Bottom Plate Blacking

[NB Willawaw]

This may seem a silly question, but it's a question that I do not know the answer to and I have been dying to ask.

 

When the time comes to black, does one black the bottom plate as well as the sides ? I seem to remember reading something somewhere that said it wasn't necessary, but this may have been in conjunction with the old working boats, not modern nb's.

 

Assuming that you have to, when you drydock, so you paint between the keel supports and then flood the drydock, move the boat a foot or two, and then paint the bits you missed ? (allowing work done to dry of course !)

[dor]

general view is not to bother. There is an argument that the water is less oxygenated, so corrosion is slower. I'm not convinced it make much difference once you get more than a few inches down. THe base plate is usually very much thicker than the sides anyway.

 

More to the point is that it will get worn away, in canals at least, quite quickly.

 

Plus it is a bu**er to do.

[David Schweizer]

More to the point is that it will get worn away, in canals at least, quite quickly.

 

.

<{POST_SNAPBACK}>

 

 

How Quickly ??? When my boat had it's last full hull survey done, the average thickness of the bottom plate was 9.8mm after 15 years and 18,000 hours of scraping it's 2ft 9ins draught around the canal system. Not too much to worry about there.

 

back to the question, the base plate is not normally blacked for the resaons alpready given, but the underside of the counter should be blacked as air gets sucked in at this point so there is quite a lot of oxygen mixed with the water , which will promote corrosion if left unprotected.

[colinjw]

I have to agree with dor on this one.

 

Apart from anything else, the bottom plate gets dragged over all kinds of stuff that any blacking would soon come of.

 

Mark, as regards flooding the drydock and moving the boat a little bit to blacken the missed bits, it may well cost you nearly double because of all the hassle involved. Apart from that the drydock bods may well laugh at you for being so pernicketty.

 

Colin

[DHutch]

We used to paint the under side of emily anne (although we left the bits she was siting on)

 

- The next time it came round to be done(3/4years), id say about 50% of the paint was still on, which i dont think is bad for a 2'8" draught

 

- i think we did it 2/3 time, and then where tooked it wasnt "normal practice" (after we'd just done it again) - the the time after that we didnt bother.

 

daniel

[ChrisPy]

How Quickly ???  When my boat had it's last full hull survey done, the average thickness of the bottom plate was 9.8mm after 15 years and 18,000 hours of scraping it's 2ft 9ins draught around the canal system. Not too much to worry about there.

 

back to the question, the base plate is not normally blacked for the resaons alpready given, but the underside of the counter should be blacked as air gets sucked in at this point so there is quite a lot of oxygen mixed with the water , which will promote corrosion if left unprotected.

<{POST_SNAPBACK}>

We (in the pipeline industry) find that corrosion on coated pipe only occurs where there is a problem with the coating (we call it a 'holiday' ) and then it is extreme point corrosion. That means a deep hole that could penetrate the steel very quickly.

 

so my advice is: if you cannot be assured of being able to maintain a full coating (you can't because it gets rubbed off) then don't do it at all. Better to suffer 0.2mm loss of metal in 15 years than to have no corrosion over 99.9% of the bottom but a hole through the plate at one point.

[Alan Saunders]

We (in the pipeline industry) find that corrosion on coated pipe only occurs where there is a problem with the coating (we call it a 'holiday'  ) and then it is extreme point corrosion.  That means a deep hole that could penetrate the steel very quickly. 

 

so my advice is: if you cannot be assured of being able to maintain a full coating (you can't because it gets rubbed off) then don't do it at all.  Better to suffer 0.2mm loss of metal in 15 years than to have no corrosion over 99.9% of the bottom but a hole through the plate at one point.

<{POST_SNAPBACK}>

I Agree!

 

There was a paper published many years ago relating to car under-seal . The researcher came to the conclusion that if the under-seal was incomplete the unprotected parts would corrode rapidly. Without under-seal point corrosion was compensated by electrolysis and the corrosion was more evenly distributed.

 

My father used to say "The painter took a holiday"!

 

Alan