How they make propeller shafts on large vessels like ocean liners or aircraft carriers waterproof considering the intense water and mechanical pressures that come into play?

Question

It seems like a grommet-type seal would fail in no time. Also, looking at an image of one of the propellers for the Queen Mary, it hit me that this is decades old, and would have to go to dry dock to be serviced, and there is no way they are doing that so… How is that even possible? I’d figure dry rot would set in somewhere along the way.

In: Engineering

Answer 1

At most, a ship’s propeller shaft might be 15’ (5m) below the surface of the water. The pressure at that depth is not intense. In fact, it’s around 7-8PSI.

Years ago, shafts were sealed with a packing gland, not unlike the one that keeps water from escaping around a valve stem under your kitchen sink, which is probably experiencing pressures exceeding 50 PSI. They were tightened until the water just dropped from the packing nut.

Nowadays, mechanical shaft seals are used and can operate leak-free for years

It’s important to remember that the pressure at a shaft penetration actually decreases as forward speed increases, since ship propellers are located aft and push the boat through the water (normally). The motion of the ship creates a negative pressure at the shaft penetration.

Source: me. Boat builder for 20+ years