The purpose of stern tube are to support the shaft and to make water tight joint where shaft enter the hull.
Fitting:-
At the fore end of the tube there is a flange which is bolted to the aft peak bulkhead. At this after end the tube has a small flange cast on it and abaft it is threaded to take a large nut. When it is in position the flange bears against the fore side of the stern frame and the but on the after side.
Construction:-
The tube itself is usually made of steel. Inside there is again a brass bush which has a groove in it running fore and aft.
Strips of lignum vitae is fitted in these grooves to act as bearing for the shaft. This strips are kept in place my help of check ring which is bolted to the after end of the tube. Small spaces are left bw the strips to allow water to come and lubricate and cool the shaft. Fore end is provided with gland packing to avoid water entering engine room. The aftermost end of the shaft is called tail end shaft and consist of a brass liner shrunk on it.
In some ships white metal bearing replaces lignum vitae and oil is used for lubrication. And special oil seals are used for sealing.
Strengthening Around Stern Tube:-
Floors are extended above the tube. If this is impossible the deep cross tie plates flanged on their upper and lower edges must be fitted above the tube. The plating of the aft peak bulkhead must be thickened or doubled around the tube. The shell plating should also be thickened in their vicinity.