Ultrasonic Thickness Testing: The Only Survey That Matters on Steel
Steel hides its condition better than any other material. One test reveals it, and it costs a few hundred dollars.
By Boatmere Brokerage Desk · Updated · 6 min read
Bring a moisture meter to a steel boat and you have brought the wrong tool. Steel does not absorb water, it loses metal, and the only way to know how much is to measure it.
What is ultrasonic thickness testing on a boat?
A handheld probe sends a pulse through the plate and times the echo off the far side. Time converts to thickness. Take readings across a grid and you get a map of where the hull has thinned.
It is quick, it is non-destructive, and it works from the outside with the boat hauled. A surveyor will take anywhere from thirty to several hundred readings depending on the size of the boat and what they find.
Why it is the test that matters
Steel corrodes from wherever water sits and air does not move, which means the inside. Bilges under the engine, behind fuel tanks, under spray-foam insulation, at the turn of the bilge where condensation runs down and stops.
None of that is visible. A steel hull can look immaculate from the dock, with fresh paint and tidy welds, and be down to sixty percent plate thickness behind a tank you cannot reach.
The readings are the only honest account of it.
How to read the numbers
You want the original plate thickness, usually available from the builder or estimated by the surveyor from the vessel's specification, and the readings as a percentage of it.
Rough guidance:
- Under 10 percent loss. Normal for an older hull. Nothing to do.
- 10 to 20 percent. Acceptable, worth monitoring, worth a modest price adjustment.
- 20 to 25 percent. Approaching the point where plating gets renewed. Price it as work coming.
- Over 25 percent. Renewal territory. On a 46-footer, replating a section runs $8,000 to $25,000 depending on access.
The pattern matters as much as the number. Uniform thinning across a whole plate is age. Deep isolated pits are worse, because a hull fails at its thinnest point rather than at its average.
What to ask for specifically
Book it as a named item, because it is rarely part of a standard survey.
Ask for readings taken at the waterline, in the bilges under the engine, at the turn of the bilge, around all through-hulls, and under any tank or insulation the surveyor can reach. Ask for the grid to be recorded as a diagram rather than a paragraph, and keep it: repeating the test in five years against the same grid tells you the rate of loss, which is more useful than any single survey.
The upside nobody mentions
A steel hull with good readings is a genuinely reassuring thing to own.
There is no osmosis to worry about, no core to saturate, no bulkhead tabbing to let go quietly. If you hit something, a welder repairs it properly in a day. And the test itself gives you a number, which is more than any other hull material offers you.
That is why the steel boats worth buying are the ones with a thickness survey already in the file. An owner who has had it done and kept it is telling you something about how the boat has been looked after.
Frequently asked questions
- What is ultrasonic thickness testing on a boat?
- A probe measures remaining steel thickness by timing an ultrasonic pulse through the plate. Readings across a grid show where corrosion has thinned the hull, which is invisible from either side until it is critical.
- How much does ultrasonic hull testing cost?
- Typically $400 to $1,200 on a 40 to 60 foot hull, depending on how many readings are taken. It is usually an add-on to a standard survey rather than included, so ask for it by name when you book.
- How much wastage is acceptable on a steel hull?
- General guidance is that up to about 20 percent loss from original plate thickness is tolerable, and beyond 25 percent plating should be renewed. Isolated pitting matters more than uniform loss, because it concentrates the weakness.
- Where does a steel boat corrode first?
- Where water sits and air cannot circulate: bilges under the engine, behind tanks, under insulation, and at the waterline. Almost never in the places you can see, which is exactly why the test exists.
