Re: Stuffing box
Posted by: Bil
Date: July 8, 2011 02:22AM
Category: Engine and propeller
Use of graphite packing depends a little on:
1. the nature of your prop shaft; and
2. whether the boat sits for long periods in seawater with no movement of the prop shaft.
The crux of the matter is that the graphite and passivated stainless steel 316 combination is not ideal in stagnant (ie low oxygen) seawater.
Theoretically, graphite will act as the cathode and ss as the sacrificial anode, with a potential difference of as much as 0.4 - 0.6 volts.
In practice, I've run a MV with an ss shaft and graphite packing. And did not notice any electrolysis of the ss. I've heard of other vessels with the same combination.
In my case, I did change back to traditional packing after realising the potential for electrolytic corrosion. The real situation is more complex than the theory - most seawater contains enough organic matter that, when captured within the shaft log (or anywhere else), decays to form hydrogen sulfide H2S. That changes the theory considerably.
BCC 116 Zygote,
Scarborough Marina, Moreton Bay, Queensland, Australia
Edited 1 times. Last edit at 07/08/11 10:37PM by Bil.