Don't forget about the zincs in the engine and generator... they are there for the same reason: the zinc on the rudder is NOT going to protect anything inside the engine room.
The bonding serves two purposes: to drain off stray currents and protect underwater metals from galvanic corrosion with the zinc. For parts up inside the engine room connected to the ocean by a hose the galvanic protection available from an external zinc is so close to zero it makes no nevermind, but the stray current protection is still really important. That's why the engine and generator zincs are so important to keep on top of seperate from the rudder zinc.
Worst case: the zinc installed in the manifold dissolves faster than the rudder zinc. But it is a piece of cake to replace. And guess what? If it dissolves quickly, that means it is needed!
It sure sounds to me like some of the original manifolds installed had some brazing issues that might have been helped by an installed zinc if they are failing in as little as 8 years.
Rose Island, Bahamas
---In amelyachtowners@..., <yahoogroups@...> wrote :
If you place a zinc on the manifold and also leave the manifold connected to the bonding system, that zinc will be the closest zinc to all items in the bonding system and will deteriorate before the rudder zincs.
Yes, it can't hurt, but I would not advise it if the manifold is connected to the bonding system.