[Public-List] stern tube question

George Dinwiddie gdinwiddie at alberg30.org
Mon Feb 4 12:34:49 PST 2019


The stern tube does not keep the water out. It allows the stuffing box 
to keep the water out at a more convenient location than glassed into 
the aft end of the bilge.

While the stern tube is sealed by the stuffing box one one end, it's 
threaded into the cutless bearing housing on the other. The cutless 
bearing housing should be caulked to not allow water to reach the 
outside of the stern tube. The inside is wet with water that comes up 
the grooves of the cutless bearing, lubricating it and keeping it cool 
while the shaft is rotating.

The engine and cutless bearing need to be aligned pretty closely to 
reduce friction, which both wears the cutless bearing and torques the 
shaft. If there's enough friction, the shaft becomes a pretzel.

The stern tube needs to be aligned well enough to clear the shaft, but 
also let the stuffing box ride on the shaft without wiggling the tube 
around. Fortunately both the original dolphinite and the 5200 that's 
often used in rework are flexible enough to accommodate any misalignment 
as you're installing things. Then the cutless bearing housing holds the 
stern tube in that alignment.

  - George [who's more familiar with this construction than I'd like]

On 2/4/19 10:38 AM, Michael Connolly via Public-List wrote:
> Hello all,
> It is my understanding that the stern tube was secured into the boat to provide a water tight passage for the prop shaft. There are no bearings in the stern tube on our boats. There needs to be enough clearance so the shaft does not rub against the tube, wearing a hole in it to cause a leak for sea water into the boat. No need for the tube to be absolutely concentric with the shaft as long as there is clearance for the length of the shaft inside the tube.
> Michael

   When I remember bygone days                         George Dinwiddie
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