According to the musician Neil Young, “Rust Never Sleeps”. Young must have been a boat owner, because certainly rust, or more generically corrosion, never stops trying to eat away at any metallic object on your boat that’s in contact with salt water.

It does this relentlessly, 24 hours a day and 365 days a year. And the process is not linear – something can look good and solid for a long period of time, and then very quickly deteriorate, developing pits and holes before starting to lose strength and eventually crumbling away.

About six months ago my fibreglass-hulled launch was hauled out for an overnight job – fitting a new transducer for the fishfinder. At the same time, we gave the hull a quick clean and checked her over. The antifoul was holding up well, just over halfway through its effective life, although the props had lost some of their protective coating. Since it was only a quick haul out, we could not re-coat the props, but they still looked solid so we expected no problems.

A typical prop nut anode

Fast-forward four months, and at the end of a crayfish dive I did my usual maintenance check of the boat’s running gear before I climbed back on board. I was horrified to find two small holes had appeared in one propeller, going right through the blades on the starboard side. The leading edges of the blades were also badly pitted and thin as tinfoil in places. Something was eating the bronze! The port side propeller, on the other hand, looked fine, with no corrosion whatsoever.

Pulling an 11m launch out the water is not a trivial process, and since we had just come out of lockdown there was a long backlog for haul outs at the marina. So, first thing was to try and identify what was causing the corrosion on the bronze propellor – and more importantly, limiting any further damage until I could pull the props off for repair.

As every boat owner knows, anodes are the ‘big guns’ when it comes to protecting against corrosion. These are usually zinc, although sometimes aluminium, and they are sacrificial metal blocks that are kept in electrical contact with every metal component on the hull. Then when saltwater sets up the inevitable galvanic process, the item that starts to corrode away first is the anode. Once it wears down you replace just that one component, saving all other metal items.


I knew my boat still had good anodes, since that is one of the items I checked during my dive. I re-checked all my electrical bonding straps, which connect the engine, drives etc to the hull and anodes. All of them seemed good, with nothing obviously loose. Time for an expert opinion. I made a call to Half Moon Bay Electrical and booked an urgent galvanic protection survey.

The process is, conceptually, quite simple. An electrical meter is used, with one probe in the water next to the boat and the other used to make electrical contact with various components of the boat. The meter measures whether current is flowing between the item and the surrounding sea, how well they are bonded together and how effectively the anode is working. The test showed all was within expected parameters, with one exception – the starboard drive shaft and propeller assembly had no electrical connection to the rest of the boat.

That drive shaft is connected to the engine through a flexible coupling, essentially a big plastic polymer ring that prevents engine vibration being transmitted to the drive. This means that there is an electrically insulating rubber surface between the metal of the gearbox and the driveshaft. Similarly, the shaft runs out of the boat through a cutlass bearing, again made from a plastic polymer material that prevents the shaft from making electrical contact with the stern tube.

A typical prop shaft anode

Quite commonly in this configuration there would be a twopiece zinc anode bolted tightly around the shaft in the water end of the assembly. However, on my vessel there is less than 10mm of shaft between the tube and propellor.

Another common protection is to have an anode that attaches directly to the prop nut, but again, my launch has no provision for this. This configuration has been unchanged for the seven years I have owned her and this issue has not arisen before.

Discussing the issue with Simon Jennings from HMB Electrical, he advised that a two-pronged approach was best. First was to get some form of electrical protection onto the shaft. The second was using a prop coating like Prop Speed or a similar product. It would physically protect the surface of the bronze.

Normally my props are well coated, and as mentioned, I have not had this issue before. But on my dive, I noticed that the starboard side had lost a good part of its coating, possibly due to poor application. And now I have this big problem.

By articulating the brush, gravity can provide the necessary wipe force on the shaft


Graphite brush soldered to the copper – not neat but solid!

Removing the props, repairing the damaged area, rebalancing them and then finally re-coating both props will happen when I can get her hauled out the water.

In the meantime, the only option for electrical protection is what is called a bonding shaft brush. Because the shaft is spinning, any electrical connection needs to make contact with a rotating surface. Electric motors do this all the time, with graphite brushes that make an electrical connection to the rotor contacts. The graphite conforms to the shape of the shaft and then provides continuous connection through a copper wire. The graphite slowly wears down over time, but since they are not metallic at least brushes do not corrode.

Unfortunately, although BEP Marine makes a generic model bonding shaft brush, I was unable to find a local supplier with any in stock. That would mean an overseas purchase, shipping costs and delays. Looking at the detailed pictures of their product, I realised it would be relatively simple to make, requiring just a strip of solid copper and suitable-sized starter motor brushes.

Finished installation.

Both of which turned out to be quite easy to source. Copper strip is readily available from industrial electrical suppliers where it is used for busbars. Again, supply issues limited what I could get, but a piece of 6mm x 19mm copper would do. Ideally, a thinner strip would have been better. I also had to buy 1.5m, so I now have plenty of spare copper for future projects!


Starter motor brushes are easily obtained from automotive stores, but I bought the biggest and cheapest version I could find on TradeMe to go with the sturdy double copper wire for soldering. For $20 I got a set of four brushes, and these proved to be exactly the right size (20mm wide and 9mm thick) for my copper strip.

Making the unit was a simple job of cutting a 50cm strip of copper, notching the end for the copper braid to fit through, then soldering the wire to the strip. This was well beyond the capacity of a soldering iron, so a small blowtorch was used to heat up the copper enough to melt the solder. And finally, I neatened up the edges and drilled some bolt holes for mounting.

Access was difficult

At this point I realised a small modification was needed. The graphite part of the brush needs to stay in continuous contact with the shaft, which means it needs to be under a small amount of pressure to allow for movement. Not too much, however, or it would wear away very quickly. Either a small spring or gravity was needed to maintain that pressure. I considered whether the natural springiness of the copper would be sufficient but decided this was too unpredictable.

Instead, I cut right through the copper strip about a third of the way along and soldered a short length of copper braid across the gap. This created an articulated joint that would allow the weight of the brush end to maintain the required contact. I scrounged the braid from a spare welding earth clamp I had at home.

The orange plastic coupling is an electrical insulator.

Installing the brushes was theoretically simple since I just had to drill two holes in the engine mounting frame and bolt it in place. But, of course, securing it was a contortionist’s nightmare, taking much swearing and sweating. Eventually the job was done, as can be seen from the pictures.

Time will tell how effective this will be, and I am not sure whether I should have painted the raw copper to prevent it also corroding? When I haul the props off in a few weeks’ time I will review the installation and make any changes needed.


Total cost was around $120, but now I have enough copper bar and two spare brushes to make another complete set. Anybody else need one? BNZ

This is how a propeller should be protected – with a thorough coating of Propspeed.