A hull survey that comes back with 3mm readings on a boat that started life at 6mm or 10mm is a horrible moment for any owner. The next question is always the same: can it be overplated, or does the old steel need cutting out and replacing? The answer depends on where the thinning is, how widespread it is, and what the boat is worth to you over the next ten years.
Overplating means welding new steel sheet, usually 6mm, directly over the existing hull plate. The old steel stays in place, the new plate is seam welded around its edges and plug welded through holes at intervals across its face so it cannot drum or trap water. It is quicker, cheaper and avoids opening the boat up, which is why it is the most common repair on the Thames and Medway.
Replating means cutting out the corroded section entirely and welding in fresh plate flush with the original hull. It is more labour, because the ballast and often the cabin floor above have to come out, the cut line has to land on sound steel, and every weld is a full strength butt joint. The result is a hull that is genuinely as good as new in that area, with no hidden steel rotting behind a cover plate.
Overplating suits localised thinning where the surrounding steel is still sound. A classic case is a baseplate at 4mm to 5mm in the bilge under the galley where a leaking tank sat for years, or a band of pitting along the waterline on one side. As a rule of thumb, if the remaining steel is 3mm or better and the area is a few square metres rather than the whole bottom, overplating is sensible.
Surveyors and insurers generally accept overplating provided it is documented, the plate thickness is stated and the welds are continuous. Expect the survey report to note it, and expect a modest effect on resale value. For a well loved boat you intend to keep, that rarely matters.
If readings are under 3mm over large areas, overplating stops being a repair and becomes a shell. Welding 6mm plate to 2mm steel is unreliable, the arc burns through, and you end up with a new skin sitting on crumbling metal that still holds water behind it. That trapped water keeps corroding the old plate and can eventually attack the new one from behind.
Replating is also the right choice where corrosion has come from the inside, for example a baseplate eaten by a wet bilge under old ballast, because overplating from outside does nothing about that. It is the better option on boats with a high value or where the owner wants a clean survey with original thickness figures. On a 57ft boat, replating the whole baseplate is a multi week job and the cost can run from the low teens of thousands upwards, against perhaps £3,000 to £8,000 for a typical overplating job on a few patches. Those figures move with steel prices, access and how much interior has to come out.
Start with a proper ultrasonic thickness survey, not a guess with a hammer. A good surveyor takes readings on a grid, marks them on a plan of the hull, and records where the pitting is worst. That plan is what a welder quotes from, so a vague verbal description will only get you a vague price.
Around Gravesend there are slipways and dry docks on the Thames and up the Medway that can lift a narrowboat, and it helps enormously if the welder sees the boat in the water first or at least gets photographs of the bilge with the floor lifted. Ask whatever fabricator you use to confirm the plate grade, the weld process and whether they will issue a written record of the work with photographs for your insurer.
In theory yes, but it is rarely wise. A second layer adds weight, traps more water and usually means the underlying steel is past saving, so replating is almost always the better choice at that stage.
Most will, provided the work is documented with plate thickness, extent and photographs, and the survey records it. Some insurers ask for a fresh survey after major hull work, so check before you book the lift.
A straightforward overplating job on a couple of patches can be done in a few days plus drying time for blacking. Full baseplate replating with the interior stripped typically takes two to four weeks depending on access and weather.
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.