Stern gland and shaft seal: a drip is normal — until it isn't

The stern gland is dripping. One person says that's how it should be, another says the boat is sinking. Both are right, and the difference between their truths comes down to which type of seal you have and exactly how many drops.

Let's sort out how to tell normal from the start of trouble.

What's actually fitted

The propeller shaft passes out of the hull, and the point where it exits needs to be sealed. There are two methods, and they behave differently.

Stuffing box (packing gland) — the classic. The hull fitting is packed with greased square-section cord, compressed by a nut. It should weep: the drops carry away frictional heat and lubricate the shaft. A dry packing overheats and eats into the shaft.

Dripless seal (shaft seal) — the modern variant: a graphite-and-stainless friction pair, held together by a bellows. It should not leak at all. A single drop here is already a signal.

Before doing anything, look and see which one you have. Packing is easy to recognize: two nuts on a thread and a stuffing box housing. A dripless seal is smooth corrugated rubber with a collar.

How many drops is normal

For a stuffing box, the reference points are:

• Underway — one to three drops per minute. That's how it's always been done and what most manuals say.
• At the dock — it shouldn't drip at all, or only very rarely.
• A steady stream, or more than ten drops per minute — time to tighten.
• Dry while underway and the housing is hot — overtightened, loosen it immediately.

Check by hand: after an hour underway, the stuffing box housing should be warm but tolerable to the palm. If it burns, it's overtightened, and the shaft is being scored right now.

For a dripless seal, the norm is simpler: dry. If it drips, either the friction pair is worn out, there's air in the system, or the water feed hose is clogged.

How to tighten a stuffing box

A fifteen-minute job, but with one condition: best done underway, engine running, so you can check the result right away.

1. Loosen the locknut.
2. Tighten the compression nut by one-sixth of a turn, no more. That's less than it sounds.
3. Tighten the locknut.
4. Start the engine, let it run at low RPM for five minutes.
5. Count the drops and feel the housing.
6. Still too much — repeat by another one-sixth turn. Never tighten it "all the way" in one go.

The rule is simple: better to undertighten and adjust again than to overtighten once.

If the nut has bottomed out and the leak isn't decreasing — the packing is spent. There's nothing left to tighten; it needs replacing.

When to replace the packing

• The nut is run all the way in.
• The leak doesn't decrease when tightened.
• Crumbs come out from under the nut instead of cord.
• The shaft is worn where the packing rides — a noticeable groove under the finger.

The last point matters more than the rest. New packing won't save a worn shaft: it will settle into the old groove and leak again. That's a haul-out job, with the shaft inspected.

Packing is usually replaced ashore. Doing it afloat is only for the experienced, and only with the seacock closed and the bilge pump ready — because for a few minutes you're opening a hole below the waterline.

What to check while you're in there

Since you're already in that compartment:

• The stern tube hose and its clamps. There should be two clamps, both stainless. If the hose has gone stiff and cracked — same logic as with through-hull fittings.
• The water feed to a dripless seal, if fitted: a thin hose from the cooling circuit. If it's clogged, the seal won't survive running dry for long.
• Shaft play. Take the shaft in hand and rock it: noticeable play points to the stern bearing, not the seal.
• Alignment. If the leak appeared after the boat was hauled or the engine mounts were changed, misalignment could be the cause.

What to keep on board

The stern gland isn't the place to improvise at sea.

• Packing cord of the right cross-section, a metre of it. It's inexpensive and doesn't go bad.
• Spare clamps.
• Wrenches to fit the packing nuts — often large ones are needed, and a standard set doesn't cover it.
• A wooden bung of the right diameter, as with all through-hull fittings.

What else is worth carrying — in the article on spares to keep aboard.

If it's leaking badly right now

1. Assess the flow. If the pump is keeping up, there's time.
2. Tighten the nut by half a turn, if it's a stuffing box. That will help temporarily.
3. Reduce RPM. The leak through the stern gland increases with shaft speed.
4. Head for shore and keep an eye on the bilge.

Don't shut down the engine without need: on many boats the leak is smaller with the engine running than with the shaft stopped while under sail.

What to remove if you need a part

Stern gland seals have strict dimensions, and "close enough" doesn't fit.

• Shaft diameter, measured with calipers.
• The old packing's cross-section — it's square, so the side dimension is needed.
• The whole assembly, so the type and layout can be seen.
• The marking on the dripless seal, if there is one.
• The engine nameplate — how to read it, which tells you the shaft line.

With this set of measurements, both packing and a complete seal assembly can be sourced, even when the boat is thirty years old and the yard no longer exists.

In short

Packing should drip: one to three drops per minute underway. A dripless seal should not drip at all. Tighten in one-sixth turns and feel the housing by hand — hot means overtightened. And if the nut has run out and the leak remains, there's nothing left to tighten: it's time to replace it.