Corrosion in Isotherm skin fitting

[Guest]

I have an Isotherm water cooled fridge where the refrigerant gas is cooled ,by the seawater, in a tube within a special Isotherm skinfitting. I have recently discovered that this tube within the skin fitting has corroded and that the only thing holding back the sea was the pipework designed to hold the refrigerant gas! Isotherm insist this is a very rare problem - has anyone had any similar experience?

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[vyv_cox]

My son owns a boat A/C and refrigeration business in Mallorca. He has always warned me against this Isotherm fitting precisely because he has experienced so many corrosion failures. It may be that later models have better metallurgy that avoids the galvanic corrosion but they are always going to be made of thin metal for best efficiency.

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[Guest]

Thanks for this info Vyv, it makes the Isotherm claim that they have only had 3 other cases of corrosion in skinfittings sound untrue. Is it possible you could put me in-touch with your son?
We had the fridge fitted in the UK when our boat was built and then went to the Med where the fridge stopped working after 2 seasons.

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[vyv_cox]

I've sent you a PM

Vyv

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[Abaker]

Recently I considered buying an Isotherm water-cooled system, but quickly gave up the idea after reading on their website that the skin fitting is made of brass. Any book or article I've read on skin fittings and plumbing says NEVER use brass below the waterline because its high zinc content will cause it to corrode and sink your boat.

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[LeonF]

Could we have more info? I am considering fitting a fridge and the water cooled system has been suggested. Is there another make that is more reliable or is this intrinsic to any such system?

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[vyv_cox]

I'm having a water cooled system also, but not the Isotherm fitting. When I discussed this post with my son he suggested that earth bonding the fitting is important and may be a cause of rapid corrosion. However, our choice for me is to have a separate supply driven by a small pump. T. Norris sell an excellent one, I had it on my calorifier supply for several years and it ran reliably whenever the engine was running, on hot seawater even though it was not designed for this duty. I shall probably take a T from the engine water supply and discharge it to the exhaust silencer, from where on my boat it falls back to the sea.

I think andart was going to contact my son, perhaps he could share his findings here?

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[Guest]

Hi Vyv,
I did contact your son and he sent me a very informative e-mail which I shall sumarise as follows:
"I have not had any problems with the Isotherm skin fitting but have had problems with the frigoboat equivalent. I would check that the fitting is correctly bonded and earthed as I have had many problems with corrosion due to the fitting not being bonded. I have also visited the frigoboat factory and discussed at length with the general manager what problems he has had with these fittings. He told me a story of Bavaria yachts who were not connecting the bond lugs to earth and the fittings only lasted 6 months before failure! That was on a production run of about 50 boats and they all had the same problem! I dont see very many of the isotherm units but imagine they can only be very much the same as the construction is very similar."

It sounds like bonding could be a key issue. Isotherm advised me that if I were to fit a replacement Isotherm system with the same type of skinfitting that this should be bonded to the prop shaft but when I told this to Northshore (the builders and original installers of our fridge) they laughed saying that this was a very dangerous thing to do and the surest way to corrode the fitting.
Northshore also told me that they now (since 2001) fit composite grp seacocks to all their yachts as the metal used in seacocks is not the same as the metal used in skinfittings such as the Isotherm one and by fitting composite grp seacocks they eliminate one source of galvanic difference.

I think the key issue is that with this type of skin fitting you can be at risk of sinking due to corrosion in the cooling pipe even when the seacock is shut.

I am now in the process of having a Wacom air cooled system fitted.

Andy

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