It's a basic problem, but it's not easy to solve: How can you let a 2-inch-diameter propeller shaft seal to pass through the bottom of your hull while allowing it to spin over 1,000 times per minute while keeping water out?
The three fundamental methods for preventing water from entering the bilge via the stern tube are stuffing boxes, face seals, and lip seals. All of these are made up of two basic parts. The first is a small piece of specialised hose that goes over the stern tube where it enters the bilge, and the second is a seal that fits over the shaft and connects to the hose.
Stuffing Boxes
The traditional marine stuffing box has been around for well over a half-century. Buck Algonquin, situated in Stevensville, Maryland, began selling stuffing boxes in the 1950s, and the product has changed very little since then, with the exception of some minor changes to the bronze alloy. The lack of modification indicates the design's simplicity and endurance rather than its obsolescence.
The compression of a packing material around the shaft is used in this form of shaft seal. This product is made of braided twine that has been impregnated with wax and tallow. The wax and tallow lubricate the twine, allowing it to be pushed against the shaft without abrading the metal.The assembly is connected to the stern tube via a short piece of heavy-duty hose. Buck Algonquin makes its own hose, which is a unique four-ply, heavy-duty variety designed to endure any torque and resist damage. Remember that no matter what sort of shaft seal you pick, it will be put below the waterline with no seacock to guard against flooding. Make sure you use the right type of hose when replacing it.
The packing material must be the proper size for the stuffing box in use, as determined by the manufacturer. Old packing must be removed with a corkscrew-like tool, and new packing installed by cutting rings to the required length and rotating the 45-degree cuts so that they do not line up. The majority of boxes have three rings. The packaging material might be classic wax and tallow twine or more sophisticated ones including Teflon and/or graphite. These choices lead us into some muddy territory, which we shall return to in a future piece. We'll stick with the tried-and-true wax-and-tallow packaging for the sake of this post.
A traditional stuffing box must drip to avoid overheating, as wax and tallow melt at around 150°. A properly installed drip irrigation system establishes a balance between temperature and drip rate. When the wax melts, it drains from the thread, beginning a vicious cycle. The box begins to drip more as it melts, indicating a shortage of packing material. Many boat owners react by over-tightening the box, causing the friction from the braid to wear away the shaft. A $3,000 shaft can be ruined if only $20 of packaging is missing.
The bronze housing should not exceed 140° at cruising speed. An infrared pyrometer measures temperature in a safe and precise manner. If it is operating at 140° or above, it is either excessively tight or does not have an appropriate flow of water. If the drip rate is high, tighten it up and re-check. If you have just repacked the box, you will need to keep a closer eye on it for the first 20 hours or so of operation.
What is a reasonable drip rate? At cruising speed, you'll most likely have to put up with four to six drips every minute. Keep in mind the various boat configurations: stern tube length, depth below the waterline, boat speed, and water flow beyond the end of the tube. The amount of water that reaches the packing gland varies greatly, and in rare situations, the conditions may result in a vacuum. When the flow of water is insufficient, water must be injected. This problem affects all of the shaft seals and will be addressed again in the end.
Maintaining stuffing boxes is simple: check the temperature and drop rate, inspect the hose for cracks or dryness, and check the tension and quality of the hose clamps. The moist and salty climate can also cause crusty build-up on the adjustable components, so maintain them clean and greased.
Traditional stuffing boxes have various advantages: They are long-lasting, dependable, field-serviceable, and easy. If you have a modest amount of replacement packaging on hand, you may repack the boat without carrying it or changing any components.
Face Seal
Face seals, also known as dripless shaft seals, compress a fixed bearing against a collar connected to the shaft. Water generates a thin coating on the contact between the stainless and the carbon, lubricating the seal. The fixed bearing is connected to the stern tube by a hose. Duramax, Nautical Specialties, and PYI are three firms that make this sort of face seal. Duramax incorporates an oil-impregnated nylon bearing, whilst Nautical Specialties and PYI employ a carbon-graphite bearing.
PYI manufactures the PSS Shaft Seal, while Nautical Specialties sells a variety of LasDrop seals, including the two face seals, Gen II and the Bellows Seal. The PSS and Bellows Seal both rely on a bellows-style hose to deliver the compression required to form a face seal. The hose's flexibility, paired with its compression, compensates for shaft movement as the propeller applies thrust. All of these seals are intended to handle small misalignment or vibration, but beyond certain limitations, the seal's functionality is jeopardised. Face seals require a water supply for lubrication and cooling.
The hose that connects the seal to the stern tube plays an important function since it is below the waterline and failure might result in sinking. The hose on most face seals provides the compression required to keep them against each other, which is why the bellows is used. The PSS hose should be changed every six years, but the LasDrop hose, consisting of military-grade vulcanised silicon, should be replaced every ten to twelve years. The LasDrop Gen II seal offers an unconventional approach. The compression is provided by a spring situated in the face seal, avoiding the need for a bellows hose. This lowers the cost of a new hose while also making service and installation easier.
The hose that connects the seal to the stern tube plays an important function since it is below the waterline and failure might result in sinking. The hose on most face seals provides the compression required to keep them against each other, which is why the bellows is used. The PSS hose should be changed every six years, but the LasDrop hose, consisting of military-grade vulcanised silicon, should be replaced every ten to twelve years. The LasDrop Gen II seal offers an unconventional approach. The compression is provided by a spring situated in the face seal, avoiding the need for a bellows hose. This lowers the cost of a new hose while also making service and installation easier.Duramax also employs a stainless spring to generate compression and, by supplying an adapter ring, avoids the usage of a hose on the stern tube. The yellow bellows on the Duramax seal are not designed to contain water, but rather to protect the spring and offer backup in the case that water gets past the adaptor. If the bellows are retaining water, there is an issue that has to be addressed.
Set screws keep the stainless collar in place on the PSS, and failures occur when the manufacturer's instructions are not followed. The collar features two threaded holes and four set screws. Those aren't extras; the instructions indicate that two set screws must be threaded into each hole. The first compresses against the shaft to keep the collar from sliding, while the second keeps the first screw from backing out.
The instructions also warn against reusing the set screws since the cupped tips flatten and lose their efficacy. These recommendations are vital. If the stainless collar on the shaft gets loose, it might slip away from the carbon face, enabling water to enter the bilges. When the faces are kept clean, face seals are dependable and simple to maintain. Particulates on the face of the seal can be damaged if the boat rests for an extended amount of time or if you run through sandy shallow water. As needed, wipe off the surface using a rag.If the carbon face seal has to be replaced (some may be refaced), you must tow the boat and remove the shaft seal from the coupling so that the replacement can slip onto the end of the shaft. The most common cause of failure is a loss of water flow to the seal, followed by particles becoming trapped between the seal faces. Replacements are rare when properly maintained, and manufacturers claim more than 20,000 to 30,000 hours of service on a single carbon seal in commercial applications.
Lip Seal
Another type of dripless gland uses a lip seal that wraps around the shaft. Lip seals are used in a variety of industrial applications, including hydraulic pumps and axle seals. A spring compresses a rubber ring against the shaft rather than two faces against each other.
Tides Marine manufactures the SureSeal (formerly known as the StrongSeal), while Nautical Specialties sells the LasDrop DrySeal. Lip seals, unlike the PSS seal and the LasDrop Bellows Seal, do not need compression. Tides Marine use an extremely tough silicon-Mylar hose with additional plies. There is no replacement intervals mentioned, however every 10 years is a decent rule of thumb.
These lip seals are designed to be drip-free, and if a drop occurs, the seal will need to be changed soon. To avoid hauling the boat and pulling the shaft, both firms provide a carrier seal, which sits on the shaft between the seal and the connection. When it's time to replace the lip seal, you may draw it out, clip it off the shaft, and slip the new one into place while the boat is still in the water. Those who remember the earlier StrongSeal will enjoy the advancements in the SureSeal design that make replacement easier and lengthen the seal's life.
For all setups, both seals require water injection. The lip seal must not dry out or it will be ruined. If you have this seal and need to be towed, you must keep the shaft from freewheeling while the engine is turned off.
Tides Marine guarantees its seals for two years or 2,500 engine hours, whereas Nautical Specialties guarantees them for three. Both businesses describe seeing commercial vessels with 6,000 to 8,000 hours on a seal on a regular basis. Because the shaft seal diameter has been lowered significantly, the lip seal can generate a wear groove on the shaft in some situations, especially with shaft seal made of lower alloys of stainless or extremely vibrating engines. To avoid or remedy this condition, consider adjusting the hose a quarter-inch forward or aft on the stern tube.
Flow of Water to the Seal
Water is required for lubrication and cooling in all three forms, dripless or not. Water inside the stern tube may suffice in some instances, but a pressurised supply of water is usually necessary. The following adjustment is not required for most traditional stuffing boxes, but if you can't keep your box cold enough, you probably need it.
Most boats get their water via the engine's raw-water loop. A tiny port is added to the loop and routed to the packing gland's hose fitting. Depending on where your engine is in relation to the waterline, the injection line may require a vented loop to prevent stern tube water from back-feeding into the engine when it is at rest.
Failure to deliver an adequate flow of water is the leading cause of face seal and lip seal failure. PYI and LasDrop propose either a vent line linked to a nipple on the seal housing or water injection for boats that do not exceed 10 to 12 knots and do not have bearings inside the shaft tube. A vented loop may also be necessary to avoid air trapping at the seal. Water from the engine's raw-water cooling system must be fed into boats that exceed 10 to 12 knots or have a bearing within the stern tube. Water injection is required for all Duramax systems.
Twin-engine boats complicate matters. If each engine directs water flow just to its own shaft seal and you only have one engine operating, the shaft on the non-running engine will freewheel, scorching the seal and possibly destroying it. One option calls for a crossover line to connect one seal to the other, allowing water to flow to both seals from either engine. You may also connect the raw-water lines from both engines to both seals. This approach adds an extra layer of safety since a cessation of flow from one engine does not result in an overheated seal. Refer to the manufacturer's instructions for all of the details provided above.
Given the importance of injected water to these seals, you should examine this component on a yearly basis. Check the hose and nipple for wear and tear, and then remove the hose from the nipple while the engine is running to ensure proper flow. Because the connections and hose are so tiny, debris can readily hinder the flow.
Seal the Deal
Boat owners have three options. A typical stuffing box is unrivalled in terms of simplicity, field service, and robustness. However, you must allow water in the bilge and monitor temperature and leak rate. Face seals can give hundreds of hours of operation if you require a dripless seal. To replace either of the primary components, a haul-out and removal of the shaft from the connection are required.
Lip seals also work well, and by placing a spare seal on the shaft, you may change a lip seal without having to drag it or deal with the shaft connection. Lip seals, on the other hand, do not equal face seals in terms of service hours. Stainless shafting has a high tensile strength but low abrasion resistance. Metal may be worn away by stuffing boxes and using lip seals. Significant shaft weakening is uncommon, but once the diameter is lowered, the shaft seal no longer against the packing or the lip.
Whatever kind you pick, keep an eye on your shaft seal as part of your engine room inspection and maintenance regimen.
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