bottom blacking?

[luctor et emergo]

Just read a post, in which the poster stated that the bottom of the hull was not blacked. This throws a few questions up.

 

Is that normal (and if not, something I wondered about before, how would you black those bits of the bottom on which the boat rest when out of the water..?)

 

If it is, why? Why black the sides, but not the bottom?

 

Does any blacking on the bottom wear off because of grounding?

[GSer]

Just read a post, in which the poster stated that the bottom of the hull was not blacked. This throws a few questions up.

 

Is that normal (and if not, something I wondered about before, how would you black those bits of the bottom on which the boat rest when out of the water..?)

 

If it is, why? Why black the sides, but not the bottom?

 

Does any blacking on the bottom wear off because of grounding?

 

 

I never black the bottom plate, but I understand that if you do you only paint the bits that are available then next time hope for supports in a different position.

 

I take a good look at my bottom whenever possible but I've seen little evidence of wear or corrosion

 

Paul

[bottle]

Just read a post, in which the poster stated that the bottom of the hull was not blacked. This throws a few questions up.

 

Is that normal Apparently so(and if not, something I wondered about before, how would you black those bits of the bottom on which the boat rest when out of the water..?)I know of one place that will black the bottom, when they do they wait for the blacking to dry, lift the boat and move it, to enable blacking the bits they missed

 

If it is, why? Why black the sides, but not the bottom? Many? theories but one is that it is unnecessary as the oxygen level that deep is reduced, so corrosion will/may not happen. From the little I know (and it is very little) most corrosion happens at the water-line

 

Does any blacking on the bottom wear off because of grounding?Yes it will scrape off if you ground, just as it will on the sides.

[WotEver]

"Yes it will scrape off if you ground, just as it will on the sides."

 

Grounding's great for de-musseling though

[ditchcrawler]

"Yes it will scrape off if you ground, just as it will on the sides."

 

Grounding's great for de-musseling though

The one I saw up in the air at Debdale (the only place I know that does it) had plenty of mussels living on it and they didn't get scraped off on the bottom.

They charge £9 50/foot + vat for sides and bottom black, I am just not sure how long they keep out of the water after the last coat of paint.

[luctor et emergo]

If it is, why? Why black the sides, but not the bottom? Many? theories but one is that it is unnecessary as the oxygen level that deep is reduced, so corrosion will/may not happen. From the little I know (and it is very little) most corrosion happens at the water-line

 

If that is the case, it seems un-neseccary to remove the whole boat from the water for blacking. Adding weight to one side should give enough lean to black an area along the waterline?

[Madmark62]

If that is the case, it seems un-neseccary to remove the whole boat from the water for blacking. Adding weight to one side should give enough lean to black an area along the waterline?

 

I have seen that done, he attached some heavy duty webbing to the far side and pulled the boat over to one side and then hung over the gunwhales and blacked the side. Thn next day he came back and repeated the process on the other side. Scary to watch, but fascinating at the same time

[Guest wanted]

I think the oxygen theory is about right, a friend of mine is an agricultural engineer with the emphasis on agricultural! He keeps his steel under water to stop it going rusty! He uses it quickly but it seems to work.

[boots]

First of all I am no expert, and as a newbie have virtually no experience...but...

 

During our search for a narrowboat we have had two separate surveyors advise that, in their experience, blacking the bottom is a very good idea....

 

Our boat is 3 years old, and the base plate has not (yet) been blacked - at the survey there was no evidence of any pitting - however, we previously walked away from another boat (built 1989) due to pitting on the base plate (If I recall correctly it was 3mm pitting in 8mm plate).

 

I accept that it is easy to 'scrape-off' the blacking from the bottom, and that it doesn't appear to corrode down there as much as the sides - but considering how important the hull condition is to the value/life of a boat & our piece of mind, we will be getting it blacked (sides & bottom) in the near future - that's just our choice.

[Lady Muck]

I never black the bottom plate, but I understand that if you do you only paint the bits that are available then next time hope for supports in a different position.

 

I take a good look at my bottom whenever possible but I've seen little evidence of wear or corrosion

 

Paul

 

I haven't bothered either - no corrosion or wear, the only pitting we've ever had was on the sides.

[alan_fincher]

I accept that it is easy to 'scrape-off' the blacking from the bottom, and that it doesn't appear to corrode down there as much as the sides - but considering how important the hull condition is to the value/life of a boat & our piece of mind, we will be getting it blacked (sides & bottom) in the near future - that's just our choice.

I think there is in many quarters an acceptance that it's better "blacked" than "not blacked", and apparently if you do, you can expect to still find a lot of it there the next time, (although most loss will doubtless have been near the sides, where it's most needed).

 

The problem is many yards do not dock or pull out boats in a way that makes it possible.

 

Most dry docks offer little space under the docked boat, and in most it still has a lot of water accumulated there or running through - not usually somewhere you would want to lie down for hours.

 

Our old marina engineer pulls them up a ramp on wheeled trolleys, but there's not room to get under when he has.

 

Finding somewhere that gets them high enough off the ground, (which I suspect will usually mean craning), that you can readily work under them will be the issue.

 

It's almost inevitable that if you do this costs will be far higher, and you are unlikely to find it at the places offering cheaper blacking, (whether you do the work, or they do the work).

 

I guess it's all about what you might spend extra over the years, versus any extra longevity you got on the base before it needed any plating.

[Machpoint005]

I think the amount of pitting is very dependant on the chemical content of your 'home waters'.

 

However, most corrosion does occur near the waterline (not a foot and a half under it) so that's where you need the protection of the blacking. We don't black the baseplate. Last time the hull was done, we used the slipway at Red Bull Services, and you can't get under the baseplate there anyway. But the Macc and the PFC are so shallow that it'd only get scraped off again, so I'll not be losing any sleep over my nicely polished bottom!

[blackrose]

The more paint you can get on your hull the better. The real reason that most people/yards don't black the baseplate is that it's bloody difficult and there will always be areas that the boat is resting on that will stay unblacked unless the boat is moved again. The oxygen theory is correct to a degree - there is less oxygen at 2' or 3' deep than near the surface, but there is still oxygen. If there isn't how did my unpainted basepate get rusty? They do rust, whatever people may say to the contrary. The idea that you will scrape the baseplate so it's not worth painting really depends which canal you are on. If you followed that rationale to its logical conclusion you might not paint the sides either...

Edited March 31, 2010 by blackrose

[mayalld]

I think there is in many quarters an acceptance that it's better "blacked" than "not blacked", and apparently if you do, you can expect to still find a lot of it there the next time, (although most loss will doubtless have been near the sides, where it's most needed).

 

The problem is many yards do not dock or pull out boats in a way that makes it possible.

 

Most dry docks offer little space under the docked boat, and in most it still has a lot of water accumulated there or running through - not usually somewhere you would want to lie down for hours.

 

Our old marina engineer pulls them up a ramp on wheeled trolleys, but there's not room to get under when he has.

 

Finding somewhere that gets them high enough off the ground, (which I suspect will usually mean craning), that you can readily work under them will be the issue.

 

It's almost inevitable that if you do this costs will be far higher, and you are unlikely to find it at the places offering cheaper blacking, (whether you do the work, or they do the work).

 

Well, at this point, billh could pop his head up and say that HE certainly knows of one of the less expensive places on the network where you can get underneath.

 

It's a sideslip, with about 4 feet headroom, and very reasonably priced for a week (was £100 +VAT 2 years ago)

[Lady Muck]

Hmm, we might do ours next time - we were in drydock a day late and it rained every day so we didn't bother, ran out of time.

 

I was pleased with the state of the hull, though, we were worried we'd have more pitting. There was quite a bit when we bought the boat, but nothing since. The boat had originally had 2 pack epoxy treatment, but it had been applied on a brand new hull that clearly hadn't been prepared properly and the whole lot came off when we pressure washed for the survey! The vendor went mad (not with us, with the boat builder!). We removed the last traces and reblacked again (this was four years ago), also adding extra anodes, following surveyors comments. All this when the boat wasn't even ours, we figured it was worth the risk rather than put the boat back in bare and have to crane out again once she was ours.

 

I'm now wondering if the pitting was due to poorly applied epoxy combined with too few anodes and being moored very close to iron pilings? Since then we got doubled up tyres so the hull isn't so close to the pilings, extra anodes and new blacking. Seems to have worked.

 

We don't have a galvanic isolator, but it seems to me now that this didn't matter anyway as the hull is sound?

Edited March 31, 2010 by Lady Muck

[gazza]

The more paint you can get on your hull the better. The real reason that most people/yards don't black the baseplate is that it's bloody difficult and there will always be areas that the boat is resting on that will stay unblacked unless the boat is moved again. The oxygen theory is correct to a degree - there is less oxygen at 2' or 3' deep than near the surface, but there is still oxygen. If there isn't how did my unpainted basepate get rusty? They do rust, whatever people may say to the contrary. The idea that you will scrape the baseplate so it's not worth painting really depends which canal you are on. If you followed that rationale to its logical conclusion you might not paint the sides either...

 

look up differential aeration corrosion.

 

the reason steel corrodes more at the waterline is because this area has LESS oxygen present than the steel above it. it becomes anodic and will corrode faster than the steel above or below it.

 

there can only be one anodic region.

 

differentail aeration is a corrosion process that is common in all sorts of application. underground pipes, car bodies, vessel holding or submerged in water etc etc. the less oxygen is a good thing is a total red herring. it is the very fact oxygen is not pressent that promotes corrosion at a specific region.

Edited March 31, 2010 by gazza

[Grace and Favour]

look up differential aeration corrosion.

 

the reason steel corrodes more at the waterline is because this area has LESS oxygen present than the steel above it. it becomes anodic and will corrode faster than the steel above or below it.

 

there can only be one anodic region.

 

differentail aeration is a corrosion process that is common in all sorts of application. underground pipes, car bodies, vessel holding or submerged in water etc etc. the less oxygen is a good thing is a total red herring. it is the very fact oxygen is not pressent that promotes corrosion at a specific region.

 

 

Soooo - - - can I ask a question please?

 

Supposing the draught on a boat was 50" . . and, in order to reduce corrosion at the water-line I gave the hull from 6" above to 20" below (the water-line) an extra six coats of blacking/epoxy/whatever . .

 

Would that mean the area to suffer anodic reaction would "move" to be without the extra painted area?

Edited April 1, 2010 by Grace & Favour

[ditchcrawler]

I have seen that done, he attached some heavy duty webbing to the far side and pulled the boat over to one side and then hung over the gunwhales and blacked the side. Thn next day he came back and repeated the process on the other side. Scary to watch, but fascinating at the same time

A chap on our moorings use to do that. He was moored at the top of the arm, Moored to one side and webbing lorry strop to the far bank and healed it over. Turned boat round and did other side.

Another chap on the mooring puts shear legs over the bows and lifts the front half, then repeats it at the stern.

[blackrose]

look up differential aeration corrosion.

 

the reason steel corrodes more at the waterline is because this area has LESS oxygen present than the steel above it. it becomes anodic and will corrode faster than the steel above or below it.

 

there can only be one anodic region.

 

differentail aeration is a corrosion process that is common in all sorts of application. underground pipes, car bodies, vessel holding or submerged in water etc etc. the less oxygen is a good thing is a total red herring. it is the very fact oxygen is not pressent that promotes corrosion at a specific region.

 

Ok, I'll accept what you say without looking it up myself - it's not my area of expertise. My point was that I know from experience it all corrodes to some degree wherever it is in the water.

[billh]

Well, at this point, billh could pop his head up and say that HE certainly knows of one of the less expensive places on the network where you can get underneath.

 

It's a sideslip, with about 4 feet headroom, and very reasonably priced for a week (was £100 +VAT 2 years ago)

OK.... head popping up. Will 5 per cent commission cover it Dave?

Scraping and painting the bottom of boats is a complete pain (literally) even with a reasonable headroom- very few folk opt to do it . We can jack the boat up off the stands ( actually the slipway trolleys ) so that all the bottom can be done, without moving the boat more than about 3inches vertically for the real masochist boater. I am minded to say strictly DIY , not something we would like to do at (almost) any price! Sorry if it sounds like an advert , more like trying to put people off painting their (boat) bottoms. Of course, if you have a wooden boat, it won't need paint underneath, just some nice gentle caulking.

If you think you can handle a 9inch angle grinder and/or needle scaler over your head long enough to clean off 400 square feet of steel bottom , then go for it!

Bill