Salt assault

Mark Rothfield | VOLUME 25, ISSUE 4
Damage may not occur until it reaches an innocent boat six or seven berths away
We shine a light on corrosion – the natural enemy of all boats everywhere.

She loosened her overalls, ruffled her long blonde hair and sighed as the breeze chilled her glistening skin. Smudges of grease mingled with delicate beads of sweat upon her brow.

“Here’s your problem,” she said, holding a rusted bolt between her fingers. “Your engine anode has corrosion …”

So, have we got your attention yet?

Corrosion. It’s as inevitable as death and taxes, but as a topic it’s about as arousing as a prostate examination. Many boat owners tend to turn a blind eye to the issue of corrosion, but it’s a fact of life that if you’ve got a boat, it’s a target for some form of corrosive activity.

When it comes to boating – and saltwater boating in particular – every boat in every bay, every outboard and trailer in every carport, corrodes at a different rate, so there’s no hard-and-fast formula for prevention. Much depends on the surroundings, what vessels are berthed alongside, what engine is fitted, how often the vessel is used, how often you wash or flush it, and so on.

One brand of diesel engine in a fibreglass hull may chew anodes faster than another brand in an aluminium hull. Tailoring your maintenance to suit the circumstance is vital but, first and foremost, you have to understand the enemy. Or more precisely, the enemies.

When an electrical current strays from an anticipated path due to improper wiring or a defective device, you get an evil blighter called “electrolysis”. This can set off corrosion.

Then there’s “galvanic corrosion”, which occurs when dissimilar metals immersed in an electrolyte (aka saltwater) generate an electrical current in much the same way as a fluid battery.

And of course, we can’t forget our old mate “rust”. Otherwise known as oxidation, rust results from oxygen combining with exposed metals to form an oxide. The main catalyst is water, especially saline water, which becomes acidic.

Just as rust never sleeps in the moist and salty marine environment, nor do galvanic corrosion and electrolysis. With proper periodic maintenance, though, you can put these bad boys to bed for a little zizz.


Marinas are meccas for electrolysis and galvanic corrosion. The more boats on an arm, the more chance that electrical current will be fizzing through the brine.

If steel and aluminium boats are berthed side by side, the two dissimilar metals will create current. Similarly, a boat with faulty electrical wiring can ‘leak’ current and cause serious grief to surrounding vessels.

Each vessel can act like the plates in a battery, and damage may not occur until it reaches an innocent boat six or seven berths away.

This ‘daisy chain’ effect also applies to marina shorepower, compounded by the fact that many boats use household-quality extension leads.

Marine technician Darin Riley, from Thermatek Marine, says leads must be marine-certified, with sufficient insulation resistance surrounding the cable. Plugs also have to be coupled correctly because every point between the power source and the vessel is a potential escape route for current.

“If you have a battery charger, it must be electronic, not the old type with a wound transformer coil. A solar trickle charger is a better option,” Riley advises.

All electrical components create a magnetic field that, in turn, causes electrolysis, so power consumption on the boat should be limited if you’re not aboard. For instance, turning off the fridge and lights will reduce the current draw.

External forces also come into play. For example, the marina on Newcastle Harbour, where I spend a lot of my spare time, has a large steel floating dock directly opposite. There are also coal ships and tugs coming and going, and factories close by. Current is rampant everywhere throughout the harbour, causing anodes on boats in the area to last as little as six months.

The general rule here is that, the higher the current, the higher the corrosion potential. Moored craft are generally better off because the charge dissipates the further it has to travel through the water.


Like a schoolyard bully, electricity always attacks the softest target. That’s why there are sacrificial zinc anodes, zinc being one of the bottom-dwellers on the galvanic table.

Anodes are inexpensive, yet arguably the most valuable item on a boat. There’s many a war story about running gear, engine legs, even yacht keels, being literally eaten away for want of a dose of cathodic protection.

Put simply, you’d rather corrosion eat the anode than the next ‘noble’ metal. Consider the relationship between a stainless steel shaft and a bronze propeller – the prop will ‘sacrifice’ itself to protect the shaft unless a softer metal (anode) is introduced. So we fit anodes as a form of frontline defence against encroaching corrosive influences.

Some shipwrights and marine electricians claim you can’t have too many anodes, arguing that there should be cup-type anodes on the shaft(s), another on the transom, and also on the trim tabs and rudders.

Others maintain that this approach harks back to 1824, when Britannia ruled the waves in her copper-sheathed ships, and a geezer called Sir Humphry Davy first delved into the properties of zinc anodes.

Metallurgy, they say, demands a modern, scientific approach.

“You can definitely ‘over-anode’ a boat, especially if it’s alloy or steel,” says marine electrician Jason Smith. “You’ll blow the paint off at the waterline and it can also promote marine growth.

“The correct way to calculate the amount of anode protection needed on any boat is to conduct a hull reference test. I launch the boat with no anodes on, then throw a probe into the water to determine the ambient voltage.

“Next step is to start introducing anodes, simply by dropping them over the side, and build up the protected level. Once you have that value, you can work out how many kilos of anodes you need to mount on the hull.”

Smith recommends having a ‘bonding system’ within the boat, whereby the main transom anode is wired directly to all the underwater components, including skin fittings.

He also monitors new anode installations, conducting a check after a month, just to be on the safe side.

“I installed some trim tabs on a fishing boat and it suddenly developed holes all through the hull. I checked it again and the numbers were fine,” Smith says. “It turned out that a stainless steel fabricator had welded rocket launchers to the hull while in the water, without earthing it properly.”


My copy of Wiring for Dummies reckons that the red wire is the positive that delivers power, and the black is negative, which closes the circuit. Thus, the black lead off the battery goes to an earth plate. Simple – or not.

Wiring up onboard electrical equipment with separate earths is a recipe for disaster, according to the experts. Many older boats have earth wires going to a metal fixture, such as the engine block, whereas you’re better off having one common earth point for every circuit.

The earth plate can be near the dashboard or switchboard, and you don’t have to run every wire directly there, but rather tap into a wire leading back. Get it wrong and it can be very expensive.

In one case I know of, a new HF radio was earthed to the engine. Over the ensuing 18 months of radio use, the saildrive leg corroded to the point where it almost fell off, and a 316 stainless steel ram on the lift-keel mechanism was also destroyed as the current tracked forward.

The repair bill came to almost $40,000, which the radio installer’s insurance covered (who says talk is cheap?). Since the earthing issue was resolved, there have been no further problems.

To stop back-feed of current from onboard devices, and to break the shorepower daisy chain, you can look at investing in an earth blocker, also known as a galvanic isolator or electrolysis blocker. Ask your friendly marine electrician.


Inboard motors may be freshwater-cooled nowadays, but most still rely on seawater going through the heat exchangers, transmission coolers and fuel coolers. Internal anodes are placed around the motor, usually where the most turbulence exists … in the case of a cooling system, near a water pump.

“Every time the motors are serviced, your mechanic should inspect the anodes. If that’s done every 200 hours and you have 50 per cent of the anodes left, then you get a ballpark figure for a maintenance plan,” Darin Riley says.

“It can take three or four service intervals to get a clearer picture, but even then, it’s virtually impossible to be completely accurate if the boat is being used irregularly. If you move the boat to a new marina, you need to do a new maintenance plan.”

Anodes tend to corrode on the inside and, if left too long, will snap when being unscrewed.

You can’t eliminate the problem; only limit it. Try shutting the seacocks to arrest the flow of water when the boat is not in use. Where possible, keep the engine bay free of water and moisture – it’s a good idea to spray the engine lightly with lanolin or a lubricant such as Inox (don’t, as I’ve seen, wash the engine with a saltwater deck wash!).

Volvo Penta produces the Neutra-Salt system that taps into the raw-water intake and neutralises the salinity, making it less potent as an electrolyte. It’s an in-water flush, in other words.

Some 45 seconds prior to shutting the engine down, you hit the switch. A concentrated solution is then injected into the cooling system, leaving a corrosion-inhibiting coating on all internal metal surfaces.

MACS (Marine Anti Corrosive System) is a product designed to reach the parts that freshwater flushing doesn’t reach in outboards. Salt deposits can not only clog the cooling system, but cause pitting corrosion in the alloy powerhead.

When flushing an outboard, it’s recommended that you leave it running long enough to open up the thermostat. Some people taste the water, others feel the temperature, but five minutes should be ample.

When it comes to external protection for outboards and sterndrives, both have anodes fitted in the form of a fin. Where possible to limit the chance of corrosion, ensure the outboard is out of the water when not in use.


The best way to combat rust is with regular maintenance to reduce salt crystalisation. That means hosing your boat off with fresh water and a mild detergent after each use, then drying all surfaces.

Most steel boats rust from the inside out because you can’t see, or reach, the areas where the problem is occurring. Treatment involves grinding affected surfaces back to bare metal and repriming, or using a rust converter to attain a corrosion-free base you can recoat.

Some steel boat owners I know swear by a rust preventive paint called POR-15 that totally seals the metal.

Altex, one of the major players in metal paint, recommends a three-phase system. Priming provides adhesion and protects the substrate, an intermediate undercoat gives an extra barrier, then you finish with a top coating (paint or antifoul).

If you can’t remove all the rust and treat it, the worst thing is painting straight over the top. It’s better to leave the rusty surface exposed so it will have a chance to dry out.

Externally, it’s advisable to sandblast the hull before treatment, but be sure to prime within a few hours. If it gets too bad, steel can be fixed by cutting out the offending area and welding in a new section.

The other thing about a steel boat is that it’s akin to a battery in the water – metal in an electrolyte. If you’re using dissimilar metals, it compounds the problem.

As a rule of thumb, use like materials for fasteners. That includes brass screws for a brass attachment, aluminium screws or rivets for an aluminium fitting.

Aluminium and stainless steel are one of the most volatile combinations because you have a hard metal up against a soft one. You must use a sealant to prevent metal-to-metal contact.

Aluminium boats don’t rust, of course, but are just as prone to galvanic corrosion. One tip is to never park them alongside a steel boat. Similarly, if you drop a sinker in the bilge of a tinnie it will eat its way through, and there’s a similar effect if you use an incompatible antifoul – a copper-based coating will quickly kill an alloy hull.


Perth-based metallurgist, Brett Maddern believes that zinc anodes are a contributing factor in wood rot and alkali attack in timber boats, not to mention paint problems and calcium build-up.

He developed the Maddox anode, which is extruded alloy rather than cast zinc, and has a more passive electromagnetic field that suits all non-metallic vessels.

“Wooden boats will see their wood rot dried up within 21 days,” Maddern claims.

With timber boats you also have to be mindful of metal fixtures. Bronze is commonly found on timber craft and it tends to get a pinkish tinge if there’s an electrolysis issue. The screws obviously need replacing and then you look at anodes.

With fibreglass hulls, osmosis is the main concern. When fitting a seacock, water can track if it gets under the laminate. Any corrosion in the skin fitting can compound the problem.


Right now you’re probably feeling pretty smug about the fact your boat is sitting on its trailer in the garage. But think again, for that steel trailer may be slowly rusting away.

Galvanising is essentially a hot-dip coating of metallic zinc, which is a form of localised cathodic protection (the zinc acts as a sacrificial anode) and a barrier coat in one. It’s quite durable, but if the zinc coating is scratched, the exposed steel can rust.

If you notice signs of light rust, clean and sand the patch, then coat with a cold galvanised spray or similar.

Springs, axles, brakes and wheel bearings are your biggest maintenance concern. Best practice is to wash your trailer with a detergent soap after every use to prevent corrosive salts penetrating.

Lubricate the most prone parts on your trailer with a silicone or teflon spray and fit bearing buddies to prevent seawater being sucked in and affecting the grease.

As with all cases of corrosion, you’ll never win the war … but you might prevail in the occasional battle. It’s mostly about keeping an eye on your boat and inspecting potential problem areas before they become major headaches

And remember, if all else fails, speak to the experts.

How to-Safety