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Blacking & the relevance of the base plate


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Hello folks

 

I am researching the above, in obtaining quotes most people only quote for blacking of the hull and not the base, if you want the base plate done that is an extra cost and you have to specify that you expressly want it carried out.

 

One marina who has its own hire fleet NEVER blacks the base of their boats and IHO thinks unless you are going to be permantly based in a marina of using rivers it is a waste as it will be scrapped off. So as a commericial/business outlook it is ok to let the boats be totally exposed to corrosion/damage; althogth he does black the hull every year therefore I assume he inspects the base.

 

Whilst the hull is subjected to 'wear & tear' the base is not ammune, very often I can here all sorts of debris being stirred up and hitting it.

 

My own marina I am now guessing from his set up, dry dock very low to the ground, who have quoted will not be doing the base and they never disussed any options; perhaps because he has none with his set up.

 

So it begs the question if and only if the base plate does not need blacking why do we need to do the hull other than for esthetics; are we just putting a surface on to scrape off again?

 

I know there is a lot of previous threads about blacking but I feel the above is enough to get some head scratching.

 

 

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I have always blacked the bottom plate on my narrowboat .When it was surveyed for insurance purposes 25 years after being built the surveyor recorded the following in his report .'' Pitting was noticed on the bottom plate up to 0.4 mm. The bottom had been painted at some time in the past and this lack of pitting is testament to the advisability of this practice. Only minor pitting up to 0.3 mm was noted on the sides,bows and swim .The steelwork is generally in remarkably good condition for its age.''

 

I know many people say painting the bottom plate is a waste of time and money but deep down I think it is because of the difficulty getting under the boat . I will continue to paint mine every two years . The baseplate is 7.5mm thick.

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Its like you say, blacking on the base plate is likely to scrape off, especially in shallow canals, so what most boat yards recommend is to just do the sides. What protects the base plate is usually the crud that has built up over the years so if you scrape this off to apply bitumen and then touch the bottom you will be exposing new steel to begin rusting.

The reason the sides are done is beacuse they will tend to rust more quickly, especially at the water line. This is because at the water line the steel is constantly getting wet, then drying, with plenty of oxygen about, all of this just promotes the rust. Also the importance of the sides being done more than the base plate is beacuse they are usually 6mm, whereas the base plate will be 10mm, so will rust through quicker if not painted.

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I have always blacked the bottom plate on my narrowboat .When it was surveyed for insurance purposes 25 years after being built the surveyor recorded the following in his report .'' Pitting was noticed on the bottom plate up to 0.4 mm. The bottom had been painted at some time in the past and this lack of pitting is testament to the advisability of this practice. Only minor pitting up to 0.3 mm was noted on the sides,bows and swim .The steelwork is generally in remarkably good condition for its age.''

 

I know many people say painting the bottom plate is a waste of time and money but deep down I think it is because of the difficulty getting under the boat . I will continue to paint mine every two years . The baseplate is 7.5mm thick.

 

My experience would suggest that you would have achieved similar result if the bottom was never blacked. Our boat was built in 1982 and the 10mm bottom was never blacked. When it was surveyed last year it revealed a few pits of up to 0.5mm

Edited by David Schweizer
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+1 to David. My Peter Nicholls is 25 years old and at the pre purchase survey the base plate was found to be in good condition. There is no evidence that the base plate has been painted but the boat has spent quite a few years on deeper water - rivers rather than canals.

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Our boat is 25 years old, it has had the sides blacked every 2 years but the baseplate has never been blacked.

 

Although it was in good condition 2 years ago, a problem - presumed to be electrical - had caused severe pitting in the sides over the past 2 years (see this thread). As a result the 6mm sides had extensive pitting up to 4mm deep, yet the 12.6mm baseplate was found to have just a few pits with the deepest being just 1.6mm deep.

 

To me this demonstrates that it would be a waste of time and money to have the bottom blacked.

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As a follow up to the OP's original question, if it isn't considered worthwhile to paint the baseplate, why don't we just paint a 1 foot wide band around the boat at the waterline and not bother with the rest of the side? It seems to be accepted that the main area of corrosion is at the waterline so why do we bother painting the rest of it?

 

I would suggest that there is as much likelihood of scraping paint off the sides either in locks or mooring in canals with shallow sloping sides as there is in scraping paint off the bottom, I mean how many times a year do you fully run aground?unsure.png

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Right up to the end of carrying many boats had thick elm bottoms, pointless in painting them. The 'dry' docks available were largely hopeless to get under a boat so nobody ever did. Boats with all metal hulls were usually on their first bottom by the time that carrying finished, (quite a lot of generalisation here by the way.) I think this is why a 'tradition' of not painting bottoms grew up. In Holland boats were and are hauled out sideways with enough room to get under the boat there was, to my knowledge, no such tradition. My own boat was hauled out in Holland last year, plonked on trestles about 4 feet off the ground and the yard pressure washed the whole underwater part. When it was dry I painted it. If I had asked them not to do the underneath they would have thought I was daft.That is the way it is done in a country with a very long tradition of metal boats. If it gets wet, paint it.

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My boat had the bottom blacked at Fox's at March, because that is the way they do it. I wouldn't like to say how well its done as they do it with the boat on the

trailer.

I have been in Debdale Marina and watched them lift boats for blacking, they also black underneath because they lift them high and can get right under them. One thing I did note was the amount of crap they pressure washed off under the base plate which obviously is not getting scraped off on the bed of the canal.

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My boat had the bottom blacked at Fox's at March, because that is the way they do it. I wouldn't like to say how well its done as they do it with the boat on the

trailer.

I have been in Debdale Marina and watched them lift boats for blacking, they also black underneath because they lift them high and can get right under them. One thing I did note was the amount of crap they pressure washed off under the base plate which obviously is not getting scraped off on the bed of the canal.

 

I emailed the previous owner, lovely guy & very helpful, he said that they used to black it them selves every 2 yrs with Intertuf but the last three times they had it done at boatyards, the last one being Fox's at March, which was last year about 6 mths before they put it up for sale. They always insisted on minimum of 3 coats but admit they were not there to see it done. The surveyor (Craig Allen) told me (& I can only say what he said to me) was that it was a c**p job and would need to be done again this year. The previous owner made a reduction to cover the cost and he also said that they never had the bottom plate blacked. Apart from the blacking there was nothing else significant in Craig's report. The hull & sides still measured between 10mm & 10.1mm & 6mm & 5.9mm respectively and although there was some pitting it was in very isolated spots and measured max 0.3mm on the base and 0.9mm on the hull. Considering the base plate has never been blacked and also no galvanic isolater was ever used athough always based in a marina I think for a 14 year old boat that is pretty damn good report.

 

As a newbie I am tending to overthink matters it is like the galvanic isolater debate, after much researching and consideration I got one, have I wasted my money/time and effort only time will tell but not in my lifetime me thinks.

 

We have had a quote from the marina we are based, hull only, for £1000 but that also includes painting the gunnels with a matt black and repainting the two colours at the stern and the decorative part at the bow. They have a proper set up and I assume, being based at the marina, they would do a good job, plus it means thay can do it whilst we are not there and saves us travelling elsewhere.

 

Difficult one blacking isn't it or is it we are making it difficult.

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1. the baseplate on modern shells is usually 10mm thick, the sides are 6mm. Pitting of say 2mm deep is more significant for the sides than the bottom.

 

2. there is increased corrosion at the waterline and just below it because of oxygen in the atmosphere or free oxygen in the upper layer of water.

 

3. electrolytic corrosion is concentrated at vulnerable locations. The sides are vulnerable where the blacking has been scraped off.

 

4. the sides are protected from electrolytic corrosion by the sacrificial anodes which are only effective where they are 'visible' to the steel - i.e. the sides, not the bottom.

 

5. the bottom will suffer corrosion, to a relatively minor degree. If it is left uncoated that corrosion will tend to be evenly distributed across the entire bottom. If it is coated the corrosion will be concentrated at the locations where the coating has been scraped off.

 

 

QED : it can be counter productive to black the bottom, if the bottom has adequate thickness (i.e. 10mm).

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1. the baseplate on modern shells is usually 10mm thick, the sides are 6mm. Pitting of say 2mm deep is more significant for the sides than the bottom.

 

2. there is increased corrosion at the waterline and just below it because of oxygen in the atmosphere or free oxygen in the upper layer of water.

 

3. electrolytic corrosion is concentrated at vulnerable locations. The sides are vulnerable where the blacking has been scraped off.

 

4. the sides are protected from electrolytic corrosion by the sacrificial anodes which are only effective where they are 'visible' to the steel - i.e. the sides, not the bottom.

 

5. the bottom will suffer corrosion, to a relatively minor degree. If it is left uncoated that corrosion will tend to be evenly distributed across the entire bottom. If it is coated the corrosion will be concentrated at the locations where the coating has been scraped off.

 

 

QED : it can be counter productive to black the bottom, if the bottom has adequate thickness (i.e. 10mm).

Surely that is an argument for not painting the sides as well then (other than a one foot band at the waterline)? unsure.png

Edited by Wanderer Vagabond
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any part that is 'visible' from the anodes should be painted. i.e. all of the sides.

 

look at it another way:-

  • the 12" below the waterline is vulnerable to aerated water.
  • the sides above the waterline are vulnerable to rusting in the damp environment
  • it is a no brainer - black it from the base to the gunwale.
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any part that is 'visible' from the anodes should be painted. i.e. all of the sides.

 

look at it another way:-

  • the 12" below the waterline is vulnerable to aerated water.
  • the sides above the waterline are vulnerable to rusting in the damp environment
  • it is a no brainer - black it from the base to the gunwale.

 

To be honest I'll be following Bee's motto, 'If it gets wet, paint it'. (due for blacking this year)

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As a follow up to the OP's original question, if it isn't considered worthwhile to paint the baseplate, why don't we just paint a 1 foot wide band around the boat at the waterline and not bother with the rest of the side? It seems to be accepted that the main area of corrosion is at the waterline so why do we bother painting the rest of it?

 

 

With that in mind has anyone ever used a rubber strake band rather than (or with) fenders??

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With that in mind has anyone ever used a rubber strake band rather than (or with) fenders??

I think that they tend to be fitted to the Otter aluminium boat, presumably because they are more susceptible to damage, don't really see the point with a steel boat since with steel against brickwork/stonework/concrete (locks) the steel will usually come out the winnerrolleyes.gif

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any part that is 'visible' from the anodes should be painted. i.e. all of the sides.

 

look at it another way:-

  • the 12" below the waterline is vulnerable to aerated water.
  • the sides above the waterline are vulnerable to rusting in the damp environment
  • it is a no brainer - black it from the base to the gunwale.

 

 

I don't know what it is, but you give a perfectly cogent set of engineering reasons why blacking the baseplate is (at best) a waste of time and money, and (at worst) can cause problems of its own, and somebody comes along and says "oh no it isn't"

 

When our boat gets blacked this year the baseplate will be left alone, but the anodes may well need replacing. I know where my money is going to be spent! (20 year old boat, 10mm baseplate).

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I don't know what it is, but you give a perfectly cogent set of engineering reasons why blacking the baseplate is (at best) a waste of time and money, and (at worst) can cause problems of its own, and somebody comes along and says "oh no it isn't"

 

When our boat gets blacked this year the baseplate will be left alone, but the anodes may well need replacing. I know where my money is going to be spent! (20 year old boat, 10mm baseplate).

 

Depending on your cruising waters you may well find your base plate so overgrown with mussels that there's no space for rust anyway. WotEver always had them about three deep completely covering the base plate.

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I don't know what it is, but you give a perfectly cogent set of engineering reasons why blacking the baseplate is (at best) a waste of time and money, and (at worst) can cause problems of its own, and somebody comes along and says "oh no it isn't"

 

When our boat gets blacked this year the baseplate will be left alone, but the anodes may well need replacing. I know where my money is going to be spent! (20 year old boat, 10mm baseplate).

This being a sort of discussion board there are likely to be a variety of opinions I would have said. Each makes their own decisions as to how they may wish to treat their boat, there isn't a 'right' way or a 'wrong' way. You may query people's decisions but it doesn't mean that you or I are right. The opinion of Murflynn I would question is why only 12 inches below the waterline is vulnerable to aerated water? When I go through a lock the aerated water comes in at the bottom of the lock and pretty much aerates all of the water under my boat (or should I say I've never noticed the bubbles stop suddenly at a depth of 12 inchesunsure.png )

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This being a sort of discussion board there are likely to be a variety of opinions I would have said. Each makes their own decisions as to how they may wish to treat their boat, there isn't a 'right' way or a 'wrong' way. You may query people's decisions but it doesn't mean that you or I are right. The opinion of Murflynn I would question is why only 12 inches below the waterline is vulnerable to aerated water? When I go through a lock the aerated water comes in at the bottom of the lock and pretty much aerates all of the water under my boat (or should I say I've never noticed the bubbles stop suddenly at a depth of 12 inches:unsure: )

But what percentage of the boat's life is spent in a lock being surrounded by all this aerated water? 1%? Less?

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But what percentage of the boat's life is spent in a lock being surrounded by all this aerated water? 1%? Less?

Depends on your cruising habits really. Over the past 3 years (since the last blacking) I've been through something over 2200 locks, at say 5 minutes a lock (possibly a bit quickrolleyes.gif ) that'd be 11,000 minutes in locks (180 hours or over 7 days continuous). Sitting in aerated water for 180 hours? might have some effect, possiblyunsure.png

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Advised to blast and 2 pack boat in 2013 due to pitting. Unfortunately due to working abroad and family crisis didn't.2017 lifted boat to do job. After blasting advised 2 pack would be waste of money as bottom was thinner than expected, and would only buy us until next docking when we would spend $$$$ getting it off to plate. Again due to work and family issues ended up overplating with 5 mm steel and then 2 pack. Cost us mega bucks. I think it's worth painting the bottom if there are pits starting because it stops further action.

And no our boat has no mains ring and isn't moored in a marina. It is left 7 months of the year unused though.

Oh and we aren't new to this we have had various boats for 36 years and till this year never painted the base plate. A boat we had in 1984 with a new bottom was rebottomed again in 2008... In spite of gallons of wax oil in the hold.

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If there's enough oxygen down there for a fish to breathe then there's enough to rust away a lump of steel.

Yes, but you will die before it happens.

 

I tend to agree that the primary reason baseplates are generally not painted is because it is difficult and it has become the accepted practice because it isn't particularly important in the wider scheme of things. It is the last part of a boat that should fail through normal corrosion by oxidisation being in the least harsh environment and the thickest piece of plate used. Despite being painted the top of the baseplate is probably more prone to corrosion than the bottom because of the catalysts to corrosion that come into contact with it and the difficulty of maintaining the original painted finish.

 

Pitting seems to be the limiting factor on most boats these days and I believe the pits are generally bright metal which would indicate they are not caused by oxidisation. A proper understanding of what causes pitting and how to prevent it is much more pertinent than worrying about whether to paint the baseplate or not.

 

JP

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