Measuring hull thickness

[Bacchus]

Hi All,

 

Does anyone have any experience or comment on the accuracy of "DIY" ultrasonic thickness meters for measuring hull thickness. (this kind of thing -- obviously no substitute for a full hull survey, but are they accurate enough to indicate whether a full survey is needed)

 

(image in case anyone is reading this after the eBay listing has ended...)

[TheBiscuits]

My usual surveyor got something similar when his "proper" one was broken.

 

He says they are only any use as a sounding hammer for bashing on the hull, and not even very good for that as they've got a soft protective case!

[Bacchus]

Thanks; I guess I thought as much... otherwise why would professionals spend hundreds on similar gauges

[Mike Adams]

I have had one of these for several years. It is accuate and gives reliable readings even from inside the hull when the boat is in the water. However much depends on the skill of the user and the surface needs to be clean. I have never found a surveyor who was worth the money they miss more than they see and are only any use in securing insurance.

[blackrose]

I think the proof of the pudding would be for someone who has one of these cheap ultrasonic gauges to do a series of tests on sections of steel of known thicknesses or compared to micrometer measurements, and report back whether the readings are in line with the +/-1% accuracy stated in the product specs. 

 

I can see them being useful for DIY measuring of small areas, but part of the skill of a surveyor examining an entire hull is knowing which points to measure and how many measurements to take across the hull, which all come together to form a broader picture of the hull at a particular a resolution of measurements.

Edited October 13, 2022 by blackrose

[David Mack]

I have one. I don't know how accurate the absolute thickness measurements are, but it is certainly capable of identifying the thinner areas, as verified by looking at the inner surface afterwards.

You do need a fairly smooth surface to get a proper reading. Liquid soap works as a connecting gel, and is easily washed off.

Maybe not as accurate as an expensive calibrated instrument, but at £70 or so why not give yourself the ability to do far more checks than a surveyor will?

4 minutes ago, blackrose said:

but part of the skill of a surveyor examining an entire hull is knowing which points to measure and how many measurements to take across the hull,

Yet the survey reports I have seen just present a list of thickness readings at about 3 ft intervals along the boat at the waterline, above the chine and somewhere on the bottom. No attempt to actually find the thin areas, which is surely what you really want to know.

[Mike Adams]

40 minutes ago, David Mack said:

but part of the skill of a surveyor examining an entire hull is knowing which points to measure and how many measurements to take across the hull,

In my experience it is just down to making measurements every metre or so. I have never found one who attemped to have a decent look  inside to see where water collects etc and investigate further. I have seen a dutch steel boat that looked great from the outside but when they lifted it the hull it cracked because it was so rotten inside.

[malp]

I have both a cheap Chinese one and an expensive Tritex.  On clean bare steel the Chinese one is accurate, but through paint it is not, whereas the Tritex is.  Both need calibrating for the material being measured, and both need some sort of gel under the probe for best results.  The difference between them is that the Chinese meters work on a single echo system,  whereas the Tritex (and other equivalents) use a more complex multi echo system.  The former cannot distinguish between paint and steel so gives a combined reading, the latter can and gives just a steel reading.

Edited October 13, 2022 by malp

[malp]

What none of these meters can tell you is how much steel thickness is left at the bottom of a pit.  So a small depth gauge should also be part of the surveyor's kit.  Many digital tyre tread depth gauges are suitable

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Edited October 13, 2022 by malp
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