Aluminum sterntube with stainless steel shaft

Hello!
I have focused for á long time about the different approches of this metal « mixture », when the shaft bearings are waterlubricated.
In order to protect shaft and props by the zinc anodes welded on the hull, the shat is generally earthed to the hull.
So, the shaft and the inside of the sterntube are in the same électrolyte, then the galvanic corrosion of the sterntube inner surface can start because the hull cathodic protection has no effect inside the sterntube.
I reviewed different solutions of protection: coating he shaft, painting the inside of the sterntubes. but perfect application of paint on the aluminum is very difficult, coating the shaft is not always possible.

could someone share his positive experience about this subject please?

 

That is no problem, the shaft is galvanically isolated from the transmission/engine by a rubber coupling or a plastic disc between coupling flanges. Alternatively, the complete propulsion unit is separated from the hull, mounted on isolating elements. The latter method is more unsafe, because you often find electrical leak paths via control wires or electrical equipment.

 

thank you for you reply.
the shaft is already and originally earthed to the hull, this is a requirement of shaft and propeller maker.

 

Might it be easier to install anodes on the shaft instead of welding them on to the hull?

What is the rationale behind the shaft and propeller maker insisting that the shaft be earthed to the hull?

 

the rationale is you connect anodes to the propeller thanks to the earthing of the shaft, for the cathodic protection.

 

built many sailing yachts, Bored 5083 and used non metallic brgs the oldest boat i built in 86, no probs, also shaft passed through P bracket no probs

 

thanks William. were the shafts connected to the hull?

 

only via the coupling AQUADRIVE and hezvy thrust bracket

 

i mean: did the shaft earthed to the hull?

 

yes
you could isolate with OMEGA , but yes earthed through engine, it is tedious to isolate, but we never had issues with electrolysis, but plenty with bubbling paint, Alodyne is the best solution there, before paint, but I will not paint my hulls now cept below WL

 

what are you building

 

if new shaft use 2205 S.S from Avesta

 

We solved this problem by making our stern tubes out of GRP.

 

thank you for the idea of Alodyne. Untill now, the inside of the sterntube stays without paint because this is very difficult to apply it correctly. The risk is if there is a crack in the paint, small surface of aluminum will be exposed face to big SS shaft surface, and then the corrosion will be very agressive for the aluminum.

 

the vessel is already built, the 5083 Al painted hull, 5083 sterntube welded to the hull and 2205 shaft.
the sterntube has 2 seawater lubricated bearings, fore and aft. due to the aft bearing, the zinc anode of the hull has no effect inside the sterntube, contrary to a quasi sister ship where there is no aft bearing.