Steel Narrowboat

[Sligoman]

Hi all

 

OK might sound like a daft question, but I am going to ask anyway.

 

What is likely to more corrosion?

 

1. A boat moored up in still water or....

2. A boat that is continuously cruising. I.E. water rushing by it.

 

Be gentle with me as I am curious to know and my way of thinking is that water moving past a boat must sort of be cleaning bits off...

 

Say no more as I might be digging a hole..

 

Thanks in advance..

 

Liam

[Bill]

I would say there is not a lot in it. However there are things which may affect the rate of erosion. For example a CC boat is likely to get scrapes and bangs during its travels which may or may not damage the blacking/protection.

[Alan de Enfield]

I'll throw this into the mix :

 

Corrosion (rust) needs three things

Oxygen / Air

Water

Steel

 

A boat continuously moving will be generating a 'bubbly' air filled stream of water running alongside the steel hull - Ideal concoction for rust ?

 

A boat continuously moored will only have the 'absorbed' oxygen in the water to act on the steel - less than ideal concoction for rust ?

 

A boat continuously moving will have its blacking eroded revealing some bare steel.

 

A boat continuously moored will have less erosion to its blacking, except at the water line when in a marina.

[RLWP]

I'll throw this into the mix :

 

Corrosion (rust) needs three things

Oxygen / Air

Water

Steel

 

A boat continuously moving will be generating a 'bubbly' air filled stream of water running alongside the steel hull - Ideal concoction for rust ?

 

A boat continuously moored will only have the 'absorbed' oxygen in the water to act on the steel - less than ideal concoction for rust ?

 

A boat continuously moving will have its blacking eroded revealing some bare steel.

 

A boat continuously moored will have less erosion to its blacking, except at the water line when in a marina.

 

We moor at the top of a busy lock flight. One side is against a vertical concrete bank, the other in regularly moving water

 

We don't get any noticeable difference side to side

 

Richard

[Athy]

I've heard it said that boats moored in marinas and connected to a shoreline electrical supply are more prone to rust. It sounds unlikely, but then I'm no scientist.

[Alan de Enfield]

Boats in Marinas do suffer more erosion of the blacking at the waterline as the concentration of diesel in the water (either from spillages or exhausts) is much higher due to the lack of 'flow' of the water

[Athy]

But diesel is an oil, and oils generally protect from wear rather than provoking it, so how does that work?

[Alan de Enfield]

But diesel is an oil, and oils generally protect from wear rather than provoking it, so how does that work?

 

Diesel breaks down blacking (which is also 'oil based') so that it 'dissolves' and breaks away from the boat leaving unprotected steel.

 

I remember reading about a case where marina moorers made a claim on a marina owner following a diesel leak - they made a claim for re-blacking.

 

Edit :

Corrosion is not 'wear', corrosion is the deterioration of a metal by oxidation or other chemical action.

Edited July 7, 2016 by Alan de Enfield

[Athy]

O.K., thanks Alan.

[peterboat]

 

Diesel breaks down blacking (which is also 'oil based') so that it 'dissolves' and breaks away from the boat leaving unprotected steel.

 

I remember reading about a case where marina moorers made a claim on a marina owner following a diesel leak - they made a claim for re-blacking.

 

Edit :

Corrosion is not 'wear', corrosion is the deterioration of a metal by oxidation or other chemical action.

Not all blacking dissolves in diesel mine doesnt

[Bobbybass]

Boats in Marinas do suffer more erosion of the blacking at the waterline as the concentration of diesel in the water (either from spillages or exhausts) is much higher due to the lack of 'flow' of the water

 

I also suffered a serious loss of blacking...down one side...where I was facing my neighbour's sink and washing machine outlets.

 

The detergents..coupled with hot water....were very effective for that.

Edited July 7, 2016 by Bobbybass

[David Mack]

I'll throw this into the mix :

 

Corrosion (rust) needs three things

Oxygen / Air

Water

Steel

 

Corrosion will also be influenced by the electrical environment. Connection to a shoreline, the hull forming part of the boat's electrical circuitry (intentionally or otherwise), fitting of sacrificial anodes, mooring alongside galvanised piling, mooring in a marina amongst other boats with any of the above. All of these will influence the electrical environment around the boat and can affect the rate of hull corrosion.

[WJM]

I've heard it said that boats moored in marinas and connected to a shoreline electrical supply are more prone to rust. It sounds unlikely, but then I'm no scientist.

That is completely true - the process is called Galvanic Corrosion - where a small current is established through the electricity supply earth wire. The cure is to install a Galvanic Isolator into the earth connection. This is a device that blocks very low voltage currents (the ones that cause corrosion over time) and allows high voltages through (the currents the earth is designed to carry away to safety).

[Athy]

That is completely true - the process is called Galvanic Corrosion - where a small current is established through the electricity supply earth wire. The cure is to install a Galvanic Isolator into the earth connection. This is a device that blocks very low voltage currents (the ones that cause corrosion over time) and allows high voltages through (the currents the earth is designed to carry away to safety).

Thank you for explaining. As a person who failed his O level Physics, I am little the wiser, but if we ever moor in a marina again I'll be sure to make enquiries about the power supply.

[Tony Brooks]

Thank you for explaining. As a person who failed his O level Physics, I am little the wiser, but if we ever moor in a marina again I'll be sure to make enquiries about the power supply.

 

It probably would not in the marina the recent posts where the OP got a shock from the hull when the boat was out of the water (pending feedback on the final cause). An isolation transformer would protect in that scenario.

[Paul C]

I would say there is not a lot in it. However there are things which may affect the rate of erosion. For example a CC boat is likely to get scrapes and bangs during its travels which may or may not damage the blacking/protection.

 

This^^^

 

The main factor in corrosion will be if the blacking is doing its job at protecting the boat, a boat which goes through locks inevitibly scrapes a little bit in a narrow lock, even if you drive it in without touching the sides. Mooring and unmooring, going through bridgeholes etc also have the potential to put scrapes on the boat but a good driver might get away with it.

[ditchcrawler]

That is completely true - the process is called Galvanic Corrosion - where a small current is established through the electricity supply earth wire. The cure is to install a Galvanic Isolator into the earth connection. This is a device that blocks very low voltage currents (the ones that cause corrosion over time) and allows high voltages through (the currents the earth is designed to carry away to safety).

But don't chain your boat up as that is another earth path.

[Sligoman]

Wow... thanks for all off the replies. Really educational and now fully understand.

 

Once again all, thank you.

 

Regards

 

Liam

Edited July 8, 2016 by Sligoman

[Rendelf]

That is completely true - the process is called Galvanic Corrosion - where a small current is established through the electricity supply earth wire. The cure is to install a Galvanic Isolator into the earth connection. This is a device that blocks very low voltage currents (the ones that cause corrosion over time) and allows high voltages through (the currents the earth is designed to carry away to safety).

 

Can anyone expand on this a little? I'm still confused. (We don't have shoreline)

 

Electricity enters the boat via insulated shoreline, and is then distributed around it via a fusebox. Does the consumer unit not route all the earth cables back through the shoreline supply? I can understand poor wiring leading to a route of least resistance into the water.... but hopefully 240v wiring isn't so shoddy. Where does the current that passes through the hull begin? Is it 'leaking' from insulated cables at levels so small to be inconsequential (in terms of safety) ? Possibly its contaminating the 12V system, which is earthed to the hull?

[Phil Ambrose]

 

Can anyone expand on this a little? I'm still confused. (We don't have shoreline)

 

Electricity enters the boat via insulated shoreline, and is then distributed around it via a fusebox. Does the consumer unit not route all the earth cables back through the shoreline supply? I can understand poor wiring leading to a route of least resistance into the water.... but hopefully 240v wiring isn't so shoddy. Where does the current that passes through the hull begin? Is it 'leaking' from insulated cables at levels so small to be inconsequential (in terms of safety) ? Possibly its contaminating the 12V system, which is earthed to the hull?

Oh you have just raised another issue, galvanic corrosion caused by stray 12v DC currents. Crikey, we are surrounded by enemies.

A galvanic isolator protects from both AC and DC.

Phil

[cuthound]

It is caused by the small voltage differences between metals in an electrolyte. Things like the piling, your boat, someone else's boat and the canal water becomes a small battery of 1-2 volts and removes metal from one to the other, in accordance with this chart.

 

http://www.splashmaritime.com.au/Marops/data/less/Shipk/Maint/Galvanic%20Scale.htm

 

That is why boats have anodes, they sacrifice themselves so protecting the steel of your boat.

 

A galvanic isolator, or isolation transformer interrupts these currents, preventing galvanic corrosion.

 

Edited to add a missing letter.

Edited July 14, 2016 by cuthound

[Rendelf]

Thanks Phil, I did greenify you.

 

So, this:

 

The galvanic isolator relies on the fact that electrolysis voltages are quite low - usually less than one volt - whereas electrical failure voltages are quite high. Silicon diodes, which are used to conduct electricity in one direction but block it in the reverse direction, have a built in forward voltage drop of about 0.6 volts. It is not like a resistor voltage drop - no current has to flow to create the drop - so below 0.6 volts it is disconnected, above this it conducts with very little resistance to current flow.

 

The isolator stops small amounts of current from flowing, because such small current draw can only mean that the hull is in the process of electrolysis? (On a 12V only system)

 

 

And this:

 

Boats in a marina plugged into shore power all act as a giant battery. They are all connected together by the green shore power ground wire, which is (or should be) connected to their DC grounds, engine block, and bonded underwater metals. If the boats are in salt water then that forms an electrolyte and the dissimilar metals connected together act as a battery, causing corrosion.

 

 

I wasn't aware that shoreline earths are connected to the engine block. This now makes much more sense.

 

A galvanic isolator, or isolation transformer interrupts these currents, preventing galvanic corrosion.

 

Sanks Cuthound. Does anyone have a stainless boat? (Perhaps only the nobility can afford them)

 

I admit that I was rather unaware of this.... anodes and electrolysis in water, yes.... but I'm uncertain how such an isolator could prevent currents from flowing if it's not installed on a cable that carries voltage. Do you weld it to the hull!?

Edited July 14, 2016 by Rendelf

[bag 'o' bones]

How does a galvanic isolator actually work? Not looked up to be fair

[cuthound]

It is a bridge of diodes, usually 4, two connected in series one way, and two in series the other way.

 

This allows it to block low voltages, such as those resulting from galvanic interaction in both directions, because the voltage is not high enough to switch the diodes on.

 

When you get an ac fault, the diodes switch on and direct the fault current to earth.

 

Often they have a capacitor across them, to shunt high frequency earth currents which result from switch mode power supplies, such as phone chargers etc.

 

Edited to add a missing "y".

Edited July 14, 2016 by cuthound

[Stilllearning]

Boats in Marinas do suffer more erosion of the blacking at the waterline as the concentration of diesel in the water (either from spillages or exhausts) is much higher due to the lack of 'flow' of the water

I had a boat with 5mm sides moored and permanently plugged in to the mains, it only moved about twice in 10 years, and once was to be blacked, so in theory it had next to no protection either from blacking or anodes. When it was sold and surveyed, the steel was still very good. The general opinion in the yard was that it was moored in a thick soup of anode-metal rich water, and my neighbours anodes were doing me a favour.