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
Author:
Updated:
Original:

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.

Related

Shelly-forward-last-day

Charter Advice for First-Timers

Never chartered? No worries. A vacation under sail can be the most memorable time of your life. That said, it also pays to be prepared by doing some reading, building your skills and listening to what the experts say. First and foremost, not all charter grounds are created ...read more

HugoBoss

Video: Vendeé Update

Last week Alex Thomson (Hugo Boss) led the fleet across the equator. As one of the class' top sailors who's been on the Vendeé Podium twice, it seemed possible that Thomson was going to grab an early lead and hold on to it all the way around the world. But early on Saturday, he ...read more

AdobeStock_229409051

Chartering Again for the First Time

It’s been a rocky road of late for the charter industry, especially here in the Western Hemisphere. First came hurricanes Irma and Maria in the Caribbean followed by Dorian in the Bahamas. There has also, of course, been the coronavirus, which burst into global prominence ...read more

01 LEAD cedaryachtclub_onedesign18_hike

An Interview with Ayme Sinclair

In recent months, US Sailing, like many organizations, has been taking a closer look at diversity to ensure it’s doing the best job it can of introducing people from all backgrounds and ethnicities to the sport. As part of this effort, this past summer it organized an online ...read more

125768940_10222759720523627_5373654001582879638_n

US Sailing Presents Adaptive Sailing Panel

On Tuesday, November 24, US Sailing’s Leadership Forum will present the latest panel discussion in their Diversity, Equity and Inclusion series. This event will focus on adaptive sailing and provide practical recommendations for organizations looking to expand their adaptive ...read more

02-IMG_5971

A Carbon Neutral Circumnav with Jimmy Cornell

Historic anniversaries have always held a special fascination for me, especially if they mark a significant nautical achievement. In 1992, on the 500th anniversary of Columbus’ would-be voyage to India, I organized a transatlantic rally that followed the historic route of the ...read more

DJI_0068

SAIL Podcast: Jimmy Cornell’s Carbon-free Circumnav

In this episode of Point of SAIL, Principal Editor Adam Cort talks with bestselling author and pioneering bluewater sailor Jimmy Cornell, who set out November 19 on yet another circumnavigation aboard a newly designed, carbon-neutral Outremer 4Zero catamaran. The voyage, which ...read more

emirates-600x

Emirates Team New Zealand Splashes the last of the AC75s

Emirates Team New Zealand unveiled its second-generation AC75 yesterday, joining the other three America's Cup teams with boats in the water. In just over 100 days, this boat will attempt to defend the Cup for the Kiwis, but there's plenty of racing between now and then, with ...read more