Just joined this forum as I can't find an earth isolation kit or information for a BUKH 24 that I'm going to get installed in my 27ft aluminium lfeboat. The starter, alternator and any other electrics need to have plastic bushes fitted to avoid the Zincs getting chewed up.
Anyone know more about how bad the issue is, although I'm going to cover the hull below the waterline with a special West System epoxy and 2 layers of glass cloth to avoid potential electrolysis issue. I do have a shore power system but that is not connected to a battery charger, it's just for heating in winter.
EARTH ISOLATION KIT FOR A BUKH
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kmorin
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- Joined: Mon Aug 18, 2008 1:37 am
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- Your location: Kenai, AK
- Location: Kenai, Alaska
Re: EARTH ISOLATION KIT FOR A BUKH
Lifeboat, Welcome to the AAB.com Forum on welded aluminum boats.
From your post I'd say you have a set of concepts regarding aluminum in marine applications that may not be as informed as you may wish to become? On other hand, perhaps my ideas and beliefs are not well founded and you can help me learn about other facts of welded aluminum boats that I currently hold true?
Anodes, zinc or the much newer and more widely used marine aluminum alloys; are meant to be sacrificial. That is; "to get chewed up"!! They're there to be the sacrifice into salt water of metal molecules taken off with stray current (normal to all powered vessels) and to substitute their own mass in place of the metal of the hull!!
ABYC, and Lloyds' standards for vessel electrical design don't provide for insulation or isolation from the hull of electrical energy; BOTH standards expect the hull's DC/neg. and AC/neutral to be bonded, intentionally, to the hull in order to guide, control, conduct the stray currents to the hull metal- which will then exit the hull THROUGH the sacrificial anodes (of any alloy) to the water.
So, isolating the engine and electrical system isn't an accepted method of dealing with stray current- that I'm aware?
Next, the idea of coating the hull with an epoxy compound? bedded in GRP fiber mesh? Unless the epoxy you propose to use has the same thermal expansion co-efficient (?) the hull-to-epoxy bond will not last long- the hull will expand and contract at alloy rates- even high solids epoxy may (don't know from personal experience) not have the matching rate- therefore the so-called protective bond of the plastic sheathing will become the source of destruction of your hull.
Especially questionable IMO, is the idea of using glass fiber cloth in the film's matrix? This implies rigidity that I don't believe will adhere long to the hull?
There are many articles/posts/threads here on the Forum about various corrosion circumstances witnessed on aluminum alloy boats. Several of them show a film of paint or improperly bedded sheath of base plates or mounts where a thin film of water was allowed under the cover of the paint or mount plate. In each case, the water, once it forms a 'thin film', becomes de-aerated and shifts ph downward to become acidic and the acid in these cells defeats the metal's self-healing oxide.
Once the oxide is broken down, the acid continues to 'eat' the metal in the form of pitting corrosion sites. This type of corrosion is not from stray current from the engine or electrical system; it comes from the 'crevice corrosion' phenomenon and is directly related to the covering layer allowing water into the narrow space between the 'un-adhered' paint/coating/metal plate and the hull's metal.
In your post it appears you currently believe this epoxy film will protect the hull? I think it will cause great damage unless you are certain the preparation to apply is correct so the adhesion is absolute (?) AND the resulting film will expand & contract with the metal?
I'd say anodes should degrade or get 'chewed up' that's how we know they're working correctly. And the marine-alloy aluminum bottom paints I've seen are pretty elastic films not rigid epoxy/glass structures. If you're bottom painting your hull (?) IMO it would better to seek out a coating designed to discourage marine growth and not to worry about corrosion that the anodes will cover?
Just my views, from building in welded aluminum and working to maintain various welded hulls since the 70's. I'd like to encourage you to do a more research into the two topics I've mentioned above and see if there's more to be learned?
Again, welcome to the AAB.com Forum and best of fortune in your lifeboat's remodel, sure would like to see photos of the boat and your work as you progress, thanks for posting.
Cheers,
Kevin Morin
Kenai, AK
From your post I'd say you have a set of concepts regarding aluminum in marine applications that may not be as informed as you may wish to become? On other hand, perhaps my ideas and beliefs are not well founded and you can help me learn about other facts of welded aluminum boats that I currently hold true?
Anodes, zinc or the much newer and more widely used marine aluminum alloys; are meant to be sacrificial. That is; "to get chewed up"!! They're there to be the sacrifice into salt water of metal molecules taken off with stray current (normal to all powered vessels) and to substitute their own mass in place of the metal of the hull!!
ABYC, and Lloyds' standards for vessel electrical design don't provide for insulation or isolation from the hull of electrical energy; BOTH standards expect the hull's DC/neg. and AC/neutral to be bonded, intentionally, to the hull in order to guide, control, conduct the stray currents to the hull metal- which will then exit the hull THROUGH the sacrificial anodes (of any alloy) to the water.
So, isolating the engine and electrical system isn't an accepted method of dealing with stray current- that I'm aware?
Next, the idea of coating the hull with an epoxy compound? bedded in GRP fiber mesh? Unless the epoxy you propose to use has the same thermal expansion co-efficient (?) the hull-to-epoxy bond will not last long- the hull will expand and contract at alloy rates- even high solids epoxy may (don't know from personal experience) not have the matching rate- therefore the so-called protective bond of the plastic sheathing will become the source of destruction of your hull.
Especially questionable IMO, is the idea of using glass fiber cloth in the film's matrix? This implies rigidity that I don't believe will adhere long to the hull?
There are many articles/posts/threads here on the Forum about various corrosion circumstances witnessed on aluminum alloy boats. Several of them show a film of paint or improperly bedded sheath of base plates or mounts where a thin film of water was allowed under the cover of the paint or mount plate. In each case, the water, once it forms a 'thin film', becomes de-aerated and shifts ph downward to become acidic and the acid in these cells defeats the metal's self-healing oxide.
Once the oxide is broken down, the acid continues to 'eat' the metal in the form of pitting corrosion sites. This type of corrosion is not from stray current from the engine or electrical system; it comes from the 'crevice corrosion' phenomenon and is directly related to the covering layer allowing water into the narrow space between the 'un-adhered' paint/coating/metal plate and the hull's metal.
In your post it appears you currently believe this epoxy film will protect the hull? I think it will cause great damage unless you are certain the preparation to apply is correct so the adhesion is absolute (?) AND the resulting film will expand & contract with the metal?
I'd say anodes should degrade or get 'chewed up' that's how we know they're working correctly. And the marine-alloy aluminum bottom paints I've seen are pretty elastic films not rigid epoxy/glass structures. If you're bottom painting your hull (?) IMO it would better to seek out a coating designed to discourage marine growth and not to worry about corrosion that the anodes will cover?
Just my views, from building in welded aluminum and working to maintain various welded hulls since the 70's. I'd like to encourage you to do a more research into the two topics I've mentioned above and see if there's more to be learned?
Again, welcome to the AAB.com Forum and best of fortune in your lifeboat's remodel, sure would like to see photos of the boat and your work as you progress, thanks for posting.
Cheers,
Kevin Morin
Kenai, AK
kmorin