Skip to main content


Leaking shaft seal

Q: Why does my shaft seal keep leaking?

A: Some shaft seals are supposed to leak a little while you’re under way. Depending upon the type of shaft seal you have, a little dripping not only might be OK, but it might be good.

If you have the old-fashioned kind with wax-impregnated, braided, squared rope — commonly called a stuffing box — a tiny amount of dripping when you’re running will help lubricate it and keep the stuffing from overheating. If you have this type and it’s leaking so much that your bilge pump frequently comes on, there may be one or more problems.

It’s possible all that is needed is to tighten the seal. These are made so that you can back off a locking nut and tighten down the stuffing box that holds the waxed rope to further compress it around the shaft. As the shaft turns it wears down the stuffing, so periodic tightening is needed. The stuffing eventually will be compressed to the extent that you must replace it. When the stuffing is new you may have to tighten the gland several times at first as it settles in. Take care not to overtighten because this could cause the stuffing to wear excessively and/or overheat.

Other causes of excessive leaking could be poor engine and/or shaft alignment, which can usually be corrected by a qualified professional. Proper alignment is important for many reasons. Another cause could be that the stuffing box has become impaired through age or electrolysis. Visual inspection may reveal this, but not always. Periodically — and whenever you suspect a problem — have the boat hauled and the shaft seal disassembled for thorough inspection.

Yet another reason the seal might be leaking is that the shaft, where it passes through the unit, has suffered crevice corrosion and has an uneven or rough surface that quickly eats away at the stuffing. Check for this by backing the stuffing box all the way off when the boat is hauled. If your shaft appears marred or rough at this point, it may be only a benign buildup of scaly material, though it could be much more serious. If it’s a scaly buildup, usually you can scrape it off with a fingernail or use Emory cloth to clean it up. If there’s any question, have it evaluated by a qualified professional.

Many new boats today are supplied with so-called “dripless” shaft glands. These are great as long as they work, but some things can cause sudden failure and flooding. If you have one of these, get to know it well and familiarize yourself with the instructions and warnings from the manufacturer.

A characteristic of some of these units is that they must be lubricated by the injection of water. If for some reason the water injection fails, they can overheat and self-destruct quickly, resulting in a large intake of water. Various things can cause failure of the water injection — for example, stoppage at the intake or failure of a supply pump (often it’s the engine’s raw water pump). If you’re being towed and your wheel is spinning freely while your engine isn’t running and pumping water into the seal, a dripless gland could overheat. Other glands are lubricated in different ways, such as by the injection of grease under pressure. Failure to service these, as with any other part of the boat, can cause big problems.

There may be another gremlin involved. A shaft seal often is connected to the shaft log by a thick, rigid hose. (The shaft log is the metal or fiberglass tube through which the shaft passes). The hose typically is installed by double hose-clamping it at its aft end to the shaft log and at its forward end to the shaft seal. The hose must be rigid enough so that it holds the seal stationary as the shaft turns within the seal. As years go by this hose may become weakened and compromised. There is a tendency for the shaft to twist the hose as the shaft turns within its seal. If the hose eventually breaks down it could cause a serious influx of water, though there usually are signs of this before it happens, such as old age, the hose becoming soft to the touch in some areas, or it simply begins to look deteriorated. Have it replaced by a qualified professional if there’s any question.

Also, the hose clamps often will rust and disintegrate, even though they’re supposed to be “stainless steel.” Being occasionally sprayed with salt water can exacerbate this. Inspect the hose clamps regularly, and be sure that they’re good, high-quality, heavy-duty all stainless clamps. This is not a place to save money.

Inherent in all of this is that much of what I’m talking about is hard to see and/or reach. Inspect these components frequently, even if it’s hard to do. I’d never have another boat with a shaft seal that I couldn’t access and inspect easily.

Have a question? E-mail it to or send it to SoundingsEditorial, 10 Bokum Road, Essex, CT 06426