Narrowboat Hull Thickness Explained: What Do 10/6/4 and Ultrasonic Readings Actually Mean?

10/6/4 is the standard steel specification for modern narrowboats: a 10mm baseplate, 6mm hull sides, and 4mm cabin. It has been the convention since the early 1990s, and it's the baseline your surveyor and insurer will judge a boat's ultrasonic thickness readings against.

Here's the part most buyers miss: those numbers are mostly a corrosion allowance, not a structural requirement. A narrowboat doesn't need 10mm of steel to float safely. The builder is giving the hull a buffer so that after decades of slow rusting, enough sound steel remains. What matters when you buy isn't the original spec — it's how much is left.

What an ultrasonic hull survey measures

An ultrasonic survey uses a handheld gauge that fires sound waves through the steel and times the echo, giving the remaining thickness in millimetres at each test point. This usually includes dozens of readings across the baseplate, sides, waterline, bow, and stern. It's non-destructive and can read through blacking, but the boat must come out of the water. Expect £500–£1,000 for a hull survey, plus the cost of having the boat brought into a dry dock.

A survey report typically presents readings like Baseplate: 9.2–9.8mm (nominal 10mm). Two patterns matter:

• General thinning — steel lost evenly across a large area. Slow, predictable, and easy to judge against the original spec.
• Localised pitting — small, deep craters of corrosion. A 3mm reading at the bottom of a pit surrounded by 6mm steel is a very different problem from 3mm across half the baseplate. Surveyors measure pit depth separately.

The 4mm rule

You'll hear that a hull is "finished" below 4mm. In practice, 4mm is an insurance convention, not an engineering limit. GJW Direct, for example, requires boats over 30 years old to hold an out-of-water survey (within the last 10 years) showing ultrasonic readings of no less than 4mm below the waterline.

Engineering opinion sits lower. An IIMS discussion paper on minimum steel thickness notes that 3mm is widely cited as the effective design minimum, that Springer built sound boats from all-over 3mm steel in the 1980s, and that insurers' 4mm requirements are inconsistent between companies. For a buyer, though, the practical reality wins: readings under 4mm below the waterline risk refused or restricted insurance, and remediation by overplating typically runs into the thousands.

What if the boat is older than the 10/6/4 era?

Pre-1990s boats were commonly built to 6/5/3, 6/6/4, or 8/5/3. So a 5mm side reading on a 1975 boat may be near-original, not heavily wasted. Always ask for the original build spec so readings are judged as steel lost, not just steel remaining. Some premium modern builders go the other way, using 12–15mm baseplates.

How fast does a narrowboat hull actually thin?

A well-maintained hull (blacked every 2–3 years, anodes renewed) typically loses steel very slowly, often around 0.1mm a year or less. Neglected blacking, missing anodes, or an unisolated shore-power connection can multiply that, which is why two boats of the same age can survey completely differently.

What should I ask the seller before viewing?

Three things: the original steel spec, the most recent ultrasonic readings (if prior survey data is available), and the blacking and anode history. A seller who can produce all three is telling you as much about the boat's care as the numbers themselves.