Prop-er job
QIf I coat a propeller with an epoxy bitumen or tar coating will it reduce or eliminate electrolysis and normal underwater corrosion?
I’ve been reading PBO since 1998 and I find the content most informative – a huge fount of knowledge in all disciplines of yachting. Gerhard Alberts South Africa COLIN BROWN REPLIES:
In theory coating a propeller will protect it from corrosion in sea water. The reality of course is not so simple.
The coating you apply must be complete to offer all-over protection. It may take two or more coats to make sure there are no pores in the coating which will otherwise allow normal sea water corrosion to carry on as normal.
In the case of galvanic or electrolytic corrosion the action could be concentrated in the exposed areas rather than being spread over the whole surface area of the propeller, leading to accelerated local corrosion effects.
Another problem is getting any coating to stick reliably to a propeller. A smooth surface, high speed through the water and sometimes cavitation all tend to make it difficult to keep a coating on a propeller.
There are some antifouling products that will stick, but the application conditions and surface preparation have to be absolutely perfect.
Adding coatings to it will also make your propeller less smooth and therefore less efficient unless you’re very careful in the coating application and finishing.
Any rough coating surface may also make biofouling more likely as the bacterial slime that starts it all can get a grip in the tiny irregularities in the coating.
A correctly fitted anode would be a far more reliable way of protecting your propeller from corrosion.
The anode size and material should be chosen to match the size of your propeller and the type of water your boat sits in.
A good polish each year will keep the surfaces smooth, making your propeller more efficient and a less secure home for biofouling organisms.
MG DUFF are experts in marine corrosion protection – you’ll find lots of useful information on their website mgduff.co.uk, as well as an online tool for ensuring you buy the right anode for the job.