It is a natural to assume that the more one has the better the protection.
Unfortunately, this is not necessarily true for wooden boats.
In fact an excess of them can be the cause of extensive decay to the timber surrounding the anode and the metal it was supposed to protect.
And as most manufacturers' instructions are aimed at the plastic boat market these can sometimes be misleading.
I wish I could give you some hard and fast rules, unfortunately it all depends on your boat, the metals which are used on her, how these interact and the amount of stay current in and around her.
For instance bronze being one of the more noble metals is a favorite with wooden boaters and because it is so noble it is considered reasonable not to bond bronze through hull fittings to the galvanic corrosion system.
However, if your prop shaft is a more noble stainless steel, the less-noble bronze will be the one to corrode away.
This will happen wherever two very dissimilar metals are either in direct contact with one another or connected by the water.
It could be a fitting secured with fastenings of a different metal or an iron ballast keel close to the plank fastenings.
However, on wooden boats the main area of concern is principally the stern gear, propellers, shafts, shaft brackets, stern tubes, rudders and keels.
Not only are these expensive to replace but they are vital to the safe boating.
And the 'zinc' must be in direct contact with metal it is there to protect in order to be effective.
So, if your propeller and shaft are of dissimilar metals, e.g. a bronze prop and stainless shaft, then you should fit a shaft anode, one for each shaft.
For the same reason on a metal rudder it should be bolted directly to the rudder.
There are plenty of wooden boat owners who are happy not to use any zincs.
They feel that their boats have a minimum of dissimilar metals, the water contact between them is not a problem and they are not concerned about stray current.
Personally I feel happier having at least one main one located on hull just below the turn of the bilge.
Whatever you decide is right for your boat there are some dos and don'ts,
The anode must be electrically connected to the metal it's protecting.
Any bonding cable should be as short as possible.
Internal fixing bolts should be above the bilge line and where they can be accessed easily.
Use plenty of silicon sealant around the fastenings this will not only reduce leaks but help reduce wood decay and corrosion.
Site them where they won't interfere with the flow across log impellers or depth transducers.
If there are mild steel bilge keels they should have separate zincs affixed.
Use a convenient bolt on the engine block to bond to one of the fixing studs.
Use bonding wire which has a low electrical resistance and is resistant to corrosion.
And remember, checking your zincs should be part of her annual check-up.
Replace them before they have eroded completely away.
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I am perfectly aware that the majority of Wooden Boat aficionados are sensible folk. However, I need to point out that I am an amateur wooden boat enthusiast simply writing in order to try to help other amateur wooden boat enthusiasts. And while I take every care to ensure that the information in DIY Wood Boat.com is correct, anyone acting on the information on this website does so at their own risk.