Anodes, bonding and dissimilar metals.

[Alpha22]

I have an elderly motor cruiser and due to damage I'm having one shaft replaced. Currently all the underwater metal is bronze, except two stainless trim tabs.

Current anodes are two pears located on the underside of the hull immediately above each shaft. The new replacement shaft will be stainless steel.

McDuff's website says anodes should not be electrically bonded to dissimilar metals and I think I almost understand why.

SO...... what anodes should I have fitted? What should be bonded to what?

I'm thinking I should add a donut on either side of each trim tab, but what to bond each pear anode to? The shafts currently are not specifically bonded in any special way.

 

[kashurst]

The usual set up is that the rudders, P brackets and all the stainless handrails etc are all electrically connected inside the boat with big flexible wires (lightning protection) and then to the anodes under the boat. Then a common point is connected to the main battery -ve terminal. This gives the boat the equivalent of an earth point through the water and ensures the most reactive metal (the anode hopefully) is electrically exposed to the other underwater metal bits. The props and shafts on old boats are considered connected to the anode circuit via the gearbox and the engine(s) - this generally works, but newer boats often have a carbon brush arrangement on the prop shaft ensuring good contact. If you don't have contact brushes the easy, safe but sure solution is to fit shaft anodes. Trim tabs usually have their own anode because connecting them electrically to the rest of the underwater metalwork is tricky because they move and any wires would fail quickly. You probably only need one anode per trim tab - on the upper face out of the water flow.

On older boats often all the seacocks are connected into the anode circuit too - but check to see what the original builder did as this varies depending on a few variables (distance from other metal parts, materials used etc). Also important to consider is where the boat is kept - fresh water or sea water. If its fibreglass/wood use zinc in the sea and magnesium in the fresh. There are aluminium alloys you can get that work in both too.

anyway that's the general idea, but it would be wise to get your anode bonding circuit checked out as like everything they deteriorate with time/water and if it fails expensive bits start dissolving (surprisingly quickly in sea water). Also don't fit more or materially bigger anodes than the original design as this can give other issues.

 

[Hadenough]

Also don't fit more or materially bigger anodes than the original design as this can give other issues.

What are the other issues? Surely bigger anodes will just need replacing less frequently?

 

[kashurst]

if the anodes are too large - or have too much surface area then the anodic current increases - this gives problems on metal boats with hydrogen being released too rapidly around the anode damaging the paint etc, increasing exposed metal so the anodes end up working harder. Don't think that's too much of an issue on fibreglass but it can affect wooden boats. I think the main issue is the anodic current is unnecessarily large so the bigger anodes end up dissolving in a similar space of time. However I stand to be corrected.

 

[Hadenough]

if the anodes are too large - or have too much surface area then the anodic current increases - this gives problems on metal boats with hydrogen being released too rapidly around the anode damaging the paint etc, increasing exposed metal so the anodes end up working harder. Don't think that's too much of an issue on fibreglass but it can affect wooden boats. I think the main issue is the anodic current is unnecessarily large so the bigger anodes end up dissolving in a similar space of time. However I stand to be corrected.

Thanks, interesting.