Read­ing Your Through-Hulls

Passage Maker - - Contents - Steve Zim­mer­man

Atrawler owner re­cently con­tacted us to sched­ule a rou­tine haul-out and bot­tom-paint job. He told us that he had some con­cerns about past dis­col­oration of the bot­tom paint around through-hull fit­tings (see photo, page 24). As it turns out, the paint dis­col­oration pointed to­ward a much more se­ri­ous con­di­tion, a lifethreat­en­ing one at that. This ex­am­ple also serves as a re­minder to pay at­ten­tion to your through-hull fit­tings each time you haul your boat.

Through-hull fit­tings get no re­spect. Hid­den be­low the wa­ter­line, slathered in­side and out with bot­tom paint, and of­ten fouled with marine growth, these fit­tings do their jobs in ob­scu­rity. When you haul your boat, the through-hulls usu­ally re­ceive lit­tle more than some scrap­ing and more bot­tom paint. But bronze through­hull fit­tings of­fer a sur­pris­ing and im­por­tant win­dow into the con­di­tion of your boat’s elec­tri­cal sys­tem. Like the ca­nary in the coal mine, through-hulls can warn you about po­ten­tially life-threat­en­ing AC elec­tri­cal leaks, dam­ag­ing stray cur­rent cor­ro­sion, bond­ing is­sues, and sac­ri­fi­cial an­ode prob­lems.

In this par­tic­u­lar case the prob­lem and the so­lu­tion in­volved mul­ti­ple is­sues. Be­fore we dive into the dan­ger­ous con­di­tion that dis­col­ored the bot­tom paint, we need a bit of back­ground in­for­ma­tion. This me­ter, used in con­junc­tion with a sil­ver/ sil­ver chlo­ride ref­er­ence cell in the wa­ter, mea­sures the volt­age of the un­der­wa­ter hard­ware. Bronze hard­ware will be pro­tected from gal­vanic cor­ro­sion with read­ings above 500 mil­li­volts. Ideally, the volt­age would not ex­ceed 750. In this photo, the high read­ing in­di­cates too much zinc. At lev­els above 1000 the ex­ces­sive volt­age might ad­versely im­pact the bot­tom paint around the through-hulls.


You might be won­der­ing what an un­der­wa­ter pipe fit­ting has to do with your elec­tri­cal sys­tem. On most fiber­glass power­boats all un­der­wa­ter hard­ware con­nects to your elec­tri­cal sys­tem through the bond­ing cir­cuit. Bond­ing refers to con­nect­ing all un­der­wa­ter hard­ware with a low-re­sis­tance, re­li­able elec­tri­cal con­nec­tion. Bond­ing achieves two goals: first, it min­i­mizes cor­ro­sion due to dis­sim­i­lar me­tals (gal­vanic cor­ro­sion); sec­ond, it re­duces the risk of an elec­tri­cal shock from a faulty shore­power sys­tem.

We’ll save the de­tailed dis­cus­sion about gal­vanic cor­ro­sion for an­other col­umn. Let’s clar­ify some ter­mi­nol­ogy. We of­ten use the terms “zincs” and “sac­ri­fi­cial an­odes” in­ter­change­ably. Zincs, how­ever, are one type of sac­ri­fi­cial an­ode, but alu­minum and mag­ne­sium also serve as sac­ri­fi­cial an­odes in dif­fer­ent ap­pli­ca­tions. For now, it suf­fices to say that if you bond all un­der­wa­ter metal, and if you have a sac­ri­fi­cial an­ode con­nected to that bond­ing cir­cuit, the an­ode will pro­tect all of the bonded hard­ware from gal­vanic cor­ro­sion. This bond­ing cir­cuit also con­nects to the boat’s DC ground wire, and that gives us a con­nec­tion be­tween your through­hulls and your DC elec­tri­cal sys­tem.

Next, let’s look into the life-sav­ing func­tion of bond­ing. Bring­ing 110-volt cur­rent (or 220) from land onto a float­ing boat cre­ates risk. Al­ter­nat­ing cur­rent al­ways re­turns to its source—in this case, the power plant lo­cated some­where on shore. It comes into the boat through the

cord and re­turns to its source by the same means. If you have an elec­tri­cal short on, say, your bat­tery charger, the case can be­come hot. The cur­rent wants to re­turn to its source and can safely do so through the green wire at­tached to that case, and then through your shore­power cord.

What if that cir­cuit has a fault some­where on your boat, on the dock, or ashore? The AC cur­rent still needs a path back to land and if you touch the case while stand­ing bare­foot or in socks against a damp hull, you be­come the path as the cur­rent passes through your body, into the wa­ter, and back to shore. And you would likely die in the process (along with any swim­mers in that path, if they hap­pen to be in fresh or brack­ish wa­ter).

To min­i­mize this risk, boat­builders and boat­yards pro­vide an al­ter­na­tive path by con­nect­ing the green wire to all of your sea­cocks and through-hulls. If the shore­power safety cir­cuit is com­pro­mised, the cur­rent has an al­ter­na­tive path to the wa­ter via the through-hulls. The good elec­tri­cal con­nec­tion through the green bond­ing wire pro­vides a pre­ferred path for the cur­rent, fa­vor­ing the low-re­sis­tance elec­tri­cal cir­cuit to the mea­ger choice your body of­fers (swim­mers are still at risk). This safety cir­cuit works so well you prob­a­bly won’t even know that you have a killer aboard, un­less your through-hulls tell you.


Armed with an un­der­stand­ing of the con­nec­tion be­tween through-hulls and your elec­tri­cal sys­tem, let’s as­sume your boat has been hauled, pro­vid­ing an op­por­tu­nity to in­spect the hard­ware for signs of trou­ble. You will be look­ing for three con­di­tions: color of the metal, pit­ting or ero­sion of the metal, and dis­col­oration of the bot­tom paint around the through-hull.

Let’s be­gin with the color of the through-hull fit­tings. Healthy bronze through-hulls ex­hibit a uni­form golden hue. When prop­erly pro­tected from cor­ro­sion they will re­tain this color for many years. We have main­tained a par­tic­u­lar sail­boat for the past 40 years, and she still has her orig­i­nal through-hulls (57 years and count­ing). Any pink­ish spots or patches in­di­cate de­te­ri­o­ra­tion of the metal al­loy, a process known as dez­in­ci­fi­ca­tion. The de­par­ture of zinc from the bronze al­loy leaves be­hind a por­ous cop­per struc­ture in a weak­ened con­di­tion. While the thick lip of the hull might have suf­fi­cient healthy metal re­main­ing, the threads which hold it in place are cer­tainly at risk. Once the gold hue turns pink­ish the fit­ting should be

re­moved and re­placed. This con­di­tion points to an in­ef­fec­tive bond­ing cir­cuit or in­ad­e­quate sac­ri­fi­cial an­odes. When the boat is launched, the bond­ing sys­tem and sac­ri­fi­cial an­ode pro­tec­tion must be checked—more about that shortly.

Also look for pit­ting or loss of metal. Stray cur­rent from your boat’s DC elec­tri­cal sys­tem can de­stroy un­der­wa­ter hard­ware very quickly. AC stray cur­rent can kill a per­son, but rarely at­tacks un­der­wa­ter hard­ware. Pit­ting al­most al­ways points to stray DC cur­rent—usu­ally a poor con­nec­tion in a wet lo­ca­tion, such as bilge pump wiring. If you see pit­ting or loss of metal, the fit­ting must be re­placed and fur­ther test­ing must be per­formed.

In ad­di­tion, have a look at the bot­tom paint sur­round­ing each through-hull. If you see a cir­cu­lar area (a halo) of dis­col­ored paint you might have too much sac­ri­fi­cial an­ode pro­tec­tion or you might have stray AC cur­rent. In this par­tic­u­lar sce­nario, the through-hull fit­ting will not be af­fected (no change in color or pit­ting). When it comes to sac­ri­fi­cial an­odes, too much of a good thing can be a prob­lem. Re­mem­ber, for a bonded boat with a com­mon an­ode, the an­ode af­fects the volt­age of all bonded hard­ware. For bronze hard­ware, 750 mil­li­volts (mV) pro­vides op­ti­mal pro­tec­tion. Bot­tom paints con­tain­ing cuprous ox­ide lose their ef­fec­tive­ness when ex­posed to volt­ages above 750 mV. When the quan­tity of sac­ri­fi­cial an­odes raises the volt­age above 750 mV, ions flow from the through-hull into the bot­tom paint, cre­at­ing dis­col­oration and re­duced anti-foul­ing prop­er­ties.


If you dis­cov­ered dis­col­oration, pit­ting, or ha­los, fur­ther checks should be com­pleted as soon as pos­si­ble. Four dif­fer­ent tests can be per­formed.

1. Bond­ing sys­tem in­tegrity: This test would be in­di­cated by pink­ish col­oration on the un­der­wa­ter bronze, es­pe­cially if the prob­lem is lim­ited to some but not all of the fit­tings. Us­ing a mul­ti­me­ter set on re­sis­tance, a marine elec­tri­cian can check the in­tegrity of your bond­ing sys­tem (max­i­mum of 1 ohm of re­sis­tance be­tween any two points). This test must be done while the boat is on land.

2. Proper ca­thodic pro­tec­tion from sac­ri­fi­cial an­odes: This test would also be in­di­cated by pink­ish col­oration on the un­der­wa­ter bronze, or if you found ha­los in the bot­tom paint. This pro­ce­dure re­quires a sil­ver/sil­ver chlo­ride ref­er­ence cell that is dropped into the wa­ter and con­nected to one side of a me­ter. The other side con­nects to your un­der­wa­ter hard­ware in­side the boat. If prop­erly bonded, all metal fit­tings will rest at the same volt­age with re­spect to the ref­er­ence cell. For bronze hard­ware, that volt­age should be ap­prox­i­mately 750 mV. If too low, the hard­ware will not be prop­erly pro­tected. Too high and you might im­pact the bot­tom paint around the through-hulls.

3. DC Stray Cur­rent: This test would be done if you found pit­ting (loss of metal) on the through-hulls. Same test as #2, but now you have to me­thod­i­cally go through the on­board cir­cuits to find out which one causes a change in the volt­age of the un­der­wa­ter hard­ware. 4. AC Stray Cur­rent: This test would be used if you found ha­los in the paint around the through-hulls. Work­ing with the shore power cir­cuits is dan­ger­ous and should be done by a pro­fes­sional. As a start­ing point, an elec­tri­cian would use a high-qual­ity spe­cial clamp-type am­me­ter placed around the shore power cord. We want to know if all of the am­per­age com­ing into the boat is re­turn­ing to shore. A few mil­liamps would be ac­cept­able, but any­thing above 30 mil­liamps presents a risk.

Let’s re­turn to that trawler with the ha­los of dis­col­ored bot­tom paint. Be­fore haul­ing the boat, we plugged in the shore power and clamped a spe­cial­ized am­me­ter around the cord. The me­ter showed 1.8 amps, or 1,800 mil­liamps. As we said ear­lier, 30 mil­liamps would be too much. We now had a smok­ing gun—an AC stray cur­rent leak into the wa­ter. The elec­tri­cian found a con­nec­tion be­tween the neu­tral and ground­ing wire on a washer/ dryer. In ad­di­tion, the generator lacked a proper ground­ing wire and re­lied on its con­nec­tion to the DC ground cir­cuit for a con­nec­tion. These con­nec­tions, and lack of con­nec­tions, al­lowed cur­rent from the neu­tral wire to re­turn to ground through the through-hulls and sea­wa­ter. This flow cre­ated the ha­los in the bot­tom paint.

Next time you have your boat out of the wa­ter, take some time be­fore paint­ing the bot­tom to in­spect your through-hull fit­tings. If cov­ered with bot­tom paint, use a scraper and some sand­pa­per to ex­pose the metal on a few of them. This sim­ple prac­tice pro­vides a valu­able look into the health of your elec­tri­cal sys­tem, bond­ing, and ef­fec­tive­ness of the sac­ri­fi­cial an­ode. If you do have a weak­ened fit­ting you can re­place it now, with the boat al­ready on land. And if the in­spec­tion points to a stray-cur­rent prob­lem, you may have saved a life.


Above: Dis­col­ored bot­tom paint around through-hulls can in­di­cate a dan­ger­ous elec­tri­cal leak in the shore­power wiring, or ex­ces­sive use of sac­ri­fi­cial an­odes. Be­low: The loss of metal from this through-hull points to DC stray cur­rent. In a less se­vere case, mod­er­ate pit­ting might be vis­i­ble.

The red­dish color of the bronze threads con­trasts with the golden hue of healthy bronze. The threads that hold the through-hull into the sea­cock can no longer be counted on to hold the as­sem­bly to­gether.

This clamp- on am­me­ter reads 1.8 amps—well above the safe thresh­old of about 30 mil­liamps. This high read­ing in­di­cates dan­ger­ous leak­age of the shore power into the wa­ter.

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