Posted: December 1, 2013
Probably Not Dripless shaft seals, despite the common belief, do require some maintenance.Advocates of dripless shaft seals are quick to point out their benefits over conventional stuffing boxes — no leaks, no flax to replace, no shaft scoring — but the zinger is always the same: “They’re maintenance free!” While it’s true dripless shaft seals require less care than a traditional packing gland, nothing on a boat is truly maintenance free.
Dripless shaft seals have become widespread in the past decade and a half, but unlike traditional stuffing boxes, when a dripless shaft seal fails, the resultant flooding is alarming at the very least, and potentially catastrophic. The goal here is to review the ways dripless shafts fail and offer some general suggestions about how to prevent such problems.
There are a number of manufacturers of dripless shaft seals and, therefore, a variety of designs. Most of the units use what’s called a face-type seal, with a flexible bellows attached to the stern tube (or a stuffing-box collar) that presses a fixed carbon/graphite flange against a rotating stainless steel rotor, which spins with the prop shaft, creating a seal between the rotor and the flange. The compression created by the flexible bellows hose plays a key role in the stuffing box’s proper, leak-free operation.
Dripless seals are popular because they don’t require adjustment after installation, and they continue to keep sea water out even if the drive train is out of alignment, producing a dry bilge and creating a safer, cleaner, more corrosion-resistant and sweeter smelling vessel.
Some dripless shaft seal products use setscrews to hold the rotor in place. If the screws lose their grip, the bellows can slowly push the rotor forward on the prop shaft. Once the bellows is relaxed, the mechanical seal between the rotor and the flange is lost, and sea water can flow into the bilge. It happens more often than you might think. According to one manufacturer’s instructions, the setscrews are never to be reused, and a new set of screws should be used each time the rotor is installed.
Another issue with a spinning rotor is that the flange and the rotor are so well mated that there is a suction effect between them, and the rotor can actually stretch the bellows beyond its relaxed position before the seal is broken. So, when the bellows finally returns to its natural, relaxed position, a tiny space is left between the flange and the rotor.
The key to preventing setscrew and rotor-movement problems is to place a hose clamp on the prop shaft up against the forward face of the rotor, so that even if the rotor works loose, the hose clamp will prevent the rotor from moving away from the flange.
The bellows performs two functions. First, it creates a watertight barrier between the stern tube and the interior of the boat. Second, it acts like a spring, pushing the flange against the rotor. Problems associated with the bellows are related to fatigue, mechanical abuse or improper installation.
Bellows fatigue can occur as the bellows material deteriorates over time. Exposure to heat, vibration, salt water, and accidental spills of fuel and fluids can take a toll on the bellows. The material can become brittle with age, and a close inspection might reveal cracks, splits or tears — problems that may not be readily apparent on the surface of the bellows. For a proper inspection, at the very least, the rotor should be moved a few inches out of the way, so it can relax and so that stretching and twisting can reveal any cracks or splitting of the material. As a general rule, if the shaft is removed, a new bellows should be installed.
The second form of fatigue consists of what is known as “compression fatigue,” where the bellows loses its resistance to compression after years of being in a compressed state. As a result of compression fatigue, the pressure between the rotor and the flange will be reduced, which diminishes the mechanical seal. Eventually, it will begin to leak.
Too many boaters have a “set it and forget it” attitude about their shaft seals, and they don’t realize the consequences of ignoring an aging or damaged unit on their vessel. While a dripless shaft seal may be “dripless,” it is definitely not maintenance free.
As with any rubber hose below the waterline, it must be inspected regularly for signs of cracks, wear, aging or chemical deterioration. PYI recommends inspecting the bellows for cracks, splits, tears and brittleness twice annually and replacing it every six years regardless of condition, at which time it also recommends the O-rings and setscrews in the stainless steel rotor be replaced. This is advice every boat owner with dripless shaft seals should heed.
Water Injection Hose
Most dripless shaft seal units have a small barb fitting on the graphite flange where a hose can be connected to ensure that water is always present right up to the rotor. From the fitting, the hose runs either to a place high above the waterline or, in the case of higher-speed vessels, into the engine’s raw-water cooling system.
The cooling water intake hose should be pulled and checked semiannually to ensure sea water is flowing unimpeded. With the engine running and in neutral, remove the water feed line, cap the seal fitting and capture the flow in a bucket. Flow should be about 1 gallon per minute at engine idle. Increase your throttle to ensure flow at all engine speeds. Dress the hose and secure it.
A new or well-maintained dripless shaft seal will do exactly what the manufacturer claims it will do: provide a vibration-tolerant, drip-free stuffing box. Be sure to follow the manufacturer’s maintenance instructions and incorporate them into your routine maintenance schedule.