If your boat has spent years on the Grand Union around Milton Keynes, a hull survey can bring unwelcome news about thin steel. Overplating is one fix, but it is not always the right one. Here is how to tell when it is needed, what it involves, and what to ask before you commit.
Most narrowboats are built to a 10/6/4 specification: a 10mm base plate, 6mm hull sides and a 4mm cabin. Older boats, particularly those from the 1970s and 1980s, often started with a 6mm base plate, so they have far less steel to lose.
A surveyor takes ultrasonic thickness readings across the hull and measures the depth of any pits. The figure that matters most is the thinnest remaining steel. Many insurers will not offer fully comprehensive cover once readings fall below about 4mm, and they commonly ask for an out of water survey once a boat is 20 to 30 years old. That insurance threshold is usually what pushes owners towards overplating.
General, even thinning is slow. Deep localised pitting is what causes most problems, and it is common on boats kept on shore power in marinas without a galvanic isolator. If you moor long term at somewhere like Peartree Bridge, Campbell Park or Cosgrove and plug in all year, it is worth checking that one is fitted.
Typical triggers for overplating include:
If the pitting is isolated, pit welding is often the better answer. Each pit is cleaned out and filled with weld, restoring the thickness without adding weight. Small patches or cutting out and replacing a damaged section can also deal with local trouble, and replacement is the more thorough repair because no old steel is left behind the new.
Overplating makes sense when the wastage is widespread. New plate, usually 6mm on the base and 4mm to 5mm on the sides, is welded over the existing hull with continuous seams so that no water can get between the two layers. Poor overplating with stitched or porous welds traps water and hides corrosion, so ask to see the seams before the hull is blacked.
Steel at 6mm weighs roughly 47kg per square metre. Overplating the base of a 57ft boat adds over a tonne and a half, which sits it around 4cm to 5cm lower in the water unless ballast is removed to compensate.
That matters on shallow stretches of the Grand Union and at silted moorings, and it brings sink outlets, engine vents and the weed hatch closer to the waterline. A competent welder will check these clearances with you and discuss removing ballast before any plate is cut. New anodes and at least two coats of blacking should be part of the job.
Price depends on boat length, how much of the hull is covered, plate thickness and access. As a rough guide, base plate overplating on a mid length boat often falls between £5,000 and £9,000, while base and sides together can run from £9,000 to £15,000 or more. Pit welding a modest number of pits is typically a few hundred pounds to around £1,500.
Dry dock or slipway hire is normally charged on top, often £200 to £400 for the lift plus a daily rate, and local yards along the Grand Union can be booked up months ahead. Always get a written quote based on the survey report, not on a guess over the phone.
A base plate only job usually takes around one to two weeks out of the water, and base plus sides can take two to three. Allow extra time for blacking to cure before the boat goes back in.
Some buyers are wary of overplated boats, because poor work can hide problems. Well documented work with continuous welds, photographs and a clean follow up survey generally reassures buyers and insurers.
Have the boat out for blacking every two to three years and look closely at the steel each time. A full hull survey with thickness readings every five to ten years is sensible, and more often on an older boat.
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.