Flexible Coupling & grounding

[trantor12020]

I've helped a friend install a flexible coupling to his prop shaft. According to the instruction a ground wire should be connected across the flanges if grounding is required. Question is, if the shaft and prop are not ground and is insulated from the rest of boat system then there shouldn't be any cathodic action to eat away the prop ...right ?

[btrayfors]

It depends...on how the rest of the grounding system is set up, on nearby boats in marinas, stray currents, etc.

However, the more important question has to do with a proper ground for the boat's AC and DC systems. There should be ONE grounding point for AC and DC systems, and it needs to be in contact with the water. Normally, this is the engine.

However, if you install a flexible coupling it's a very good idea (imperative) to make sure the shaft and prop are grounded to the engine.

You protect the prop and other underwater metals with sacrificial zincs.

Bill

[Maine Sail]

This is what the ABYC has to say. ABYC is NOT mandatory but a suggested good standard based on years of combined experience by its many voting members.

From ABYC P-06 standard:
6.5.5.2 If a non-conductive flexible coupling is used, an alternative means of grounding the shaft must be provided.

If the engine is acting as the ground point, which it is on most boats being built, then you'll either need to jump over the coupling with a wire or find an alternative means of grounding the shaft. They are fairly clear that the shaft should be grounded, they actually used the words "must be grounded".....

[trantor12020]

So even if I leave the shaft and prop "floated" (not ground) it has detrimental effect ? Where would the leakage current flow to and from ?

[Maine Sail]

Dissimilar metals submerged in electrolyte/salt water will create galvanic issues on their own. Stainless shaft/bronze prop/electrolyte..

[trantor12020]

Ok, just to be on safe side, I'll get my friend to loop a small wire across the flanges. Thanks for the heads up.