ELECTROLYSIS on prop but not Bonded Skin Fittings

[Cheoy Lee 41]

I have a CHEOY LEE 41 which after 35+ years has suddenly developed electrolysis on the prop blades and tips, I have owned her for the past 17 years. A new Beta engine was fitted 3 years ago. The boat has never had an anode since being built. All the skin fittings have a copper wire bonded/attached but due to the build it has not been possible to trace where the wires go to !! The engine is an insulated earth return model. The skin fittings are bronze and show no signs of corrosion inside or out.

I would rather not go fit an anode, as she has up to now, spent 2 - 3 years in between haul outs [got a very good anti foul] if I go down the anode route this will obviously have to change. The summer is spent on an island pontoon mooring without power, some of the winter is spent in a marina with shore power, which has not caused issues in the past.

Any suggestions ideas or facts would be very welcome.

Many thanks for any advice

 

[Tranona]

Presumably a new prop was fitted with the new engine. If that is the case, then if it is a fixed blade, it is almost certainly lower quality "bronze" than the original - most modern props are actually a brass, even though they are called bronze. If that is the case you will need an anode, and the usual way to do it is to use a large pear type, bolted through the hull and bonded to the shaft, either by attaching to the gearbox housing (and if necessary with a bridge wire over the coupling if it is the flexible type), or using a device with brushes in contact with the rotating shaft.

There should be no need for anodes for the skin fittings if they are bronze, although in the past they were often bonded, unnecessarily.

If you are spending time on shorepower you should ensure you have a galvanic isolator in the shorepower circuit.

You may well find the anode lasts more than a year, but you won't know until you try. You can always supplement it with a hanging anode when the boat is moored.

 

[Topcat47]

If you have a flexible coupling, you need to bond across it. I use galvanised wire mesh straps similar to those found on cars. Otherwise you will need a shaft anode close to the prop.

 

[vyv_cox]

If, as Tranona suggests, a new prop was fitted three years ago you may now have discovered the main reason why we do not recommend bonding. It sounds as if the metallurgy of the new prop is inferior to that of the bronze skin fittings, so the prop is now protecting them and corroding preferentially. An anode is definitely the answer as it will corrode preferentially to all of the metals it is connected to. A shaft anode is far easier if you can fit one. Mine lasts at least four years, so it may not be necessary to haul out more frequently than you do now.