The relation between corrosion of propeller shaft and zinc anode for cathodic protection of propeller is investigated by mathematical calculation. A summary of results is as follows:
1) At present, most of propeller shafts on steel ships are disconnected electrically by oil film in the bearing at their running state. As electric current path will be made from zinc anode to sea water-propeller-propeller shaft-sea water-stern tube-ship hull-zinc anode in such a case, the electric current leaving shaft sleeve will cause considerable corrosion of shaft sleeve. It is recommended to keep good electrical contact between shaft and hull by brush and slip ring.
2) Potential of inner surface of stern tube has much influence on the corrosion of shaft sleeve. The more negative the potential of stern tube, the lesser the corrosion of shaft sleeve.
3) Good paint coating on the interior surface of stern tube will prevent corrosion current from shaft sleeve considerablly.
4) If propeller is cathodically protected by zinc anode attached on the propeller boss, corrosion of shaft sleeve will be much reduced.
5) On wooden vessels, zinc anode should not be attached directly on the stern surface of stern tube. Otherwise corrosion of shaft sleeve will be increased if electrical resistance between brush and slip ring became larger.

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Prevention of (erosion) corrosion on the propeller and it's shafting was examined with a stern model and discussed the results.
In the case of running ship, protecting current to propeller (white arrow in Fig. 1) is used to be checked, and stray current (black arrow) arises in stern tube at the same time.
To prevent these troubles, several methods were ascertained. The results were as follows:
1) It is favorable for both propeller and it's shafting to make sure the connection between the intermediate shaft and bearing by the ring and brush.
2) Painting on the inner side of stern tube (especially copper alloy parts in it) would decrease the stray current mentioned above.
3) Zinc anodes attached on the boss of propeller are effective, but the effect are insufficient to the vicinity of blade tip. However, this foible is diminished by the coating on the inner half of propeller.
4) In case that cathodic protection by external anode would be used for the ship, it is favorable to connect this anode to the shafting directly, and not to the ship's hull.

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