I have owned my E28 for about two years and sail in the San Diego area. Just recently I noticed what appears to be galvanic corrosion forming on the shaft packing area just inside the hull. I do not see any evidence of zincs anywhere in the area, and nothing in the owner's manual mentions zincs on the shaft inside the hull. I have studied the example in Calder's Boatowner's Mechanical and Electrical Manual, but I don't see any evidence of that arrangement on my prop shaft, but I am not sure it even applies in my case. I am not a mechanic, so if this gets too complicated I will have to pay to have it fixed. If it is a simple fix, I would like to take care of it myself. Any suggestions?
Propeller Shaft Assembly Corrosion
Bonding
Gary,
I am assuming you see the corrosion on the stuffing box. The rest should either be gelcoat/glass or bonded. I am planning to bond the bronze stuffing box (see my other note on E34 drivetrain rebuild) to the common grounding point on the engine. Having a zinc inside the boat doesn't seem to address the issue as you are not closing the loop with the electrolyte sea water outside. Ideally, one should have perhaps a shaft brush to pick up any charges there as well.
Gary,
I am assuming you see the corrosion on the stuffing box. The rest should either be gelcoat/glass or bonded. I am planning to bond the bronze stuffing box (see my other note on E34 drivetrain rebuild) to the common grounding point on the engine. Having a zinc inside the boat doesn't seem to address the issue as you are not closing the loop with the electrolyte sea water outside. Ideally, one should have perhaps a shaft brush to pick up any charges there as well.
gareth harris
Sustaining Member
I have never connected all the through hulls etc. to a common ground as per Nigel Calder, as the boat maintenance world seems to be divided about its benefit. The best I can figure out the physics of it: there may be electric fields in the water, from stray wiring on the dock or other boats. Since copper is a much better conductor than salt water, bonding all your metal points of contact with the water (through hulls, prop, lightning conductor etc.) may induce voltages between them, and much higher voltages than are created by galvanic action.
Nigel Calder does not address this issue, but when I was on the hard on Shelter Island a few years ago, the contractor working on my boat showed me one of his other projects, which had major problems in the fibreglass around every through hull, as an example of what he felt was attributable to using the Calder method.
This is a case where experience is best, so maybe some other folk will have different opinions.
Gareth
Freyja E35 #241 1972
Nigel Calder does not address this issue, but when I was on the hard on Shelter Island a few years ago, the contractor working on my boat showed me one of his other projects, which had major problems in the fibreglass around every through hull, as an example of what he felt was attributable to using the Calder method.
This is a case where experience is best, so maybe some other folk will have different opinions.
Gareth
Freyja E35 #241 1972
zinc it good!
I am not getting into the bronze vs. marelon debate but I happen to have Marelon, ant I like it
. If I take the argument Gareth makes one step further, then ever piece should be zinc-ed wherever dissimilar material touch, and maybe everywhere else just in case there is stray current. The argument that you might be connecting points that are at different potential due to being in different places in the surrounding electric field seems correct.
In the case of the packing box, these points are presumably close together, and the shaft itself should be bonded to the engine anyway. Additionally grounding the solid portion of the box should just assure that the green look doesn't happen. I have a brand new stuffing box and I do care (somewhat).
I am not getting into the bronze vs. marelon debate but I happen to have Marelon, ant I like it
. If I take the argument Gareth makes one step further, then ever piece should be zinc-ed wherever dissimilar material touch, and maybe everywhere else just in case there is stray current. The argument that you might be connecting points that are at different potential due to being in different places in the surrounding electric field seems correct. In the case of the packing box, these points are presumably close together, and the shaft itself should be bonded to the engine anyway. Additionally grounding the solid portion of the box should just assure that the green look doesn't happen. I have a brand new stuffing box and I do care (somewhat).
Zinc = good
If I may comment on Chris' statement about the virtue of zincs....
When we put in our FrigoBoat refrigeration setup last year, the seller/distributor *strongly* reccommended that we go with the zinc-equipped "keel cooler" (they are listed in the brochure with and without). This is a sintered bronze plate that contains a loup of copper that carries off the heat from the evaporator (or is it the compressor? sometimes I am a bit of a Luddite).
Anyway, he said that whether a boat used bonding or not, they still preferred to sell only the version with the two little zincs on it.
Our boat has no bonding and never did. The only external metal is the lead keel, and the shaft/prop/strut and this keel-cooler for the Frigoboat system. We do also have a zinc on the prop and also the shaft.
Sorry if this is getting off topic.
Loren in PDX
If I may comment on Chris' statement about the virtue of zincs....
When we put in our FrigoBoat refrigeration setup last year, the seller/distributor *strongly* reccommended that we go with the zinc-equipped "keel cooler" (they are listed in the brochure with and without). This is a sintered bronze plate that contains a loup of copper that carries off the heat from the evaporator (or is it the compressor? sometimes I am a bit of a Luddite).
Anyway, he said that whether a boat used bonding or not, they still preferred to sell only the version with the two little zincs on it.
Our boat has no bonding and never did. The only external metal is the lead keel, and the shaft/prop/strut and this keel-cooler for the Frigoboat system. We do also have a zinc on the prop and also the shaft.
Sorry if this is getting off topic.
Loren in PDX
Dave Hussey
Member III
fiberglass electrolysis
Gareth, in the boat you mentioned, what sort of damage did you see to the hull? Was the fiberglass deteriorated by stray current? I am aware that wood can become damaged due to fastenings effected by galvanic corrosion, but I never would have thought fiberglass was susceptable.
On another note: Woudn't it be terrific if the 'experts' could agree on the best method of protecting the expensive underwater bits which, should one fail, could sink our boats. I wired my thru hull fittings together and to the engine block, with the assumption that, this being a low resistance conductive path (lower than the path of conductivity through salt water at least) there would be no difference of potential between the dissimilar metals, and therefore, no current flow. God, I hope I'm right.
Dave Hussey
Gareth, in the boat you mentioned, what sort of damage did you see to the hull? Was the fiberglass deteriorated by stray current? I am aware that wood can become damaged due to fastenings effected by galvanic corrosion, but I never would have thought fiberglass was susceptable.
On another note: Woudn't it be terrific if the 'experts' could agree on the best method of protecting the expensive underwater bits which, should one fail, could sink our boats. I wired my thru hull fittings together and to the engine block, with the assumption that, this being a low resistance conductive path (lower than the path of conductivity through salt water at least) there would be no difference of potential between the dissimilar metals, and therefore, no current flow. God, I hope I'm right.
Dave Hussey
gareth harris
Sustaining Member
Dave - I wish I could give you a good answer, but I do not know enough about the chemistry of fibreglass.
The description I was given, from someone who knew a lot about boats but not about science, was that it was a problem caused by electical potential related to the connected bronze through hull. I would have expected myself that the resistance in the hull would be so huge it would not care about voltages across it in the vicinity of bronze; and that it is more likely that water managed to penetrate the fibreglass through the laminates, coming in from the hole where there was no gel coat, after the through hull had been eaten away.
I think the reason there is no consensus, is that if your boat is anchored well away from shore, your only concern is galvanic action, for which connecting all through hulls gives them protection; if however you are tied to a dock with all sorts of stray voltage, galvanic voltage is the least of your worries.
A well built bronze through hull, or stuffing box, will last for decades submerged in salt water, although it will not be shiny.
Gareth
Freyja E35 #241 1972
The description I was given, from someone who knew a lot about boats but not about science, was that it was a problem caused by electical potential related to the connected bronze through hull. I would have expected myself that the resistance in the hull would be so huge it would not care about voltages across it in the vicinity of bronze; and that it is more likely that water managed to penetrate the fibreglass through the laminates, coming in from the hole where there was no gel coat, after the through hull had been eaten away.
I think the reason there is no consensus, is that if your boat is anchored well away from shore, your only concern is galvanic action, for which connecting all through hulls gives them protection; if however you are tied to a dock with all sorts of stray voltage, galvanic voltage is the least of your worries.
A well built bronze through hull, or stuffing box, will last for decades submerged in salt water, although it will not be shiny.
Gareth
Freyja E35 #241 1972