Electrical Isolation of prop shaft

[GeorgeTina]

I have recently installed an R and D coupling between the gearbox and prop shaft on my yacht. This means the prop shaft has now no electrical connection to the engine. Does this matter ?. I can fit an earthing strap if required but it seems to me that having no electrical connection would reduce the chance of corrosion to the gearbox bearings. Any comments please.

 

[eyehavit]

[ QUOTE ]
This means the prop shaft has now no electrical connection to the engine. Does this matter ?.

[/ QUOTE ]
If there are disimilar metals and salt water involved, e.g. bronze prop/ss shaft and salt water it will matter.
Either connect the shaft to the existing anodic protection system or fit a shaft or prop anode.

 

[roly_voya]

Whats the inboard bearing if any? I had the same problem of a flexible coupling isolating engine block but having a bronze prop on S/S shaft which will cause corrosion. On my set up the inboard bearing screws to a bronze shaft log and has a bronze inner bearing in it, when I checked it with the meter there is a good electrical conection accross the bearing and I was able to connect the bearing housing to the anode to provide shaft protection leaving the engine block isolated

 

[pampas]

R&D supply a very stiff foil which, when rolled tightly slides and is compressed by the two couplings (Placed in the centre hole), didnt like that idea so made straps to span bolts,one on each 1/2 and opposite in order to maintain balance.

 

[nedmin]

My flex coupling was strapped across ie bonding shaft and gearbox output but I found I was getting galvanic action on the prop,when I put a meter across the prop shaft to earth I was getting a small current, this could only have been the oil in the gearbox bearings insulating the continuity! I mounted a brush holder and brush to run on the metal rim of the coupling,continuity was made and now all ok.Galvanic probs can be a sod to cure sometimes.I once moored alongside steel piling and would you believe the anodes wore down more on the side away from the piling.

 

[fisherman]

I foolishly installed a R&D coupling without bridging it for a season and now have limited life left in the prop, i.e it can't be repaired next time I dink it. prop nut disappeared as well.
I drilled the centre of the straps in the coupling and put in an 8mm stainless bolt with lock nuts.
Against all advice someone put a lorry starter on a Ford in a boat. It did not have an insulated return. The boat had all new anodes fitted, went from Newlyn to the IoM, no anodes left on arrival. It's why I often bang on about residual current getting into the wet.

 

[VicS]

[ QUOTE ]
If there are disimilar metals and salt water involved, e.g. bronze prop/ss shaft and salt water it will matter

[/ QUOTE ] Bronzes and stainless steels are relatively close on the galvanic scale so generally there is no problem with a bronze prop on a stainless shaft.

If there is no problem obvserved with the prop and the shaft then there is no need to try fixing it.

'Manganese bronze' is, however, sometimes used for props. It is not in fact a true bronze but a high tensile brass and will be susceptible to dezincification. If that is the case then it will become pinkish in colour due to the loss of the zinc and then cathodic protection becomes necessary. A shaft anode is one solution or a hull anode located nearby is the alternative. If a hull anode is chosen then it is essential than a good low resistance electrical connection exists between the anode and the prop. The pitfalls of achieving this by relying on a path via the coupling and the gearbox have already been highlighted leaving a brush assembly on the shaft immediately inboard of the stern gland as the most likely method to be successful.

 

[Birdseye]

Just installed an R&D myself. Not bothered with an electrical bridge - just put an anode on the shaft.

I suspect there will be an electrical contact anyway, from the P bracket through a short distance of salt water.

On my previous boat, the battery negative was not connected to anything and neither were any of the sea cocks. The engine was solidly connected to the shaft with one small anode to protect. never had any trouble with erosion of the fittings, so I wonder whether my present boat with everything wired up will be a problem.

 

[PCUK]

I simply used a piece of 4mm domestic cable and crimp terminals to bridge the coupling. One on each side to maintain the balance. Worked fine when checked with a multimeter and cost about 10p!

 

[PCUK]

I later used an MG Duff Electro-Eliminator having changed the coupling for another type that didn't allow the simple bridging as above. Both worked perfectly. Just had the ten year old prop' serviced and no excess corrosion (Thank goodness as a new one is currently £1,500!!!)