What Type for Your Boat?
How Many and Where to Fit them.
These are the ones that protect mechanical components such as raw water cooled engines and heat exchangers.
Usually they screw into the water-jacketing and need to be replaced at specified intervals.
Stern drives and outboard motors, because they are usually made of aluminium require aluminium anodes rather than zinc.
For exterior hull protection there are three main types of Anode material.
It is best to fit protection to suit the waters in which you normally moor your boat.
Zinc is the most common type, because it is the simplest and cheapest material available that would do the job.
However when used in freshwater, zinc forms a coating of zinc hydroxide that insulates it and stops it from working.
The Magnesium anodes are often used in freshwater but tend to disappear very quickly when used in saltwater and wooden boats can be overprotected by very active magnesium.
A recent innovation is an alloy of aluminium, zinc and indium which I am told lasts longer and provides better protection.
However, Zinc still remains the most commonly used on 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.
This will happen wherever two very dissimilar metals are either in direct contact with one another or connected by the water.
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.
Martyr Anodes Volvo PentaTM Anode Kit - Zinc CMDPHKITZ - $21.99
Retail Price: $32.79
You Save: $10.80And the anode 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 anodes 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 anodes should be part of her annual check-up.
Replace them before they have eroded completely away.