How do I replace the stern gland (shaft seal) on a Hallberg-Rassy Monsun 31?

Replace the Monsun 31's complete propeller-shaft seal with the boat ashore, after identifying the fitted gland and measuring the shaft and stern-tube connection. A traditional packed gland may need only adjustment or fresh packing; fitting a replacement rubber lip seal normally means removing the shaft coupling and sliding the seal over the shaft's forward end.

Identify the fitted gland

Find the leak first. Dry the assembly, then trace the water to the shaft interface, hose connection or stern tube. Repacking will not cure a split hose or loose connection.

Monsuns have had decades of engine and sterngear changes. The boat's name or build year alone will not identify the replacement.

• Packed gland: a metal body with a compression nut and locknut, or a follower tightened by two nuts. Some arrangements have a greaser.
• Rubber lip seal: a moulded rubber body clamped to the stern tube, commonly a Volvo Penta type or an aftermarket equivalent.
• Mechanical face seal: a bellows with a stationary sealing face and a stainless rotor secured to the shaft, such as a PSS.

Do not confuse the propeller-shaft gland with the rudder's greased packing arrangement. They seal different components and need different procedures.

Before ordering a replacement seal, measure the shaft diameter with callipers and record the stern-tube outside diameter beneath the hose, available installation length and any water-feed connection. Distinguish 25 mm from 25.4 mm: nominally similar sizes are not interchangeable.

Prepare and release the shaft

Have the boat securely supported ashore and isolate the engine starting circuit before disconnecting the shaft.

1. Photograph the installation and mark the shaft's position relative to the coupling so you can restore its original position.
2. Check clearance between propeller and rudder before moving anything. You need enough movement and working space to remove the coupling hub and gland. Do not push the propeller against the rudder.
3. Unbolt the shaft coupling from the gearbox flange. Identify how its hub grips the shaft: split clamp, taper or keyed parallel bore. Check for retaining screws or nuts.
4. Support the shaft, release its retainers and remove the hub using the specified extraction method. Undoing the flange bolts alone does not free the shaft from the hub.

A seized coupling can take most of the time. Use a suitable puller or call a marine engineer. Hammering the shaft or levering against the gearbox risks expensive damage. If clearance is insufficient, you may need to remove the propeller or dismantle further.