If a hull survey has flagged thin steel, the next question is always the same: what will plating actually cost? The honest answer is that it depends on how much steel needs replacing and where your boat is, but the ranges are predictable once you know what drives them. Here is what we tell owners who ask us at our Gravesend workshop.
Most narrowboat plating jobs fall into one of three brackets. Localised patch plating, where a surveyor has found isolated pitting near the waterline or around a skin fitting, typically runs from £400 to £1,500 including materials. Partial overplating of the baseplate or lower hull sides on a mid-length boat usually lands between £3,000 and £7,000. A full bottom overplate on a 57 to 62 foot boat, using 6mm steel over the existing baseplate, commonly comes in between £6,000 and £12,000 once dock time is included.
These are working ranges, not quotes. Steel prices move, access varies enormously, and two boats of the same length can need very different amounts of preparation. Anyone quoting a firm price without seeing the survey report and the boat out of the water is guessing.
Steel thickness and coverage are the big ones. Overplating in 6mm plate costs more in material than 4mm, but most surveyors and insurers now expect 6mm on baseplates, so it is rarely worth economising there. The area to be covered matters more than boat length as such: a 45 foot boat needing full bottom and lower side plating can cost more than a 60 foot boat needing the baseplate only.
Then there is access. The boat has to come out of the water or into dry dock, and that facility charges by the day or week, typically £300 to £600 a week on the Thames and Medway. Preparation eats hours too: grinding back old blacking, cutting out badly wasted sections rather than plating over them, and dealing with internal ballast or fuel tanks that sit against the area being welded. Welding near a diesel tank or gas locker needs careful isolation, and that time goes on the bill.
Overplating means welding new steel over the existing hull. It is quicker and cheaper, and done properly with fully sealed continuous welds it is accepted by most insurers. The downsides are added weight, a slightly reduced draft margin, and the fact that you can no longer inspect the original steel underneath. Some surveyors will note it on future reports, which can affect resale.
Replating means cutting out the wasted steel and welding in new plate flush. It costs more, often half as much again, because of the extra cutting, fitting and internal disruption. But the hull ends up as good as original, with no hidden layer. For a boat you plan to keep twenty years, replating the worst areas and overplating the rest is often the sensible middle ground, and a decent welder will talk you through that split rather than defaulting to one answer.
For boats on the Medway, the lower Thames or the Kent coast, the practical constraint is finding dock space and a coded welder available at the same time. Book the dock first, then the welding, and allow slack: once the hull is blasted or ground back, more wastage often shows up than the survey predicted, so build a 15 to 20 percent contingency into your budget.
Ask any welder for evidence of previous hull work, confirm they will provide continuous welds rather than intermittent stitching below the waterline, and get the scope in writing: plate thickness, coverage area, weld type and whether blacking afterwards is included. Blacking or two-pack coating on the new steel adds a few hundred pounds but skipping it wastes the whole job.
A full baseplate overplate typically takes one to two weeks in dock, depending on preparation and weather if the work is outdoors. Patch repairs can often be done in a day or two.
Most will, provided the work uses adequate plate thickness, continuous sealed welds and is documented, ideally with a follow-up survey. Check with your insurer before the work starts, not after.
If the survey shows wastage confined to the baseplate or a few areas, plating usually costs far less than the boat's value and buys decades of life. If the sides, bottom and bulkheads are all thin, get a second opinion before spending, because costs can spiral once cutting starts.
Tell Michael about your job. Drop a quick message with what needs welding, where it is and a photo if you have one. He will come back the same day.