Systems+Engines

Isolated Prop Shaft

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Gary McKinney of Loon Lake, Washington, asks:

I have installed a shaft-saver coupling on the prop shaft on my sailboat and have realized the shaft is now isolated from the ground. Is this a good thing, or should I put a grounding strap on the coupling?

Nigel Calder replies:

The primary objective is to tie your shaft and propeller into your bonding circuit and an attached zinc anode to prevent galvanic corrosion. This requires an almost electrically perfect connection from the shaft to the zinc, so the critical issue is the electrical path from your shaft-saver to the zinc, which should be via a #8 AWG or larger cable with good connections at both ends. In order to protect the shaft and propeller, you need not tie the engine into this circuit—i.e., you don’t need the ground strap on the coupling. This is done to create a path to ground in the event of a lightning strike. The science in this respect is murky, so I’ll leave it up to you to decide if it is necessary!

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