Over plating questions

[Kudzucraft]

A quick intro before my question. We are a couple of Yanks that will retire in a few years. We are researching buying a Narrow Boat and spending a a couple of years CC'ing the canals and rivers. I have been raised around the water and boats. I am very familiar with wooden boats, runabouts and a little experience with the wood equivalent of your GRP cruisers here in the states. But no experience with Steel Hulled boats. The are just not common inland.

 

I understand what over-plating is. I know it is because of rust and that metal has become thin.  What I am trying to grasp is what are the causes? That is other than of the obvious lack of maintaining the boat, such as not blacking regular. Then I know shoreline power can cause problems.  When I see a 20 year old boat that has been over-plated it scares me a little. Not that it has been repaired but makes me question how it was cared for, or rather not cared for? 

 

If you maintain a boat reasonably, what is the expected life of a hull? How often should you expect to need to replate? I know water and steel are at odds with each other so there is a constant battle going on there. Just trying to educate myself and better understand this subject.

 

 

[Alan de Enfield]

14 minutes ago, Kudzucraft said:

If you maintain a boat reasonably, what is the expected life of a hull? How often should you expect to need to replate? I know water and steel are at odds with each other so there is a constant battle going on there. Just trying to educate myself and better understand this subject.

 

The correct answer is (probably) somewhere between 5 years and 200 years.

 

There are so many contributory factors ranging from what type of water it is in (clean, chemical pollution, brackish, etc etc), how often it is 'blacked' (is the blacking touched up when scratched / damaged), is it moored in a marina, or next to boats that are connected to the mains-electricity, is it moored against a steel piling wharf ? does it have the correct size, numbers and type of anodes.

 

It is always the 'lowest' type of metal (least noble) on the galvanic scale that is attacked first - normally on a steel narrowboat it will be the hull itself (mild steel) whilst on a GRP or wooden boat it will be the Bronze Propeller.

 

 

 

It appear to me that 'electricity' pays an increasing role in corrosion and pitting - particularly 'mains electricity'.

 

 

Edited May 10, 2019 by Alan de Enfield

[eid]

This from a survey carried out last year on a 17 year old boat:

 

 

Quote

The hull ..... appears to be originally fabricated from nominal 10mm steel plate.

..........
From the sample areas selected, ultrasonic measurements show the plate thickness to be between 9.6mm and 10mm, which are within
acceptable limits.
Moderate pitting corrosion is present and the maximum pit depth measured was approx. 1.2mm. Deeper pitting may be
present in the areas where the marine growth was not removed for inspection.

 

 

I wouldn't touch a 20 year old, overplated narrowboat with a barge-pole! Unless it was ridiculously cheap, meaning I could sell it easily at the end of the 2 years.

Edited May 10, 2019 by eid
removed noise

[Keeping Up]

You just can't tell. Take the case of my boat for example. Owned by myself from new, immaculately maintained with regular blacking every 2 years. Surveyed at 23 years old, prior to fitting a new engine, everything was perfect and the deepest pits in the 6mm sides were less than 0.5mm. Then surveyed at 24 years old at the request of the insurance company, the whole of each side was extensively pitted to a depth of 4.5mm; as the surveyor said, in a few more months I'd have been the owner of a submarine.

 

I still don't know the cause. Mains fault on the shoreline (which had a GI) perhaps, but it didn't affect the other 4 boats on the line. An earth loop via the new engine's exhaust seems possible. After emergency overplating there's been no further deterioration since.

[Neil2]

3 minutes ago, Keeping Up said:

You just can't tell. Take the case of my boat for example. Owned by myself from new, immaculately maintained with regular blacking every 2 years. Surveyed at 23 years old, prior to fitting a new engine, everything was perfect and the deepest pits in the 6mm sides were less than 0.5mm. Then surveyed at 24 years old at the request of the insurance company, the whole of each side was extensively pitted to a depth of 4.5mm; as the surveyor said, in a few more months I'd have been the owner of a submarine.

 

I still don't know the cause. Mains fault on the shoreline (which had a GI) perhaps, but it didn't affect the other 4 boats on the line. An earth loop via the new engine's exhaust seems possible. After emergency overplating there's been no further deterioration since.

You hear stories like this all the time.  The problem is that steel narrowboats, relatively speaking, haven't been around that long and there is a dearth of real research into the causes of rusting/pitting.  Personally I still believe that investing in a more effective barrier than blacking will remove most of the uncertainty, but for many owners seemingly the aggravation of getting the boat grit blasted outweighs the benefits.  

 

I've said before that I wouldn't buy an overplated boat but I would consider buying one that needed overplating.  Although rust is the enemy, the great thing about steel is that it is relatively easy to repair and even the most desperate cases can be salvaged - it's the cheapest way into narrowboat ownership. 

  

 

 

[Alan de Enfield]

2 hours ago, Kudzucraft said:

I understand what over-plating is. I know it is because of rust and that metal has become thin.  What I am trying to grasp is what are the causes? 

Interesting article on the dangers of overplating (or 'doubling' as it is also known) :

 

https://www.iims.org.uk/

 

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

 

Just to note : The MCA is the Maritime & Coastguard Agency who are responsible for safety and registration of all commercial boats and seafarers in UK waters 

 

https://www.gov.uk/government/organisations/maritime-and-coastguard-agency

 

But as the author notes overplating is the accepted quick fix for leisure boats.

 

 

 

 

Edited May 10, 2019 by Alan de Enfield

[roland elsdon]

This subject has been covered in great detail in the past.

Having overplated my boat i would buy an overplated boat but only if

1) i knew who had done it

2) the old steel had been properly prepared first

3) it had plug welding or cut out welding between the two plates.

 

the front of my boat has 3 bottoms on some places some rivetted some welded a notional 20 mm thick, but of course being a narrowboat has had corrosion from the inside out, that purpose made canal cruisers should not get. My main object now is to keep the inner plate as good as possible , if the inner plate rots through then i will be in a worse place as then rain water can get between the plate and overplate

[Bee]

I think Alan's piece above takes quite a harsh view of overplating. There are problems overplating sea going boats as the plating can detach from the frames because of salt water under the cabin sole, I would not be happy with overplating on a sea going vessel without a very good look at the thing and whether the frames are still touching the plating and attached.  Narrowboats are massively strong and the 'frames' are not quite so vital to keeping the boat in shape. Most of the plates are flat and overplating from the waterline down to the bottom squarish edge is perfectly reasonable. For a narrow boat I reckon that overplating the bottom is OK but its not brilliant. Personally I would do it to my own boat but I might not be happy to buy a boat like that. Until very recently most boats had a couple of coats of bituminous paint slapped on the sides and the bottom was unpainted. More recently better paint has been used and the bottom is painted. this is obviously better but there are plenty of boats going around with not much paint on them and a surveyor should give his opinion on this. If you buy a boat that's OK then (I think) epoxy paint will keep it OK. As far as electrical damage to metal is concerned I don't know much about it but I put my faith in paint.

[Kudzucraft]

Answers are all I appreciated, every one of them, great information. 

 

Keeping Up,  your example is actually very helpful or maybe enlightening is the better word. Sounds like most issues are related to Shore power and/or being in Marina with Shore mains. Am I correct in thinking that someone CC'ing would have less risk of sudden serious damage? Of course I realize that anything is possible.

 

Our goal, which being a few years off will may change, is buying a boat that is is good shape. Full survey will be done or I won't buy.  Expecting to spend a few weeks in a marina, fitting it out to suit us.  After that the goal is just CC the system. Depending on winter weather we might moor up for 2-3 months in a Marina? We are thinking we would like to do this for a couple of year and then sell the boat and move on to some new adventure.

Edited May 10, 2019 by Kudzucraft

[Kudzucraft]

Just to keep this on track, I probably not interested in a boat with a overplated bottom.  Replacing a section wouldn't bother me much if it was done right.  Ideally I will wait for a boat in good shape and I want to know about how to keep in it good shape so when we get ready to sell it we are going to be facing a big repair/cut in price. 

[Neil2]

3 minutes ago, Bee said:

I think Alan's piece above takes quite a harsh view of overplating. There are problems overplating sea going boats as the plating can detach from the frames because of salt water under the cabin sole, I would not be happy with overplating on a sea going vessel without a very good look at the thing and whether the frames are still touching the plating and attached.  Narrowboats are massively strong and the 'frames' are not quite so vital to keeping the boat in shape. Most of the plates are flat and overplating from the waterline down to the bottom squarish edge is perfectly reasonable. For a narrow boat I reckon that overplating the bottom is OK but its not brilliant. Personally I would do it to my own boat but I might not be happy to buy a boat like that. Until very recently most boats had a couple of coats of bituminous paint slapped on the sides and the bottom was unpainted. More recently better paint has been used and the bottom is painted. this is obviously better but there are plenty of boats going around with not much paint on them and a surveyor should give his opinion on this. If you buy a boat that's OK then (I think) epoxy paint will keep it OK. As far as electrical damage to metal is concerned I don't know much about it but I put my faith in paint.

I agree, whoever wrote that piece doesn't know much about the typical narrowboat.  Ok working boats and replicas of same might be built like "proper" boats but modern narrowboats are really just an overbuilt steel box where the "framing" is really just to support the internal fittings.  The chine of a flat bottom narrowboat is for all practical purposes sacrificial not necessarily because of rust but the general rough and tumble of canal life, and there will be scores of 6mm bottom boats that have had the chines overplated it's accepted practice.  As for the comments on buoyancy freeboard etc I don't think that would bother most narrowboaters but most of the boats that have been overplated will be 6mm baseplates and have the facility to remove ballast.   Come the day when your 25mm bottom Hudson starts to wear out you may be in trouble but if there are two words that will probably never occur in the same sentence it's "overplate" and "Hudson".

 

6 minutes ago, Kudzucraft said:

Answers are all I appreciated, every one of them, great information. 

 

Keeping Up,  your example is actually very helpful or maybe enlightening is the better word. Sounds like most issues are related to Shore power and/or being in Marina with Shore mains. Am I correct in thinking that someone CC'ing would have less risk of sudden serious damage? Of course I realize that anything is possible.

 

Our goal, which being a few years off will may change, is buying a boat that is is good shape. Full survey will be done or I won't buy.  Expecting to spend a few weeks in a marina, fitting it out to suit us.  After that the goal is just CC the system. Depending on winter weather we might moor up for 2-3 months in a Marina? We are thinking we would like to do this for a couple of year and then sell the boat and move on to some new adventure.

It sounds like a great idea and the great thing about narrowboats is they are so easy to sell, you might even get your money back.  Lots of folk now winter for about three months in a cozy marina just don't get too cozy with the mains electric and free wifi eh? 

[Neil2]

3 minutes ago, Kudzucraft said:

Just to keep this on track, I probably not interested in a boat with a overplated bottom.  Replacing a section wouldn't bother me much if it was done right.  Ideally I will wait for a boat in good shape and I want to know about how to keep in it good shape so when we get ready to sell it we are going to be facing a big repair/cut in price. 

Of course you could do what I have done and go for an aluminium boat.  No rust just lots of lovely shiny metal and forget about that horrible blacking stuff.  The only real downside is the initial price but ally boats hold their value very well so if you only expect to keep the boat for a couple of years it's as safe as, er, houses... 

 

[Tony Brooks]

14 minutes ago, Neil2 said:

Of course you could do what I have done and go for an aluminium boat.  No rust just lots of lovely shiny metal and forget about that horrible blacking stuff.  The only real downside is the initial price but ally boats hold their value very well so if you only expect to keep the boat for a couple of years it's as safe as, er, houses... 

 

Ah but what are anodes made of for brackish water and if there is a mains fault on an adjacent steel  boat what is the risk to your hull. Just putting the other side.

[Alan de Enfield]

2 minutes ago, Tony Brooks said:

if there is a mains fault on an adjacent steel  boat what is the risk to your hull.

Two words :

 

Aluminium Dust !!!

 

 

[Captain Pegg]

2 hours ago, Neil2 said:

I agree, whoever wrote that piece doesn't know much about the typical narrowboat.  Ok working boats and replicas of same might be built like "proper" boats but modern narrowboats are really just an overbuilt steel box where the "framing" is really just to support the internal fittings.  The chine of a flat bottom narrowboat is for all practical purposes sacrificial not necessarily because of rust but the general rough and tumble of canal life, and there will be scores of 6mm bottom boats that have had the chines overplated it's accepted practice.  As for the comments on buoyancy freeboard etc I don't think that would bother most narrowboaters but most of the boats that have been overplated will be 6mm baseplates and have the facility to remove ballast.   Come the day when your 25mm bottom Hudson starts to wear out you may be in trouble but if there are two words that will probably never occur in the same sentence it's "overplate" and "Hudson".

The structural principles of the hulls of historic and modern narrow boats are pretty much the same. I wouldn’t want to rely on the integrity of the cabin as part of the structure of the boat so in that sense the internal members of the hull are very much more than glorified floor bearers.

 

The chines (and that little tab which featured in the Brentford sinking thread) carry an external weld at the foot of the side plates (or stem post in the case of the tab). They are oversized to protect that weld rather than the side plates directly. Hence there is good reason they are not cut or ground flush so they aren’t truly sacrificial. As someone pointed they are vestiges of the fabrication process in the sense that the surplus baseplate is cut from around the hull but they are necessary ones.

 

JP

 

 

Edited May 10, 2019 by Captain Pegg

[Captain Pegg]

5 hours ago, Kudzucraft said:

A quick intro before my question. We are a couple of Yanks that will retire in a few years. We are researching buying a Narrow Boat and spending a a couple of years CC'ing the canals and rivers. I have been raised around the water and boats. I am very familiar with wooden boats, runabouts and a little experience with the wood equivalent of your GRP cruisers here in the states. But no experience with Steel Hulled boats. The are just not common inland.

 

I understand what over-plating is. I know it is because of rust and that metal has become thin.  What I am trying to grasp is what are the causes? That is other than of the obvious lack of maintaining the boat, such as not blacking regular. Then I know shoreline power can cause problems.  When I see a 20 year old boat that has been over-plated it scares me a little. Not that it has been repaired but makes me question how it was cared for, or rather not cared for? 

 

If you maintain a boat reasonably, what is the expected life of a hull? How often should you expect to need to replate? I know water and steel are at odds with each other so there is a constant battle going on there. Just trying to educate myself and better understand this subject.

 

 

Very simply;-

 

If you aren’t particularly budget constrained buy a boat that isn’t and doesn’t need overplating.

If you are budget constrained don’t let overplating be a barrier between you and what is otherwise the boat you want.

 

I own a 50 year old boat with 32 year old overplating. My genuine expectation is that it will outlast me and I’d like to think I’ve got at least 25 more years boating to do. Most steel boats last for many decades and there is no general expectation that any one boat should ever need overplating.

 

JP

[MtB]

4 hours ago, Tony Brooks said:

Ah but what are anodes made of for brackish water and if there is a mains fault on an adjacent steel  boat what is the risk to your hull. Just putting the other side.

 

 

They have a nasty habit of melting away to nothing in a fire, too...

 

 

 

 

Actually, if the temperature gets high enough the aluminium actually burns, IIRC. 

 

 

[Neil2]

6 minutes ago, Mike the Boilerman said:

 

 

They have a nasty habit of melting away to nothing in a fire, too...

 

 

 

 

It's still floating though...

 

[cuthound]

8 minutes ago, Mike the Boilerman said:

 

 

They have a nasty habit of melting away to nothing in a fire, too...

 

 

 

 

Actually, if the temperature gets high enough the aluminium actually burns, IIRC. 

 

 

 

Braunston 2017?

[MtB]

1 minute ago, Neil2 said:

 

It's still floating though...

 

 

True!

 

An afternoon with a pop rivet gun and it will be good as new....

 

 

 

1 minute ago, cuthound said:

 

Braunston 2017?

 

Yes, 25th June 2017.

 

 

[Neil2]

Thank goodness for that water ballast otherwise it would have been a write off.

[Captain Pegg]

25 minutes ago, Mike the Boilerman said:

 

 

They have a nasty habit of melting away to nothing in a fire, too...

 

 

 

 

Actually, if the temperature gets high enough the aluminium actually burns, IIRC. 

 

 

Anything burns if the temperature gets high enough. Aluminium is used as the fuel in solid fuel rocket boosters but you’d do well to burn it domestically.

 

JP

[Alan de Enfield]

31 minutes ago, Neil2 said:

 

It's still floating though...

 

It is very shallow water - it sunked !!

[LEO]

Hi,

Note you mention carrying out 'improvement' work in a marina - take care on this point, many marinas now have high residential occupancy levels and have strict rules about the amount/scope of DIY work allowed.

If you intend cruising the whole network take care with the length of boat as some canals have length restrictions.

 

Good luck!.

 

L

[blackrose]

14 hours ago, Tony Brooks said:

Ah but what are anodes made of for brackish water and if there is a mains fault on an adjacent steel  boat what is the risk to your hull. Just putting the other side.

 

Yes, being less noble, aluminium is far more susceptible to galvanic corrosion so you must make sure the AC system is isolated if using shore power. 

 

If money was no object I'd like a stainless steel boat!

Edited May 11, 2019 by blackrose