How thin is too thin ?

[Narrowboat Nimrod]

30 years ago your average narrowboat might have had a few dim lights in the cabin, a water pump and a bilge pump. Now people want a fridge, washing machine, microwave and a tv ect. Lots more scope for stray current and hulls to be eaten away in short order if not correctly protected. Most boats now have a galvanic isolator fitted but is it fitted correctly? How often is it tested? Pulling it out of the water every couple of years is the only sure way of checking your hull is not suffering. 

[cuthound]

5 hours ago, Narrowboat Nimrod said:

30 years ago your average narrowboat might have had a few dim lights in the cabin, a water pump and a bilge pump. Now people want a fridge, washing machine, microwave and a tv ect. Lots more scope for stray current and hulls to be eaten away in short order if not correctly protected. Most boats now have a galvanic isolator fitted but is it fitted correctly? How often is it tested? Pulling it out of the water every couple of years is the only sure way of checking your hull is not suffering. 

 

Personally I test my GI at least 2-3 times a year and after every circuit breakervtripping jncident. Takes about a minute using a digital multimeter set to doide test mode.

 

Much better than pulling the boat out of the water and finding the hull is pitted from galvanic corrosion.

Edited May 30, 2019 by cuthound
Clarification

[ditchcrawler]

I just don't connect mine to the electrics 

[MtB]

6 hours ago, cuthound said:

 

Personally I test my GI at least 2-3 times a year and after every circuit breakervtripping jncident. Takes about a minute using a digital multimeter set to doide test mode.

 

Much better than pulling the boat out of the water and finding the hull is pitted from galvanic corrosion.

 

Is there much evidence that a GI stops galvanic corrosion totally?

 

Or more to the point, are there any cases when one failed to protect?

 

[blackrose]

On 29/05/2019 at 09:43, Alan de Enfield said:

Why would that affect the hull thickness ?

(In fact it would probably cause less problems than one on 'fresh water' that is full of diesel and stray electrical worms, scraping over the shopping trolleys & bed of the canal and rubbing along the steel & concrete walls at every lock and every time you moor up for the night.

 

Narrowboats have a much harder life than 'sea-boats' (maybe that is one reason why (leisure) sea-boats regularly only have 4-6mm steel and canal-boats have 10mm+)

 

But a typical steel canal boat has a couple of coats of bitumen slapped onto the sides, an unpainted baseplate and a few magnesium anodes. Take one of those out and leave it in a much more efficient salt water electrolyte and you'll soon see the effects on the hull. That was the point of the OP's rhetorical question.

 

It's only the baseplate of modern canal boats that are regularly 10mm, the sides are 6mm, the same as some seagoing boats. Also not all of us rub along against concrete at every lock or mooring. The sacrificial baseplate overhang might scrape if fenders aren't deployed.

Edited May 30, 2019 by blackrose

[howardang]

1 hour ago, Mike the Boilerman said:

 

Is there much evidence that a GI stops galvanic corrosion totally?

 

Or more to the point, are there any cases when one failed to protect?

 

Please forgive a stupid question - I am no electrical expert- but do any of the battery/electrical  management systems such as Victron include galvanic protection built to the system? Our share boat has a Phoenix inverter charger, and I should know the answer but I'm afraid I don"t. I have a vague memory of someone once  mentioning that this was the case but it was a long time ago, and fortunately (touch wood) there is no sign of any pitting- probably because of the regular blacking and good anodes.

 

Howard 

Edited May 30, 2019 by howardang

[cuthound]

9 hours ago, Mike the Boilerman said:

 

Is there much evidence that a GI stops galvanic corrosion totally?

 

Or more to the point, are there any cases when one failed to protect?

 

 

A GI works by interrupting the earth connection until the voltage across he diodes has risen sufficiently to allow them to conduct.

 

Therefore as long as he galvanic voltages (different earth potentials) remains below the diode turn on voltage there can be no earth connection and thuscno galvanic corosion.

 

This is why an isolating transformer is preferred by some as it interupts  the earth connecion permanently.

[Momac]

There is a point at which the hull would become structurally unsound and a failure would occur due to a stress caused perhaps by an impact . 

The structural analysis of that would be complex. I doubt anyone has done that analysis on a narrowboat.

 

It seems 4mm is the insurance industry standard and that has proven to be satisfactory in practice.

Therefore less than 4mm is too thin.

 

[Naughty Cal]

On 29/05/2019 at 17:07, Bee said:

Boaters have not got into the frequent docking habit yet. Its all about keeping that film of paint on. B&Q cheap white gloss would do if it stayed on, Most of us need to think of annual docking or maybe every couple of years as necessary maintenance

Some of us don't have a choice.

 

Every 12 months for us. Don't want it to be any more frequent then that!

[Alan de Enfield]

9 hours ago, blackrose said:

But a typical steel canal boat has a couple of coats of bitumen slapped onto the sides, an unpainted baseplate and a few magnesium anodes. Take one of those out and leave it in a much more efficient salt water electrolyte and you'll soon see the effects on the hull. That was the point of the OP's rhetorical question.

 

But in a well maintained boat that wouldn't happen - would it.

Yes - a 'sea-water' boat without paint and anodes will corrode.

 

I might just as well say "if I don't repair the scratches on my car it will get rust and eventually penetration / corrosion holes".

[noddyboater]

On 29/05/2019 at 12:07, Mad Harold said:

A tip I got from an old boater was to give a  boat one coat of blacking every year.

The reasoning behind this was that the blacking should still be intact after a year,and also you will only need one day in the dry dock.The nearest to me charges £175 for the first day,and £90 per day thereafter.

Doing this,I would guess that hull corrosion would be minimal.

This is really misleading advice, as it all depends on what you’re painting over every year. I shared a dock with a boater who did exactly this, with bitumen on his (very expensive) hull that was a few years younger than mine. After jet washing he was amazed at the condition of my hull, compared to his that had quite extensive pitting around the water line. Mine has been blacked twice in 20 years, the difference being it was grit blasted and 2 packed before any corrosion took hold.

There’s no point sealing in layers of scale and pitting with a coat of cheap bitumen yearly, get it “bottomed” and start again. 

[MtB]

1 hour ago, MartynG said:

There is a point at which the hull would become structurally unsound and a failure would occur due to a stress caused perhaps by an impact . 

The structural analysis of that would be complex. I doubt anyone has done that analysis on a narrowboat.

 

It seems 4mm is the insurance industry standard and that has proven to be satisfactory in practice.

Therefore less than 4mm is too thin.

 

 

This is flawed logic. Too thin for what, other than to get insurance?

 

I hold that 4.1mm would also prove 'satisfactory in practice', in that boats would not fall to pieces on hitting a lock side, as would 3.9, and probably 2.9. Lots of the small Springers were built from 3mm steel and they aren't noted for sinking on hitting stuff. 

 

 

 

 

 

[noddyboater]

On the subject of galvanic isolation- 

My mooring used to be at a well known northern boatbuilders yard. Their shells were stored in the water awaiting fitting out, with a token roller round of comastic for protection. This always left areas of bare steel around rubbing strakes etc that quickly rusted, and showed when they were eventually lifted out for fitting. 

Us moorers received a letter one day stating that we must show evidence of a GI being fitted on our boats as they were making his new ones corrode! 

[WotEver]

9 hours ago, howardang said:

do any of the battery/electrical  management systems such as Victron include galvanic protection built to the system? 

No. A GI is a separate unit installed between the shore supply socket and any internal boat wiring. 

[Naughty Cal]

39 minutes ago, Mike the Boilerman said:

 

This is flawed logic. Too thin for what, other than to get insurance?

 

I hold that 4.1mm would also prove 'satisfactory in practice', in that boats would not fall to pieces on hitting a lock side, as would 3.9, and probably 2.9. Lots of the small Springers were built from 3mm steel and they aren't noted for sinking on hitting stuff. 

 

 

 

 

 

No they sink without hitting stuff on the Fossdyke. One sank on the long term moorings and stayed sunk institu for about 12 months!

[Alan de Enfield]

9 minutes ago, Naughty Cal said:

No they sink without hitting stuff on the Fossdyke. One sank on the long term moorings and stayed sunk institu for about 12 months!

Maybe one of the drifting dead-deer hit it ?

[Naughty Cal]

17 minutes ago, Alan de Enfield said:

Maybe one of the drifting dead-deer hit it ?

It probably would have holed it if it had hit it given the state of the hull when it was recovered!

[Momac]

3 hours ago, Mike the Boilerman said:

 

This is flawed logic. Too thin for what, other than to get insurance?

 

Too thin to get insurance would be an issue for me.

[howardang]

4 hours ago, WotEver said:

No. A GI is a separate unit installed between the shore supply socket and any internal boat wiring. 

Thank you for the reply. It was a long time ago but I thought I had been informed that the battery/charger management system had either a GI or an isolation transformer as part of the package. Obviously my memory has let me down - old age I suppose

 

Howard

[WotEver]

9 minutes ago, howardang said:

Thank you for the reply. It was a long time ago but I thought I had been informed that the battery/charger management system had either a GI or an isolation transformer as part of the package. Obviously my memory has let me down - old age I suppose

 

Howard

It’s possible that either a GI or IT was included as part of the package but it’s a separate unit from the inverter/charger.