A question that comes up with surprising regularity among sailors is whether their sacrificial anodes are doing their job correctly — and whether the rate at which those anodes are being consumed is normal. It is a question worth taking seriously, because the answer tells you a great deal about what is actually happening beneath the waterline.
Bonding Systems: Lightning Protection or Corrosion Protection?
When a viewer recently wrote in to describe their custom-built sailboat, they detailed a copper bonding strap running through the boat with conductors going to all the metal strainers and other fittings that should normally be bonded. The boat carried zinc anodes on the propeller, propeller shaft, and bow thruster — but nothing else. No hull anode, no strut anode. The viewer wanted to know whether this was normal.
My first reaction was that this sounds less like a galvanic corrosion protection system and more like a lightning protection system. For many years, one widely used approach to lightning protection involved an internal copper bonding strap to which all metal fittings were connected, with an external strap tied through to a grounding plate or strip on the hull. If the strainers and other metal components in this boat are tied into a bonding system but are not directly in contact with seawater — and there is indeed some kind of external grounding plate connected — then there may be no corrosion protection role for that bonding system at all. The galvanic protection work is being done entirely by the individual anodes on the propeller, shaft, and bow thruster, each of which is protecting its own isolated underwater fitting.
This is an important distinction. A bonding system intended for lightning protection and a bonding system intended for corrosion protection are not the same thing, and conflating the two leads to real confusion. If this boat's internal copper strap has no external anode tied to it, and some of those bonded fittings are in contact with seawater, then there is a genuine problem — and an anode needs to be added to that system without delay. We would need more information about the specific installation to say definitively which situation applies, but the question of whether there is an external grounding component is the key one to resolve first.
Why Different Anodes Wear at Different Rates
The viewer also noted something interesting: the propeller zinc was needing replacement mid to late season, while the bow thruster and propeller shaft anodes were still well above 50% remaining. Is that unusual? Not especially. And understanding why helps clarify how galvanic protection actually works.
When underwater metal fittings are not electrically connected to one another — as appears to be the case here — each anode is working in isolation, protecting only its immediately adjacent metals. The propeller itself is typically made of some variety of bronze, which has a relatively high copper content, and the anode protecting it is usually a fairly small zinc collar fitted to the end of the propeller boss. The combination of surface area, alloy composition, and local water flow around a spinning propeller can drive a meaningfully higher rate of zinc consumption than you might see at a stationary fitting like a bow thruster bracket. The rate of zinc consumption on propeller anodes is genuinely variable from boat to boat — changing a propeller zinc once or twice a season is not at all unusual, and is not in itself a cause for alarm.
What matters is whether the anode is being consumed before the metal it is protecting suffers any attack. If you are finding the zinc exhausted and the propeller still in good condition, the system is working although you should have replaced the anode well before it was entirely consumed. If you are finding the zinc gone and pitting beginning on the propeller itself, then the anode is undersized or the interval between changes is too long.
The Case for Switching to Aluminium Anodes
Before we close, there is one more factor worth raising. We have been talking about zinc anodes throughout, but it is well worth knowing that aluminium anodes can now replace zinc anodes in almost every application on a boat. Wherever you would fit a zinc, you can fit an aluminium. The material lasts significantly longer, which means less frequent replacement and better long-term value.
The one rule you must follow is that you cannot mix and match within a connected system. You could not, for example, fit a zinc shaft collar on the propeller shaft and an aluminium anode on the propeller if those two components are metallically connected. However, if your underwater fittings are electrically isolated from one another — as they appear to be on this boat — you have more flexibility. You could run aluminium on the propeller and propeller shaft while keeping zinc on the bow thruster, because they are not tied into a common system. Where aluminium is available for your fitting type, it is genuinely worth making the switch.
Learn More About Corrosion Protection
If you want to go deeper on bonding systems, lightning protection, sacrificial anodes, and galvanic corrosion prevention, all of these topics are covered in detail in the Advanced Marine Electrics course. Understanding how these systems interact — and when a bonding system is protecting your boat versus simply grounding it against lightning — is the kind of knowledge that prevents expensive underwater surprises.
