Does my baseplate need gritblasting?

[RiverRo]

Looking for a bit of advice...I’m looking at buying a narrowboat which comes at a good price, however the survey recommends gritblasting and painting the baseplate, because apparently it is thinner than usual and has ‘widespread pitting’ There seems to be two schools of thought when it comes to baseplate maintenance from what little research I’ve done, ie painting or leaving as is because of thickness/lack of oxygen etc.  I would prefer not to have to blast and paint it if I didn’t have to because of the expense.

 

Here’s an excerpt from the survey for more details....

 

‘The base plate had a nominal thickness of 8mm, but point thickness measurements indicated some diminution. The majority of readings were above 7mm, the lowest reading being 6.1mm.
Widespread pitting, not uncommon on narrowboat base plates, was widespread, resulting in shallow craters with an estimated depth of 0.5 – 1mm, but no loss of structural strength was detected with hammer testing.

 

1. Given the comparatively thin base plate, grit blasting and applying a protective paint coating would greatly reduce the chance of serious diminution in the future. However, boatyards are reluctant to take on this work due to the limited access beneath the hull.

 

[roland elsdon]

Yes .boats rust from both sides , when you sell it will be worse.

[Alan de Enfield]

I wouldn't bother.

At 6mm thickness it still has more metal than most boats built in the 80s had to start with.

Many, many boats with 6mm bottoms from new are still floating, as will yours be in 10 - 20 - 100 years time.

 

Every year at the 'surveyors symposium' they look for a 'failure / recommendation of the year' for them all to report on.

It is currently "painting the bottom".

Edited August 28, 2019 by Alan de Enfield

[dor]

How old is the boat?  If it has lost 2mm in say 20 years, it has a long way to go before it becomes critical.

[RiverRo]

She’s 30 years old this year 

[Alan de Enfield]

Just now, RiverRo said:

She’s 30 years old this year 

In which case you may have to do whatever your insurance company demands - they tend to insist on surveyors recommendations.

 

Alternatively insure it 3rd party only when no survey or 'thickness' requirements are required.

[BEngo]

Getting it blasted and a good two pack epoxy coat properly applied will ensure you can forget about it for at least 10 years.  After that it may need another blast, but more likely damage repair work here and there followed by a recoat.

 

Otherwise your guess is as good as mine. Eventually it will need overplating, but whether that will be in 5 years, 10 years or 15 no one can say.

 

Blasting is quite expensive, and the benefits only arrive if you hang onto the boat, as no one pays a premium for a SH epoxy painted hull. Keep it 5 years and I think it is worth it, just.  At 10 years to me it is well worth it.

YMMV.

 

N

[RiverRo]

Thanks for all the info. Reckon I’ll just get her blackened for now and see where we are in a few years as unsure whether we’ll be in her long term 

[Sea Dog]

3 minutes ago, RiverRo said:

Thanks for all the info. Reckon I’ll just get her blackened for now and see where we are in a few years as unsure whether we’ll be in her long term 

You ask a tough question to which there is no agreed answer. That said, I'd hazard a guess that most folk don't even black the base plate. They either believe that being a couple of feet or so below the surface means that any corrosion will be so slow as to be negligible, or else think that ensuring all the base plate is coated on a boat sitting on sleepers is simply too difficult for a disputed reward. Others do go the extra mile, but many think whatever is applied gets scraped off anyway on shallow canals.  Yer pays yer money...

[nebulae]

I had a boat grit blasted underneath,many years ago. The boat was 10 years old, The grit blasting went through the bottom in one place. I think the bottom was 6mm from memory.

[roland elsdon]

Yep.

we were advised to grit blast. ( was our 5 th boat and we had never bothered before.

Due to circumstances left it 3 years without blasting. (It was 6mm with pits) Blasted it , re measured- down to 5.5 in places with 1.5 mm pits.

That meant it was 4 mm and would need overplate re bottom in a few more years ‘wasting’ the griblast and two pack .

£10000 later... we had a new 2 packed baseplate.

its 12 mm thick in places.

As an open boat the bottom is pitted from the inside which makes it more risky, but do you know there is good bilge protection on what you are buying.

 

metal thickness testers are also notoriously unreliable. They are ok on new plate but not on old. 

On my last survey the plate measured 4 mm in one place. I didnt believe the tester. One big hammer blow later and it was 0 mm

I think the real value of blasting and two pack is it kills the active rust and fills the pits with paint

[rusty69]

Ours is 6mm with pits. I recently asked our surveyor about epoxy coating v saving the money for an overplate. He suggested not to bother with the epoxy. 

[MtB]

On 28/08/2019 at 11:19, BEngo said:

Getting it blasted and a good two pack epoxy coat properly applied will ensure you can forget about it for at least 10 years. 

 

I'd suggest you can still forget about it for ten years even if you don't do anything to it....

 

 

[blackrose]

11 hours ago, nebulae said:

I had a boat grit blasted underneath,many years ago. The boat was 10 years old, The grit blasting went through the bottom in one place. I think the bottom was 6mm from memory.

 

Well it obviously wasn't 6mm where the grit blasting penetrated it!

 

If you'd lost 4 or 5mm thickness from your baseplate in 10 years something wasn't right.

[blackrose]

11 hours ago, roland elsdon said:

Yep.

we were advised to grit blast. ( was our 5 th boat and we had never bothered before.

Due to circumstances left it 3 years without blasting. (It was 6mm with pits) Blasted it , re measured- down to 5.5 in places with 1.5 mm pits.

That meant it was 4 mm and would need overplate re bottom in a few more years ‘wasting’ the griblast and two pack .

£10000 later... we had a new 2 packed baseplate.

its 12 mm thick in places.

 

 

12mm in places? You're not making the mistake of adding the new plate to the old and quoting the combined figure are you?

 

If a hull is overplated then the original plate was too thin in places and should be disregarded. It's only the new plate that counts.

Edited August 30, 2019 by blackrose

[MtB]

6 hours ago, blackrose said:

12mm in places? You're not making the mistake of adding the new plate to the old and quoting the combined figure are you?

 

Roland can speak for himself but IIRC, Fenny is a town class grand union boat which he had re-footed and re-bottomed seven or eight years ago at enormous expense. I rather doubt it was overplated. 

 

I suspect the "12mm in places" is an acerbic comment meaning the new baseplate started life at 12mm and most of it probably still is.

 

 

[roland elsdon]

It was overplated actually. We had no choice. What was supposed to be 10 day grit blast and 2 pack became 3 week overplate. It was at an appalling time . Just enough time post work to return to mooring then back overseas to work to pay for it.

12 mm was what the stupid tester came up with on the  1963 doubled front plate.

they are not reliable. A drill hole got it to 6mm..

[MtB]

11 minutes ago, roland elsdon said:

It was overplated actually. We had no choice. What was supposed to be 10 day grit blast and 2 pack became 3 week overplate. It was at an appalling time . Just enough time post work to return to mooring then back overseas to work to pay for it.

12 mm was what the stupid tester came up with on the  1963 doubled front plate.

they are not reliable. A drill hole got it to 6mm..

 

Apologies for getting it wrong Roland. I knew there would be more to your comment about 12mm than face value though! 

 

 

 

 

[roland elsdon]

I think the real issue is people examine from the outside and measure pits from one side. They forget ( or avoid ) the issue of internal corrosion. Any boat wet at any time internally will rust from the inside. I have taken mms of scale from hull bottoms inside cabins.

many builders flashed over a coat of oxide on hull internals. That doesnt seal the steel. Condensation water pump leaks shower leaks are all enough to cause internal rotting

when the invisible pit from the inside meets the visible but unpainted pit from the outside the result is inevitable.

 

With our hull now overplated on the bottom my attention is keeping the top surface ie the old bottom dry and sealed.

[nebulae]

On 29/08/2019 at 15:32, nebulae said:

I had a boat grit blasted underneath,many years ago. The boat was 10 years old, The grit blasting went through the bottom in one place. I think the bottom was 6mm from memory.

Cant say whether there were any special circumstances which caused the problem,as I had only just bought the boat. The anodes were O.K. I was told that it may have been a piece of carbon grit rolled into the plate?  The bottom was badly pitted all over.  Has anybody ever had a boat sink because the bottom plate has rusted through? Maybe we worry to much?

[RiverRo]

Thanks for all the suggestions andy sorry to hear of the horror stories...to update, we’ve found the money to get the grit blasting and epoxy, figured it’s the safest bet....presumably if any holes do appear after blasting they would have rusted through eventually anyway, and the idea of over plating doesn’t sound too appealing, now or further down the line....presumably any holes that did appear would be noticed and welded? Or would this require over-plating? (we’re going with a well reviewed boat yard so presumably they’d check steel integrity after blasting, but I will make sure to talk to them about it) The hull survey does mention evidence of rusting from the inside suggesting there were issues there that have since been dealt with, as it now appears dry... If we were to sell the boat down the line I think a history of epoxy coating would satisfy any buyer doubts (?) Also, we’re moving the boat to Ireland so I’m not sure we’d have the same level of boatmanship over here when it comes to narrowboat maintenance, so probably best to get it done before shipping 

[Sea Dog]

On 30/08/2019 at 19:00, nebulae said:

Has anybody ever had a boat sink because the bottom plate has rusted through? Maybe we worry to much?

There was someone posting on here a couple of years ago who had his boat grit blasted and epoxied  in a marina up Cheshire way. The boat was returned to his mooring by the company and it sank overnight. The grit blast had apparently left it like a pepper pot and they'd llegedly painted over it. Sordid business that I believe did not end well with the insurance either iirc. 

[nebulae]

On 30/08/2019 at 19:00, nebulae said:

Cant say whether there were any special circumstances which caused the problem,as I had only just bought the boat. The anodes were O.K. I was told that it may have been a piece of carbon grit rolled into the plate?  The bottom was badly pitted all over.  Has anybody ever had a boat sink because the bottom plate has rusted through? Maybe we worry to much?

The holes in the bottom of my boat were repaired by localised welding. Worked all the time I had the boat. Less drastic than plating the bottom and a lot cheaper