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Hull overplating?


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Hi all,

 

I know a fair amount about boats in general - been owner of several fibreglass sailboats over the last 12 or so years, do all own work/maintenance on them, etc. but know nowt to speak of about steel hulls so would welcome any advice re buying a boat that's been overplated at some time. One concern I'd have would be the possibility of water between the original plate and the newer if the welding wasn't 100% and how do the devices that are used to measure hull thickness deal with overplating, e.g.

 

All other things being equal would you choose the older boat with what looked to be a viable, well maintained hull or the one that had been overplated?

 

Any and all advice welcome and thanks in advance...

 

Dave

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6 minutes ago, Crewcut said:

All other things being equal would you choose the older boat with what looked to be a viable, well maintained hull or the one that had been overplated?

 

All other things won't be equal.

 

A well maintained older hull will always be more desirable and hence more expensive than an identical fitout in an overplated hull.  Both are more valuable than an identical fitout in a knackered, not overplated hull.

 

 

 

In practice, you'd be unlikely to find three identical boats with hulls graded as the good, the bad and the ugly but the principle holds!

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11 minutes ago, Crewcut said:

One concern I'd have would be the possibility of water between the original plate and the newer if the welding wasn't 100% and how do the devices that are used to measure hull thickness deal with overplating, e.g.

 

What you will need is a proper marine surveyor who is used to checking overplated steel boats.

 

@Alan de Enfield will likely post the report on a badly overplated boat which sounds to me like it was written by a surveyor with very little experience of correctly overplated boats.  TL;DR it sank in the Thames in a matter of seconds but the people on onboard got rescued.

 

 

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18 minutes ago, TheBiscuits said:

 

What you will need is a proper marine surveyor who is used to checking overplated steel boats.

 

@Alan de Enfield will likely post the report on a badly overplated boat which sounds to me like it was written by a surveyor with very little experience of correctly overplated boats.  TL;DR it sank in the Thames in a matter of seconds but the people on onboard got rescued.

 

 

 

 

No, I've given up worrying - let folk buy what they want, when it sinks it's their money not mine.

You can only repeat yourself so many times before thinking 'why bother'.

 

All they need to do is a search and 100s of relevant threads will appear as if by magic.

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Stoke Boats did a grand job on mine a few years back, I visited often to see what they were doing and how it was going. I'm still afloat...

The previous overplating was a bit shoddy but still lasted over twenty years before failing.

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2 hours ago, TheBiscuits said:

 

What you will need is a proper marine surveyor who is used to checking overplated steel boats.

 

@Alan de Enfield will likely post the report on a badly overplated boat which sounds to me like it was written by a surveyor with very little experience of correctly overplated boats.  TL;DR it sank in the Thames in a matter of seconds but the people on onboard got rescued.

 

 


the boat that sank in the tidal Thames did not sink because the overplaying was bad but due to water getting into vents in the hull , not helped by the weight of the overplating lowering the boat.

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Posted (edited)

Thanks for the replies, pretty much as I thought. @Alan de Enfield - I didn't spot the search facility though did have a good trawl to see if I could find  any recent, relevant threads before I posted.

 

Edit: Now found and read the report, very illuminating indeed...

 

All the best,

Dave.

Edited by Crewcut
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1 hour ago, Crewcut said:

Thanks for the replies, pretty much as I thought. @Alan de Enfield - I didn't spot the search facility though did have a good trawl to see if I could find  any recent, relevant threads before I posted.

 

Edit: Now found and read the report, very illuminating indeed...

 

All the best,

Dave.

 

I'll post another one seeing as you made the effort to look; (posted so you are aware of some of the issues involved and can make you own decision).

 

I would personally not buy an overplated boat, I'd rather buy one that needed overplating and have it done by someone who knows how to do it properly, not jusgt a quick-patch by a local moble welder and just to get the boat so it can be sold.

 

As for insurance purposes they will only take the thickness of the 'new' steel into account and wil not 'add it to the original'.
 

 

Feature article written by Alan Broomfield MIIMS, who tackles the thorny subject of overplating on steel hulled vessels, in particular Dutch barges and Narrowboats.

It is common practice when in the field surveying steel vessels to find mild steel plates welded to the hull, a practice regularly carried out on leisure vessels as a permanent repair. If any defects are found on the shell of a metal boat during a survey, surveyors are all too quick to recommend that the area concerned be overplated. Marine surveyors who deal with steel vessels will find that very often – Dutch barges and canal boats in particular – are frequently heavily overplated and should remember at all times that such overplating does NOT constitute a repair. It merely hides the defect.

I have recently seen an overplating welded job done to an existing doubling plate on a Dutch barge moored on a gravel tidal mooring. The result was a two foot crack in the second over plate allowing water to down flood between the plates nearly sinking the vessel which was only saved by the occupants having sufficient bilge pumps to keep her afloat until she could get into dock.

I feel overplating should never be allowed on an existing doubling plate even though such bad practice is often found. It is a very bad practice and should be condemned and highlighted within our reports. If doubling or overplating is found on a vessel, the marine surveyor should remember the Law of Unintended Consequences.

Wherever possible, doubling or overplating should be avoided and any defective steel cropped out and renewed. It should never be carried out on round bilges and never doubling over existing doubling plates. However, one occasionally sees this and it should be strictly taboo.

Doubling or overplating can only ever be regarded as bad practice, a cheap bodge job and is intellectually dishonest. It is often carried out on leisure vessels to cover over areas of pitting which is not necessarily the best solution. Pitting, if small in area and localised, is often best dealt with by back filling the pits with welding rather than extensive overplating. Pitting on non structual interior bulkheads can often be satisfactorily filled with a plastic metal paste such as Belzona but this method of repair should not be used on shell plating. Plastic metal should only be used on single pits on water/ballast tank plating or in areas where heat is not allowed or unsafe (fuel tanks).

Finally, the marine surveyor should remember that overplating, though a common practice, is often carried out without thought as to the unintended consequences.
We should realise that it adds weight to the vessel’s structure without adding much compensating volume and, as a direct result, the vessel necessarily sinks lower in the water. It also has a number of other unintended and often unrealised side effects.

1. By increasing the draft, it reduces the available freeboard and, therefore, the amount of reserve buoyancy.
2. It also, therefore, reduces the transverse metacentric radius (BMT), and slightly, increases the height of the centre of buoyancy (KB) usually with very little compensating reduction in the height of the centre of gravity (KG) so that the end result is a reduction in the metacentric height (GM) and a negative alteration to the characteristics of the statical stability curve i.e. a reduction in the maximum GZ value and the range of positive statical stability. [The average metacentric height of a narrowboat is about 150 mm (6 inches)].
3. It may also, depending upon where the overplating is sited, alter both the longitudinal trim and the transverse heel of the vessel with further indeterminate alterations in her statical stability curve.
4. It lowers the deck edge immersion angle and, therefore, any downflooding angle(s).
5. The double plating is usually not secured to the primary supporting structure – the shell side framing. It is also rarely fitted with centre plate plug welds and is dependent only on the edge weld for security.
6. The double plating is secured only at its edges and the greater the area of plate, the smaller the length of the attachment weld per unit area and, therefore, the greater the stresses in those welds.
7. The corrosion or pitting, being the reason for fitting the doubling plates, means the corrosion or pitting will still remain there and, if it is on the inside of the original shell plate, will still be increasing. Doubling, therefore, is merely hiding the problem, not repairing it.

The marine surveyor should remember that time spent considering the consequences of his actions is never wasted. A lot (too many) of boats, particularly inland narrow boats and private pleasure boats, are doubled or over plated to various degrees in both terms of area and quality of welding and finish. When presented with a vessel that has a length of 6 mm plate some 250 mm or so wide welded astride the normally laden waterline, it is not unreasonable to conclude that the plating in way has severe corrosion or pitting (for whatever reason) and that somebody in the past has recommended overplating as a cure.

At this point the marine surveyor’s mind should go into cause and effect mode and ask “How extensive was the defect? Could it have been more simply rectified by grinding out and back welding an area of pitting? Was the corrosion arrested before the doubling was fitted?” That said many of those questions are academic as the answers to most of them are well and truly hidden from view which only leads to speculation. In cases where the marine surveyor finds the situation described applied to both sides of the hull, another question arises – “Did both sides of the vessel’s hull exhibit the same degree of damage or was the double plating simply applied to both port and starboard sides to ensure maintenance of lateral stability or appearance?”

If the plate is badly pitted or where the actual thicknesses, as measured, of bottom or side shell plating fall below allowable minimum, the metal structure in way requires remedial treatment within time limits to be laid down by the marine surveyor. It is, in the author’s opinion, (and for that matter also apparently that of the MCA who will not allow doubling plates of any size – particularly on passenger boats – to be fitted except as a ‘get you home’ emergency measure) far better to crop out such thin areas back to metal of an acceptable thickness and renew the plate in way although it is accepted that that is more difficult, time consuming and costly

 

Edited by Alan de Enfield
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58 minutes ago, Alan de Enfield said:

 

I would personally not buy an overplated boat, I'd rather buy one that needed overplating and have it done by someone who knows how to do it properly, not jusgt a quick-patch by a local moble welder and just to get the boat so it can be sold.

 

 

But then you would never be able to sell it as no one would buy a boat thats been overplated, would they?

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31 minutes ago, ditchcrawler said:

But then you would never be able to sell it as no one would buy a boat thats been overplated, would they?

 

 

Plenty of people about who think overplating makes a hull as good as new. 

 

One even occasionally sees boats advertised as having a "new hull", which one enquiring usually turns out to be an overplated hull.

 

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1 hour ago, ditchcrawler said:

But then you would never be able to sell it as no one would buy a boat thats been overplated, would they?

 

"There is them as has experience, and them as have yet to acquire experience."

 

Otherwise I was pretty much going to respond much as MtB did, so I won't.

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1 hour ago, MtB said:

 

 

Plenty of people about who think overplating makes a hull as good as new. 

 

One even occasionally sees boats advertised as having a "new hull", which one enquiring usually turns out to be an overplated hull.

 

I’ve a brand new hull 😎

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4 minutes ago, Goliath said:

I’ve a brand new hull 😎

How can it be brand new when its 150 years old 😀 and what you have done is a sort of hybrid between overplating and replating.

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41 minutes ago, Alan de Enfield said:

 

"There is them as has experience, and them as have yet to acquire experience."

 

Otherwise I was pretty much going to respond much as MtB did, so I won't.

I was only going by your point of view, you wouldn't buy an overplated boat but you would expect others to.

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5 minutes ago, dmr said:

How can it be brand new when its 150 years old 😀 and what you have done is a sort of hybrid between overplating and replating.

Composite 👍

 

😃

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1 minute ago, Goliath said:

Composite 👍

 

😃

 

Have you got a wooden bottom?

 

I'm sure there is a potential joke there, lets see what the form can come up with?

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Some bits of my boat are 84 years old, others some are 50 years old, some 10 and others 6. I am unsure what to worry about. I’m sure some of the 84 year old bits are better than some of the 10 year old bits. Oh hang on there are some 22 year old bits too. There’s also some bits that were taken off in 1960, don’t know where they are, or if I should be worried about them someone could clone my boat and then their boat might rust through.

 

There’s some 5 mm overplating on the bottom, and some 6 mm overplating on the bottom with some 5 mm on top of that. Does that make 16 mm and what about the 5/16 th bits from the 1930s.


That’s what I know about.

Stress stress stress, or;

 

 

Survey - dock -paint- check - repeat.

 

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On 18/05/2022 at 17:22, Tracy D'arth said:

I would not own an overplated boat unless I knew who had done the work. And there are very few people whom I would trust to do it properly.

I agree completely with this and I'm a welder/fabricator of 40 years standing. There's welding and then there's welding!

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