Double Shaft

Alan Therrien of Boxford, Massachusetts, asks:

“The two zincs on my prop shaft were eroding quickly last summer, so I hung a zinc guppy over the side and attached it to the backstay. I measured the current between the backstay and the guppy with my multimeter, which read between .04 and .07 amp of current with all battery connections removed. Where is this current coming from? Is it the result of galvanic action?

My starting and house batteries are grounded to the engine block, and I use an Off-One-Both-Two switch to control them. The DC system and the boat’s lightning-protection system are also grounded to the engine block. I get a solid reading for conductivity from the engine ground through the shaft to the water.

My boat is on a mooring, and when I change my location in the mooring field the current readings remain the same. All my through-hulls are bronze, the shaft is stainless, the prop is bronze, and the fin keel is lead. No other metal is attached to the hull. Am I missing something?”

Nigel Calder replies:

A zinc guppy’s effectiveness comes from a low-resistance electrical path from the guppy to the backstay to the engine block (through a bonding wire) and down the propeller shaft to the propeller. To check the resistance, pull the guppy out of the water and measure the current between it and the propeller shaft; a quality digital multimeter in the ohms mode should give you a reading below 1 ohm. If not, measure the current from the guppy to the backstay, from the backstay to the engine, and from the engine to the shaft. When you discover where the resistance is occurring, eliminate it by cleaning all the terminals.

If there is a low-resistance path between a zinc anode (the guppy) and the metal it is protecting—in this case, the propeller and the shaft—the amount of galvanic current generated is a function of the relative surface areas of the metals involved; in your case it could be 40 to 70 milliamps, especially if the lightning bonding system on your boat includes the bronze through-hulls. In general, I do not recommend putting bronze through-hulls in a bonding circuit precisely because of the additional zinc consumption they cause.

To measure galvanic current you must have a very sensitive clamp-on DC ammeter; if not, you’ll need to break the bonding circuit inside the boat and connect your multimeter, in the milliamps mode, across the break.

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