Hull Pitting

[snaps]

Hello,

 

I have been concerned by the amount of pitting on my 53 ft Narrowboat, and it is due to come out again soon for blacking.

 

The pitting puzzles me as I have had it in 3 different locations in the 5 years of its life from new, the last 2 being an on line mooring where the boats near to me never move and have no one living on them, they may have the electric connected for battery charging only. I use the boat most weekends for cruising (not live aboard) where I run 12v and 240v permanently over that weekend. When moored up during the week the boat is not connected to a landline and everything is off.

 

When I black the boat I prepare it well, 3 coats of international bitumin with a brush, I have had luck with the weather and have it out a week to dry.

 

I have noticed the bolt on anodes have not gone down much (still on the same from new) could the fact they are bolt on mean they have a bad connection to the hull?

 

Do I need to cross bond the 240 and 12v? which I have heard about.

 

Will it get less with the eventual build up of paint over the years.

 

Should I get a galvanic Isolator?

 

Any help appreciated.

 

Regards

 

 

Si

[sarahavfc]

Get the hull gritblasted and 2-packed.

[Ex- Member]

Get the hull gritblasted and 2-packed.

 

Was going to say the same. Bitumen is weak at the waterline and with a lot of mooring in a marina it's not going to last long.

[Grace and Favour]

Si - -

 

What others above said - - - -

 

But I would definitely get a galvanic isolator - or - to do the job properly - an Isolation Transformer!

 

Link

[Timleech]

Si - -

 

What others above said - - - -

 

But I would definitely get a galvanic isolator - or - to do the job properly - an Isolation Transformer!

 

Link

 

They say the boat is not (usually) connected to a land line, neither of those is of any use unless so connected.

 

Tim

[Grace and Favour]

They say the boat is not (usually) connected to a land line, neither of those is of any use unless so connected.

 

Tim

I'd missed that

[Guest]

I believe that weld on anodes are best. My previous boat had bolt on ones, and I wasn't impressed.

 

I have spoken with the people who run our local paint shed, who black many boats in a year. They tell me there is sometimes no rhyme nor reason as to why some boats "pit" more than others, but that shorelines seem to be a common denominator.

[snaps]

Hello

 

Thanks for the replies

 

How often should the Anodes be replaced.... I would say my bolt on ones are about 30% reduced in 5 years?

 

Si

[Guest]

Hello

 

Thanks for the replies

 

How often should the Anodes be replaced.... I would say my bolt on ones are about 30% reduced in 5 years?

 

Si

There is no easy answer really. My weld on ones needed replacing after 6 years which were about 50% spent. There are so may variables.

[Timleech]

Was the hull painted with the millscale still on the steel? If so, that may be part of your problem. Apart from providing a very poor surface for painting (shiny and hard), once it starts to come off (which it will) there can be some sort of reaction between the bare steel and the remaining millscale, which leads to pitting.

 

Tim

Edited April 19, 2013 by Timleech

[snaps]

Tim,

 

I think you have hit the Nail on the heat there it was left with the scale on....... and then a Kamastic put on top.

 

I have since put International Bitumin on top which you can do but not the other way round.

 

The only cure would be sanblast I guess, I may use some etching type primer on the bare metal next time though see how that goes if not I will then plan for a sand sblast next time.

Si

[peterboat]

Hi why didnt you stay with comastic? it is much more durable and isnt effected by diesel etc. There wasnt much damage on the waterline on my boat last time it was out and this year I am hoping for the same. But I would go for grit blasting and two pack or zinger coating if it is still available. Are your moorings next to steel pilings? if so move the boat always ends up being anode!! or it does here.

 

Peter

[ditchcrawler]

Si - -

 

What others above said - - - -

 

But I would definitely get a galvanic isolator - or - to do the job properly - an Isolation Transformer!

 

Link

Tim beat me to it

Edited April 19, 2013 by ditchcrawler

[blissbee]

hello, i am selling a boat and the survey is back and the buyers are saying that it will need to be sand/ grit blasted and epoxy painted as she is pitted and more than expected for a boat of 2.5 years old. What causes pitting? What is the best option to remedy it, ensure the boat doesn't deteriorate rapidly? And is epoxy the same thing as 2pack?

 

It has just recently been cleaned, scraped and blacked with 2 coats of bitumen. she's a 45 ft widebeam. Can you tell me also i) a rough cost for sand/ grit blasting? ii) a rough cost for epoxy 2 pack? i am trying to get a sense of what work is needed and what I can allow for when we re-negotiate the price.

 

Thank you!

[Chris Pink]

It doesn't 'need' grit blasting and epoxy ( yes, 2 pack). This is an option that you don't need to pay for. There may be remedial work needed for the pitting but epoxy is NOT that. It's a separate issue.

[alan_fincher]

Are your moorings next to steel pilings? if so move the boat always ends up being anode!! or it does here.

 

How does that work then?

Edited April 19, 2013 by alan_fincher

[matty40s]

How does that work then?

something to do with an Antony M fitout...

[Guest]

How does that work then?

It is a curious one, but I have heard it said that mooring next to pilings can affect a steel hull. My previous boat was moored next to pilings for some time, and was (still is) fine. I did used to keep tyres between the boat and the pilings.

[alan_fincher]

It is a curious one, but I have heard it said that mooring next to pilings can affect a steel hull. My previous boat was moored next to pilings for some time, and was (still is) fine. I did used to keep tyres between the boat and the pilings.

For some reason each time these topics re-emerge I struggle to remember previous arguments.....

 

(OK, I accept it is probably an age related thing!)

 

I can see that if your boat is on a land-line, and hence electrically connected to earth, that issues are possible.

 

Where I'm struggling is to understand how anything can happen in the case described here, where the boat spends most of its time not connected to anything.

 

If someone has an explanation why they believe a steel boat can be damaged by being moored against piling, when it is not connected to a shoreline, I'd be grateful to be reminded of it!

[Guest]

For some reason each time these topics re-emerge I struggle to remember previous arguments.....

 

(OK, I accept it is probably an age related thing!)

 

I can see that if your boat is on a land-line, and hence electrically connected to earth, that issues are possible.

 

Where I'm struggling is to understand how anything can happen in the case described here, where the boat spends most of its time not connected to anything.

 

If someone has an explanation why they believe a steel boat can be damaged by being moored against piling, when it is not connected to a shoreline, I'd be grateful to be reminded of it!

Like you, I struggle to understand it. I believe Gibbo has commented on this in the past, but can't remember what his views were.

 

I do wonder how much of hull corrosion is of chemical nature though. I would guess that many canals are a cocktail of nasty chemicals, stirred up by motor boats.

[Mat B]

If someone has an explanation why they believe a steel boat can be damaged by being moored against piling, when it is not connected to a shoreline, I'd be grateful to be reminded of it!

 

My mooring has galvanised scaffold poles so similar to pilings, there are fenders between boat and scaffold poles.

 

Two different metals in an electrolyte form a cell. (Mild steel hull, galvanised steel poles.)

 

A multimeter shows a voltage exists between the boat hull and the poles, therefore if the boat was allowed to touch the poles a current would flow and corrosion would take place below the water line.

 

m@

[Grace and Favour]

My mooring has galvanised scaffold poles so similar to pilings, there are fenders between boat and scaffold poles.

 

Two different metals in an electrolyte form a cell. (Mild steel hull, galvanised steel poles.)

 

A multimeter shows a voltage exists between the boat hull and the poles, therefore if the boat was allowed to touch the poles a current would flow and corrosion would take place below the water line.

 

m@

Can the water not carry (an electric) current?

[Chris Pink]

There needs to be another connection to make the circuit. Eg a landline.

 

So keeping them separate doesn't in itself prevent corrosion. Especially as any connection would be high impedance and not good contact.

 

If I was paying proper attention, Gibbo, agreed with by Idle Loddon, said that a boat setting up a galvanic circuit with a metal pontoon could involve another boat in close physical proximity even if not electrically connected. Whether or not this gainsays the use of a transformer / GI I'm not sure.

[Mat B]

Can the water not carry (an electric) current?

 

Yes - the metals in the water are creating the voltage; so the circuit is through the water, the mooring, the boat and back to the water.

 

 

 

There needs to be another connection to make the circuit. Eg a landline.

 

So keeping them separate doesn't in itself prevent corrosion. Especially as any connection would be high impedance and not good contact.

 

 

True, but keeping them separate must help. The resistance of any connection will of course depend on paint, etc.

 

 

 

If I was paying proper attention, Gibbo, agreed with by Idle Loddon, said that a boat setting up a galvanic circuit with a metal pontoon could involve another boat in close physical proximity even if not electrically connected. Whether or not this gainsays the use of a transformer / GI I'm not sure.

 

Several boats and pontoons in proximity will set up a very complex situation with steel, zinc, bronze and other metals, currents may be flowing all over the place.

 

A transformer / GI breaks the circuit between the boat hull and the earth spike associated with the mains supply.

 

m@

[smileypete]

Like you, I struggle to understand it. I believe Gibbo has commented on this in the past, but can't remember what his views were.

 

I do wonder how much of hull corrosion is of chemical nature though. I would guess that many canals are a cocktail of nasty chemicals, stirred up by motor boats.

 

Possibly a marina could be much worse, going back to your earlier comment marinas could be a common factor.

 

I think a galvanic isolator with status monitor is a good middle ground together with sufficient anodes, next up from that a decent grit blast and 2 pack job.

 

cheers, Pete.

~smpt~