing to allow enough slack in the water injection hose to allow the seal to “float” freely (using a hose that is too stiff can have the same effect, ideally type A or B fuel hose should be used, while clear PVC hose should not be used) can also lead to chronic leakage.
Another face seal installation issue worthy of mention is the relationship between the shaft and the shaft log, the tube through which the shaft passes. If the shaft is not parallel with and centered in the log, the seal faces will have difficulty achieving a reliable watertight seal. I’ve encountered this scenario many times, and the stuffing box itself is often pointed to as the culprit, when in fact the problem lies with the original engine and shaft installation.
Yet another method of achieving a drip-free seal involves the use of a lip seal. Lip seals have been around for almost as long as internal combustion engines; they are used to seal crank and transmissions shafts, as well as in a variety of other industrial applications. Because so many are manufactured, they have been virtually perfected.
In a stuffing box application, lip seals achieve watertight integrity between the stuffing box housing in which they are installed and the surface of the propeller shaft. For this reason, in order for such a seal to work, the shaft must be clean, smooth, and free of all scratches, nicks, pitting, or corrosion, at least in the comparatively small area where the seal will make contact.
Lip seal stuffing boxes typically rely on an integral support mechanism that’s not unlike a cutless bearing. Made up of a synthetic material into which flutes or grooves have been cut to facilitate water flow, it centers and steadies the stuffing box on the shaft. As a result, this type of seal tends to be somewhat less sensitive to misalignment between the shaft and shaft log.
Like face seals, lip seal stuffing boxes are reliant on cooling water to provide lubrication and cooling. Unlike face seals, that water supply must be pressurized rather than passive. For all seals, the water injection flow rate should be verified at the time of installation and again periodically thereafter. Because of its small diameter, typically 3/8 inches, it’s not unusual for this supply to become clogged with silt or “crumbs” from decaying zinc anodes located in heat exchangers (for this reason, some prefer to tap water supplies prior to the heat exchanger). Lip seal injection hose should meet the manufacturer’s requirements. Like the face seal, lip seal stuffing boxes must be allowed to float freely, overly stiff hose or hose that leaves little or no slack will limit this necessary movement. When installed properly, lip seals can be expected to provide a thousand hours or more of leak-free service.
Regardless of which type of seal is used, temperature measurement remains the most accurate method of ensuring proper operation. Like conventional stuffing boxes, dripless seals should not operate at a temperature that is greater than 30 degrees Fahrenheit above that of the water in which the vessel is operating.
VARIATION ON A THEME, TWIN SCREW
With one caveat, most twin-screw installations, both lip and face seals should be equipped with parallel cooling water plumbing. This arrangement ensures that, when operating on one engine, windmilling shaft’s seal will remain lubricated and cool. The caveat involves the method by which water is drained from the engines’ exhaust systems. If portions of the exhaust are higher than the engine’s riser, then such a crossover method must include isolation valves to prevent catastrophic flooding of the engine’s cylinders. If your vessel is equipped with such a crossover, make sure you or a professional review the exhaust system design to determine if such flooding when operating on one engine is a possibility.
Dripless stuffing boxes have made the advent of a dusty bilge a reality. With less water in the bilge, corrosion and odors can be reduced. Because the bilge should normally be dry, any water that does show up warrants immediate investigation.