On a boat, corrosion works silently, below the waterline, in the bilges and behind the electrical panels. By the time you see it, it has often already cost you a part, an unplanned haul-out or a failure.
An invisible problem with a real price tag
Owners face two difficulties. First, corrosion hides: an anode wasting too fast, a saildrive leg under attack or a connector turning green behind a bulkhead won’t catch your eye on a stroll around the deck. Second, its causes overlap, so parts get replaced without the root cause being dealt with, and the problem comes back.
The cost shows up in replacement parts, unscheduled haul-outs, hours spent searching, and risk. A failed through-hull or seacock is water coming in. A seized steering sheave is a boat that stops answering the helm properly just when you need it.
Three mechanisms worth understanding
- Galvanic corrosion: two different metals connected and immersed in seawater form a battery, and the less noble one dissolves. Anodes, made of zinc, aluminium or magnesium depending on the water, are there to be that sacrificial metal.
- Stray-current electrolysis: an insulation fault aboard, or current coming from the dock, sends current through the water and can eat away underwater parts very quickly.
- Crevice corrosion in stainless: stainless steel needs oxygen to protect itself. Trapped under a seal, inside a stanchion base or a damp fitting, it rusts from the inside without warning.
Weak points, area by area
Saildrives and propellers
A saildrive leg is aluminium, a metal very prone to galvanic corrosion. Watch its anode and the propeller anode where fitted, the condition of the leg’s paint, and never put copper-based antifouling on aluminium. The seal between saildrive and hull also has a service life set by the manufacturer, so record when it was fitted. On shaft-driven boats, the same care goes to the shaft anode, the propeller and the stern gland.
Through-hulls and seacocks
Work every seacock regularly: one that no longer turns won’t close on the day you need it to. Check that through-hull, valve and tail materials are compatible, look at the hoses, and make sure the clamps are doubled.
Deck stainless
Stanchion bases, the arch, chainplates, fittings: rust streaks or a cracked seal often point to stainless working in a confined space, and sometimes to a leak that needs fixing at the same time.
Shore power and wiring
In a marina, the shore power lead connects your boat to the earth of the whole marina, and so to your neighbours. A galvanic isolator or an isolation transformer limits that exchange. Aboard, look for green terminals, untinned wire and connections sitting in damp bilges.
A simple monitoring routine
- Photograph the anodes at every haul-out and note their wear: an anode that wastes abnormally fast, or not at all, is a warning sign.
- Measure protection potential with a reference electrode and compare against the manufacturer’s values, or have a marine electrician do it if in doubt.
- Check the electrical system’s insulation, especially after any change or new equipment.
- Once a year, inspect stanchion bases, fittings, sheaves and steering parts, taking apart whatever needs it.
- Record everything: date, finding, photo, action. Repeated observations are what reveal a trend.
Steer clear of false good ideas too: adding anodes at random, bonding parts to ground without an overall plan, or painting antifouling over an anode. A painted anode protects nothing. If wear changes suddenly after electrical work, look at that work first.
Aboard myCloud: steering seized by rust
During the refit we found the steering cables and sheaves seized with rust. Nothing dramatic from the helm: the steering simply felt heavier, and you get used to anything. Once removed, the sheaves no longer turned freely and the cables were rubbing. The whole system was overhauled. In our case the fix was mechanical, but the cause was the same as everywhere else aboard: salt, moisture and nobody looking. The lesson is straightforward: metal parts hidden in lockers and bilges, where air barely moves and salt water lingers, deserve a scheduled inspection, not just attention once something starts to bind.
The same applies to the generator space under the cockpit floor, which was re-glassed and repainted during the refit. A warm, damp compartment speeds up corrosion of everything inside it, from hose clamps to electrical connections.
In the myCloud boat management app, every anode, seacock and steering component has its own equipment record tied to an area of the boat, and the checks come back as scheduled maintenance with reminders before they fall due. The checklist of points for each area guides the inspection, and each finding is logged as a job, with its documents.
In the logbook. For each anode: date fitted, condition when removed, photo. After two or three haul-outs you know what normal wear looks like on your boat, and you notice straight away when it changes.