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The dangers of overplating
Posted on March 17, 2017 by News Hound
Surveyors take note – overplating does not constitute a repair on a steel hull
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.