Anodes that vanish in a season, pitted prop shafts and green deposits on underwater metal are usually a symptom of your boat sharing a shore power earth with every other boat on the pontoon. Different metals sitting in the same electrolyte, with a copper path between them, form a battery, and the least noble metal pays for it.
Galvanic corrosion is slow and predictable. Stray current corrosion, caused by a DC leak finding its way to the water, is far faster and far more damaging. Telling them apart matters because the fixes are completely different.
Your shore power earth connects your underwater metal to the earth of every other boat on the same supply. If a neighbouring boat has more noble metal in the water than you do, your anodes, and then your shaft, skin fittings and outdrive, protect their boat as well as yours.
That is why a boat can be fine on a swinging mooring and eat anodes on a pontoon.
A DC positive finding a path to water through bilge water, a damp connection or a poorly insulated pump can strip metal in days rather than seasons. If anodes have gone in weeks, or corrosion is concentrated around one fitting, treat it as urgent and get the DC system leak tested.
Do not simply fit larger anodes and carry on, that hides a fault that can eventually put a hole in the boat.
A galvanic isolator blocks the low DC voltages that drive galvanic corrosion while still allowing the safety earth to do its job in a fault. It is the usual first step for a boat kept on shore power.
An isolation transformer goes further, separating the boat's AC system from the shore supply completely. It costs and weighs more, but on boats with a lot of underwater metal, or on supplies of uncertain quality, it is the thorough answer.
You can safely record anode condition each lift, note when the wastage started, and check whether it correlates with a change of berth or a new appliance. Anything involving measuring leakage, opening AC enclosures or fitting isolators is qualified work, because it sits directly on the safety earth.
Most owners plan on replacing them annually, but the honest answer depends on berth, metals and use. What matters is the change, if they suddenly go far faster than they used to, something has changed electrically.
It usually slows it, which is a useful diagnostic clue, but it is not a fix, and it leaves you without charging. A galvanic isolator or isolation transformer solves it properly.
It is fitted in the shore power earth conductor, so incorrect installation can compromise the safety earth. This is one to have installed and tested by a marine electrician.
We test for stray current, check the shore power earth path and specify isolation that suits your boat and berth.