Troubleshooter: Understanding Shaft Seals

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It’s a simple problem, though not an easy one to solve: How do you allow a 2-inch-diameter propeller shaft to penetrate through the bottom of your hull, give it enough freedom to spin over 1,000 times a minute, yet not allow water to pass through the hole?

Stuffing boxes, face seals, and lip seals are the three basic ways to keep water from pouring into the bilge via the stern tube. All of these consist of two main components. First, there is a short length of specialized hose that fits over the stern tube where it enters the bilge and, second, there is a seal that fits around the shaft and attaches to the hose.

Stuffing Boxes

The conventional marine stuffing box has been around for more than half a century. Buck Algonquin, based in Stevensville, Maryland, started selling stuffing boxes in the 1950s and, other than some tweaking of the bronze alloy, the product has changed very little in all that time. The lack of change reflects the simplicity and durability of this design rather than its obsolescence.

This type of shaft seal relies upon compression of a packing material around the shaft. This material consists of braided twine impregnated with wax and tallow. The wax and tallow provide lubrication so that the twine can be compressed against the shaft without abrading the metal. A short length of heavy-duty hose connects the assembly to the stern tube. Buck Algonquin manufactures its own hose—-a special four-ply, heavy-duty type—designed to withstand any torque and protect against wear. Remember, regardless of the type of seal you choose, they are all installed below the waterline, with no seacock to protect against flooding. When replacing the hose, make sure you use the correct type.

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Face seal from PSS. The bellows style hose compresses the carbon bearing against the stainless collar.

The packing material has to be the correct size for the stuffing box in use, as identified by the manufacturer. Old packing must be pulled out with a corkscrew-like tool, and new packing fitted by creating rings cut to the proper length, with the 45-degree cuts rotated so that they don’t line up. Most boxes utilize three rings. The packing material can be the traditional twine with wax and tallow or more modern versions utilizing Teflon and/or graphite. These alternatives take us into some murky waters and we will revisit them in a future column. For the purposes of this article, we will stay with the tried and true wax-and-tallow packing.

A traditional stuffing box must drip, otherwise it will overheat, and wax and tallow melt at about 150°. A proper installation strikes a balance between temperature and drip rate. Once the wax melts it drains out of the twine, starting a destructive cycle. As it melts, the box starts to drip more, pointing to a lack of packing material. Many boat owners respond by over-tightening the box, causing the friction from the braid to begin wearing away the shaft. Missing just $20 worth of packing can ruin a $3,000 shaft.

At cruising speed, the bronze housing should not exceed 140°. An infrared pyrometer provides a safe and accurate way to monitor the temperature. If it is running at 140° or higher, then it is too tight or lacks an adequate flow of water. If you have an excessive drip rate, then tighten it some and re-check. If you have recently repacked the box, you will need to monitor it more closely for the first 20 hours or so of run time.

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LasDrop Gen II face seal. A spring compresses the bearing against the seal, eliminating the need for a bellows-style hose.

What is an acceptable drip rate? Realistically, you will most likely have to live with about four to six drips a minute at cruising speed. Keep in mind the varied configurations of boats: length of the stern tube, depth below the waterline, speed of the boat, and water flow past the end of the tube. The amount of water that reaches the packing gland varies quite a bit, and in some cases the conditions might create a vacuum. In cases where water flow is inadequate, water must be injected. This issue applies to all of the seals and will be addressed again at the conclusion.

Maintenance requirements for stuffing boxes are minimal: monitor the temperature and drip rate, inspect the hose for cracks or dryness, and the tension and condition of the hose clamps. The damp and salty environment can also lead to crusty buildup on the adjustable parts, and it is good practice to keep them clean and lubricated.

Conventional stuffing boxes offer several advantages: they are durable, reliable, field-serviceable, and simple. If you carry a small amount of replacement packing with you, it can be repacked without hauling the boat or replacing any components.

Face Seals

Often referred to as dripless, face seals compress a fixed bearing against a collar that is attached to the shaft. A thin film of water forms on the interface between the stainless and the carbon, lubricating the seal. A hose connects the fixed bearing to the stern tube. Three companies manufacture face seals of this type: Duramax, Nautical Specialties, and PYI. Duramax uses an oil-impregnated nylon bearing while Nautical Specialties and PYI incorporate a carbon-graphite bearing.

PYI produces a product marketed as the PSS Shaft Seal and Nautical Specialties offers a line of LasDrop seals, including the two face seals, Gen II and the Bellows Seal. The PSS and the Bellows Seal rely upon a bellows-style hose to provide the compression needed to create a face seal. The hose flexibility combined with its compression compensates for movement of the shaft when thrust is applied by the propeller. All of these seals have been designed to accommodate minor misalignment or vibration, but beyond these limits the effectiveness of the seal will be compromised. Face seals must have a source of water for lubrication and cooling.

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The yellow bellows on this Duramax face seal protects a compression spring. This bellows should not contain any water.

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The hose connecting the seal to the stern tube fills a critical role: It is below the waterline and failure could lead to sinking. On most face seals, the hose provides the compression needed to hold them against each other, which is the reason for the bellows. The PSS hose should be replaced every six years, while the LasDrop hose, made of military-grade vulcanized silicon, has a 10- to 12-year replacement interval. The LasDrop Gen II seal takes a different approach. A spring located in the face seal provides the compression, eliminating the need for a bellows hose. This reduces the cost of a replacement hose and simplifies service and installation. Duramax also uses a stainless spring to provide compression and avoids the use of a hose on the stern tube by providing an adapter ring. The yellow bellows on the Duramax seal is not intended to hold water, but is there to protect the spring and to provide backup in the event water does make it past the adapter. If you notice that the bellows is holding water, then there is a problem that must be addressed.

On the PSS, set screws hold the stainless collar in place, and failure to follow the manufacturer’s instructions leads to failures. The collar has two threaded holes and comes with four set screws. Those are not spares—the instructions specifically state that two set screws must thread into each hole. The first one compresses against the shaft to keep the collar from slipping, and the second one prevents the first screw from backing out.

Additionally, the instructions caution against re-using the set screws, because the cupped points become flattened and lose their effectiveness. These guidelines are critically important. If the stainless collar becomes loose on the shaft it can move away from the carbon face, allowing water into the bilges. Face seals are reliable and easily maintained when the faces are kept clean. If the boat sits for long periods or if you run through sandy shallow water, particulates on the face can damage the seal. Clean the surface with a rag as needed. If the carbon face seal needs replacing (some can be refaced), you will need to haul your boat and pull the shaft from the coupling so that the replacement can slide onto the end of the shaft. Loss of water flow to the seal is the number one cause of failures, followed by particulates getting caught between the seal faces. Properly maintained, replacements are not common, and the manufacturers report more than 20,000 to 30,000 hours of use on a single carbon seal in commercial applications.

Lip Seals

Another variety of dripless glands relies upon a lip seal that fits around the shaft. Lip seals are common in industrial applications, such as hydraulic pumps and axle seals. Instead of compressing two faces against each other, a spring compresses a rubber ring against the shaft.

Tides Marine produces the SureSeal (an earlier version was called the StrongSeal) and Nautical Specialties offers the LasDrop DrySeal. Unlike the PSS seal or the LasDrop Bellows Seal, lip seals do not require compression. Tides Marine uses an exceptionally durable silicon-mylar hose with extra plies. No replacement intervals are specified, but every 10 years would be a good rule of thumb.

These lip seals are meant to be drip-free and if a drip develops then the seal will have to be replaced in the near future. To avoid the need to haul the boat and pull the shaft, both companies offer a carrier seal—a spare that rides in place on the shaft between the seal and the coupling. When the time for replacement comes, you can pull out the old lip seal, snip it off the shaft, and slide the new one into place, even with the boat in the water. Those familiar with the older StrongSeal will appreciate improvements in the SureSeal design which make replacement easier and extend the life of the seal.

Both seals require water injection for all installations. The lip seal cannot run dry or it will be quickly damaged. If you have this seal and need to be towed, you must prevent the shaft from freewheeling if the engine will not be running.   

Tides Marine guarantees its seals for two years or 2,500 engine hours and Nautical Specialties for three years. Both companies report regularly seeing commercial vessels with 6,000 to 8,000 hours on a seal. In some cases, especially with shafts made of lower alloys of stainless or excessively vibrating engines, the lip seal can create a wear groove on the shaft, preventing the seal from seating properly, since the shaft diameter has been reduced slightly. To prevent this situation, or to correct it, try moving the hose a quarter of an inch forward or aft on the stern tube.

Water Flow To The Seal

Dripless or not, all three types need water for lubrication and cooling. In some cases, water inside the stern tube is adequate, but a pressurized source of water is generally required. Most conventional stuffing boxes do not require the following modification, but if you cannot keep your box cool enough, you probably need it.

For a source of water most boats tap into the engine’s raw-water loop. A small port is added to the loop and directed to a hose fitting on the packing gland. Depending on the location of your engine relative to the waterline, the injection line may require a vented loop to keep the stern tube water from back-feeding into the engine at rest.

Failure to provide a good flow of water is the number one cause of failure to face seals and lip seals. For boats that do not exceed 10 to 12 knots and which have no bearings inside the shaft tube, the PYI and LasDrop recommend either a vent line attached to a nipple on the seal housing, or water injection as described earlier. The vent line prevents air entrapment at the seal and a vented loop might also be required. For boats that run above 10 to 12 knots or boats that have a bearing inside the stern tube, water must be injected from the engine’s raw-water cooling system. Duramax specifies water injection for all installations.

Twin-engine boats present a complication. If each engine directs water flow only to its respective shaft seal and you have to run on only one engine, the shaft on the non-running engine will freewheel, overheating the seal, and likely ruin it. One solution specifies a crossover hose from one seal to the other, so that water from either engine will flow to both seals. You also can plumb the raw-water line from each engine to both seals. This method provides an extra measure of protection, since the loss of flow from one engine would not lead to an overheated seal. For all of the details described above, refer to the manufacturer’s instructions.

Given the importance of injected water to these seals, your annual maintenance should include inspection of this component. Make sure the hose and nipple remain in good condition and remove the hose from the nipple with the engine running to make sure there is good flow. The fittings and hose are small in diameter and debris can easily obstruct the flow.

Seal The Deal

Boat owners have three good choices. For simplicity, field service, and ruggedness, a conventional stuffing box can’t be beat. But, you’ll have to accept water in the bilge and keep an eye on temperature and drip rate. If you want a dripless seal, face seals can provide thousands of hours of operation. Replacement of either of the main components will require a haul-out and removing the shaft from the coupling.

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Lip seals also do the job effectively, and by mounting a spare seal on the shaft, you can replace a lip seal without hauling or dealing with the shaft coupling. However, lip seals do not match face seals in terms of service hours. Although stainless shafting has high tensile strength, it has low abrasion resistance. Stuffing boxes and lip seals can wear away metal. Significant weakening of the shaft is not common, but once the diameter has been reduced, the shaft will no longer seal against the packing or the lip.

No matter which type you choose, be sure to keep an eye on your shaft seal as part of your routine engine room inspection and maintenance program. n

Steve Zimmerman is the president and founder of Zimmerman Marine, which operates four boatyards in Maryland, Virginia, and North Carolina. Zimmerman has been repairing and building boats for almost four decades.

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