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Residual Plate Thickness


Ambull

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1 minute ago, TheBiscuits said:

 

Gritblasting, (optionally) pits welding up and 2-pack epoxy should be doable for a few grand depending on where you get it done.  If you really like the boat and are worried about the pits this should sort it for the long term.

 

Does the seller have a previous survey you could compare yours to?

Yes there is a survey from two years ago, it states a residual plate thickness of 8.3 mm. So I take it that it hasn’t gotten any worse since then

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Just now, Ambull said:

Yes there is a survey from two years ago, it states a residual plate thickness of 8.3 mm. So I take it that it hasn’t gotten any worse since then

 

In that case, and if the baseplate was 2-packed, a previous owner has sorted it out - I'll bet they fitted the galvanic isolator too.

 

If you like the boat, buy it ASAP. 

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12 minutes ago, Ambull said:

Hi Sea Dog,

 

What made me doubt him was that 20 percent of the baseplate just seems a lot for a relatively newish boat that has been 2 pack epoxied, has sacrificial anodes, galvanic isolator and seems immaculate in every other way

He really ought to have seen more baseplates than most of us and those of us who have seen more baseplates than him haven't seen this one! 

 

I'd be aiming to find out whether you can trust his professional opinion rather than trying to second guess him at the possible expense of losing a boat you like. If you do find you can't trust his opinion, it'd be better to do that check before engaging your next surveyor so that you get one who's opinion you are going to be happy with.

 

Having said that, buying a boat can a nerve-wracking time, so I do totally understand why you're asking the question! I wish you all the best with your decision. :)

 

Edited by Sea Dog
Replaced a so with a do!
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19 minutes ago, LadyG said:

I'd be thinking about installing a galvanic isolator if to be kept in a marina on shore power, its not difficult. 

 

 

Did you read any previous posts ?

 

... a relatively newish boat that has been 2 pack epoxied, has sacrificial anodes, galvanic isolator and seems immaculate in every other way

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Has the hull abd base plate been epoxied FROM NEW, or was it applied after some corrosion was found ?

 

If it was applied subsequent to corrosion being identified it may be worth considering Microbial Corrosion (as mentioned earlier by Tony Brooks)

 

If a hull is found with evidence of microbial attack, it is necessary to deal with it to try to prevent it recurring. A simple solution is for the whole area to be washed with copious amounts of high pressure fresh water. When dry the area affected should be coated with a strong bleaching agent (sodium hypochlorite) diluted 1:4 with water and left for twenty four hours. Afterwards a second high pressure fresh water wash is necessary followed by recoating. This will probably remove around 90% of the microbes but the only real solution is to blast back to bare steel and to treat any inaccessible areas such as tack-welded rubbing strakes as best one can with the bleach solution before applying the next stage of the coating process. The main problem is that the microbes can continue to live beneath the existing paint coatings and once sealed in with a fresh blacking, the lack of oxygen and light is the perfect environment for them to thrive leading to a risk of corrosion from the inside out. No coatings are entirely proof against a microbial attack from the exterior. Minute pinpricks, mechanical damage below the waterline are all opportunities for the microbes to penetrate the steel and commence the process from the outside in..

 

See attached :

 

 

 

MICROBIOLOGICAL-CORROSION.pdf

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Maybe it is 3/8 inch (9.5mm) plate not 10mm. 

 

Personally I would be concentrating a lot more on the condition of the sides below water which are likely to be 6mm or 1/4 inch (6.35mm). 

 

I've not investigated it but I would be incredibly surprised to find canal boats which had failed hulls due to the baseplate when it was 66.7% thicker than the sides. 

 

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Getting out of my pay grade now but I would have thought a good surveyor could  comment on whether the pitting is electrolytic or "microbial" was the epoxy applied to to the baseplate or just the sides if the former did it cover (fill) the existing pitting nicely and prevent further pitting as the 2 year old survey implies in which case if it has halted the deterioration fine good to go. Is the epoxy still in good condition especially in the pits it should be after 4 years and if the pits are full of paint all should be good. I think you need to have a further chat with your surveyor.

We don't know how much this boat is selling for, it is a seller's market at the moment there are going to be a lot of boats out there far worse than this at prices that would a year ago have been ridiculous.

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If, and its a big if, it is microbial corrosion then simple blasting may not be enough. Having prepped the steel it may require a microbe killing wash before coating. It seems to me that many blackers are not up to speed on recognising this form of corrosion or knowing how to deal with it. See Allan's attachments.

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In my opinion this is no reason to not buy the boat, that is still loads of thickness left. However It needs to be stopped and established whether or not its ever been painted properly. Many (most?) builders don't make a very good job of the underneath at all and 'docking' often means painting the sides. If it left the builders with a coat of something black and cheap on it then a subsequent coat - and one coat is not enough - of epoxy probably didn't stick very well. I don't know much about microbial stuff or galvanic thingys but there are very many boats going around with a less than adequate painting history. 

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7 minutes ago, ditchcrawler said:

Ask 100 people and 75 will say don't paint the base plate, its a waste of time there is no oxygen down there. I get mine painted whenever I can

I bought a colecraft in 2001, the boat was built in 1981 of 6 mil baseplate and sides. I bought it from a great bloke who had owned it for 18 years since it was 2 years old and he was honest enough to say he had NEVER had it out or blacked it in those 18 years. I bid him a very low price which he accepted as we both new the steel would be shot somewhere or other due to lack of maintenance. As always I bought without a survey. First trip I did was up to Johny Pinders for an out of water check of the hull. He did a thorough check and the worst bits he found were 5.7 mil. I had him two pack it to keep it near new as it was. The only slight wear was the chines at the arse end and a strip of rubbing strake was welded along for a small length this was where it had ground away on its linear mooring over many years. Weird things are boats.

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16 hours ago, Ambull said:

Yes there is a survey from two years ago, it states a residual plate thickness of 8.3 mm. So I take it that it hasn’t gotten any worse since then

Ah, this changes things, along with the epoxy.

Last owner has already dealt with the problem so buy it.

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18 minutes ago, matty40s said:

Ah, this changes things, along with the epoxy.

Last owner has already dealt with the problem so buy it.

 

 

But has it ?

 

The two surveys, 2 years apart, show that for the latest survey the extent of the 'rash' has doubled from 10% of the baseplate to now 20% of the base plate.

 

I'd suggest that this shows the 'problem' has not been resolved.

 

@Ambull Have it properly checked over (by someone whio knows the various types of corrosion) before buying.

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3 hours ago, ditchcrawler said:

Ask 100 people and 75 will say don't paint the base plate, its a waste of time there is no oxygen down there. I get mine painted whenever I can

Frankly, I don't paint mine, but you're not wrong - there's more than the odd fish or two who would agree that there's is still oxygen at even more than 3 feet down!

 

(We can agree we shouldn't paint fish, but for entirely different reasons!)

Edited by Sea Dog
Missed a than.
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16 hours ago, Ambull said:

Yes there is a survey from two years ago, it states a residual plate thickness of 8.3 mm. So I take it that it hasn’t gotten any worse since then

 

16 hours ago, TheBiscuits said:

In that case, and if the baseplate was 2-packed, a previous owner has sorted it out - I'll bet they fitted the galvanic isolator too.

 

If you like the boat, buy it ASAP. 

 

Hypothesis 1: first owner neglected it, second owner fixed it up with reasonable measures, third owner lost the history or isn't sharing?

 

2 minutes ago, Alan de Enfield said:

The two surveys, 2 years apart, show that for the latest survey the extent of the 'rash' has doubled from 10% of the baseplate to now 20% of the base plate.

 

I'd suggest that this shows the 'problem' has not been resolved.

 

Hypothesis 2 (weaker because I don't know how surveyors do this): this 10% / 20% number is rather ballpark. If it were mine I would be giving it more attention, maybe with photos?

 

@Ambull If you don't buy it and it's 55~60ft NB, please PM me. Maybe I can... share the cost of your survey with you, or something?

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7 minutes ago, Alan de Enfield said:

 

 

But has it ?

 

The two surveys, 2 years apart, show that for the latest survey the extent of the 'rash' has doubled from 10% of the baseplate to now 20% of the base plate.

 

I'd suggest that this shows the 'problem' has not been resolved.

 

@Ambull Have it properly checked over (by someone whio knows the various types of corrosion) before buying.

I will do that Alan, thank you

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5 minutes ago, wakey_wake said:

Hypothesis 2 (weaker because I don't know how surveyors do this): this 10% / 20% number is rather ballpark. If it were mine I would be giving it more attention, maybe with photos?

Not a daft idea - a further surveyor might have said 15%. Subjective. If you aren't happy to judge it yourself, best to get a surveyor you trust... and trust him.

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