Copper plating a propeller

I own Cecilia, a small motorsailer that I keep in North Wales, used primarily in winter when we are not sailing in Greece. Soon after buying the boat we hauled her out for various work and found that the existing propeller was cracked, possibly by being forced onto a shaft with an unmatched taper.

I was in the situation that I needed to buy a replacement propeller. I contacted propeller specialists Castle Marine at Caernarfon who, rather surprisingly, advised that my cheapest option was to buy a replacement from USA, made in Michigan. The prop is three-bladed with dimensions 18 x 14 inch, DAR 42% and having a brand new one in pristine condition required some research into the best antifouling I could buy. Since I use the boat as a test-bed for various projects I decided that this might be the ideal opportunity to investigate copper plating it.

Copper sheathing of ships’ hulls for antifouling was first suggested by Charles Perry in 1708, although the first experiments were not made until 1761. It was soon found that copper performed very well in preventing the growth of weed by the formation of a poisonous compound copper oxychloride that deterred the marine organisms forming the fouling. Over time the film washed away, taking away any growth that did attach itself. It seemed to me that copper plating a propeller should provide similar benefits, which led me to consider any problems or disadvantages. The only other example of a propeller plated in this way known to me is in Australia, where according to a poster on the forums it has been totally fouling free for more than a year.

The first question was whether the copper plate would be hard enough to resist the erosive nature of flowing seawater. Pure metals are generally soft by comparison with alloys made from them and erosion is perhaps the greatest concern. My research found that plating by the acid copper method would result in the hardest and thickest coating that was also capable of being polished, a further aid to anti-fouling.

The next question was whether corrosion of the propeller might be induced by the application of copper plate. Copper is more anodic than any of the common propeller materials: manganese bronze, which is a brass with small additions of other metals, tin bronzes such as LG1 that are used for more expensive propellers and the aluminium bronzes or nickel-aluminium bronzes used for high performance propellers. Fortunately the galvanic voltages between these alloys and copper is low, so the corrosion rate of the copper should be negligible. If any galvanic action does take place it will be the copper plate, rather than the propeller, that is consumed.

I contacted a number of companies via Google but the only one who showed much interest was Lathco in Sheffield, who offered to apply a 90 micron thick coating of hard copper for a total sum of £108. Sheffield is not too far away for me to deliver the prop to them and I collected it in less than a week. I was delighted with the result.

As my primary sailing location is Greece during the summer time, Cecilia is launched late in the year and used in winter: for the first season with the new propeller she lay in a drying mud berth at Port Dinorwic. We managed to use her several times before Christmas 2015, mostly under engine, but the poor weather after New Year meant she was used less than we hoped until mid-April, when we motored and sailed her. She was lifted out at Dinas Marine in May 2016 after six months afloat. I was delighted to see that the propeller was discoloured, as expected, but spotlessly clean, whereas the hull and rudder were moderately fouled. Clearly one winter in a mud berth was not much of a test but looking around the dock at boats in similar circumstances showed that most other propellers were fouled quite badly.

At this point in my life fate took a hand. I was diagnosed with an aggressive form of prostate cancer towards the end of 2016. The radiotherapy treatment that I would undergo would prevent us from sailing in Greece for the whole of the 2017 season, so we decided to base Cecilia in Pwllheli marina, where she would be convenient to use if I was not feeling entirely 100%. We relaunched in April 2017 and motored round the Lleyn peninsula to our new base at Pwllheli. At this stage my treatment had not begun, although I was undergoing various scans, tests and injections.

Unfortunately, my treatment was not as comfortable as we had hoped and other lifestyle changes intervened, with the result that Cecilia lay almost unused for the next two years, other than as a floating caravan in the marina. It was not until August 2019, two years and four months since launching, that we were able to lift out for a check on the progress of the propeller antifouling. As expected the hull was heavily fouled with black slime, shell and copious numbers of mussels, all of which was scraped off using garden hoes by the yard gang prior to pressure washing. Most pleasing though was the condition of the propeller, with only relatively few barnacles to show for the long exposure to the marine environment. Photo shows the prop before pressure washing.

My only action for the propeller was to scrape off the barnacles, easily done, and lightly abrade with 240 grit paper. The plating appears to be in good condition, no pitting or any other form of corrosion. One blade has a slightly rough edge, which may be mild plating loss or corrosion, not enough to worry about. Cecilia goes back in the water early in September 2019, hopefully with many years of fouling-free service ahead.