Hull Thickness Test Afloat, is it Possible?

[Atticus Finch]

Hello knowledgeable people of canal world.  I already own a boat so I don't see the point in paying a surveyor to tell me that I need some repairs to the hull, when I can put the money towards the repairs.

I have access to the inside of the hull, as I will be refitting the boat out and my question is: do ultrasonic thickness measurement gauges work when there is water on the other side of the metal? In other words, can I survey the hull myself from the inside?

Many thanks for your input in advance.

 

[Mike Adams]

I purchased a Chinese one for about £50 off Ebay. It is accurate to about 0.1mm and works just as well with the hull in the water. It probably wouldn't pick up small pitting and if has a lot of scale on the inside you need to remove that first. It's going to tell you how much metal is left. The results were the same as I got from the outside when the boat is out of the water.

[Atticus Finch]

3 minutes ago, Mike Adams said:

I purchased a Chinese one for about £50 off Ebay. It is accurate to about 0.1mm and works just as well with the hull in the water. It probably wouldn't pick up small pitting and if has a lot of scale on the inside you need to remove that first. It's going to tell you how much metal is left. The results were the same as I got from the outside when the boat is out of the water.

That is brilliant, thanks Mike. I'll at least be able to establish whether or not she needs to come out of the water before spring, thanks again.

[Ex Brummie]

Whilst it may tell you a reasonably accurate thickness where you test, as pitting occurs from the outside, you really need to see it visually.

[blackrose]

I've never used an ultrasonic thickness tester myself but I do have a background in materials testing. I was always led to believe that there was a big difference in the accuracy of cheap ultrasonic testers and the more expensive sets. However, perhaps things have changed and the cheap Chinese jobbies are ok these days?

 

Anyway, don't forget that when you take a measurement, assuming it's an accurate measurement, you're only getting one data pinpoint which means very little by itself unless it can be set into the context of the entire plate. So you need hundreds of those pinpoints across the hull in order to make up a picture of what's going on across any particular plate and the entire hull. That's where the skill of the surveyor comes in. He knows that he can't measure every square inch of steel so it's about taking enough measurements to give enough overall resolution.

 

What you're suggesting isn't a bad idea, but as Ex Brummie has said, without being able to look at the outside of the hull to see things like wear and corrosion, you might well miss some areas that you should have tested. Also if you were testing from the outside you'd remove the blacking from the contact area. Since you can't do that if you're measuring from the inside you might be measuring a mm of blacking and assuming it's steel. I guess that could be the case if a surveyor was measuring from the outside too and there was a coating on the inside, but I bet a good surveyor with good equipment could calibrate his gear to distinguish between the two.

Edited November 30, 2020 by blackrose

[blackrose]

As I suspected, the accuracy of different testing kit varies greatly. 

 

Go onto Amazon and you'll see for yourself. 

 

The one I've shown in the picture below says: ”When the given thickness over 20mm, the accuracy is ±1%; when the given thickness less than 20mm, the accuracy is ±5%"

 

+/- 5% is a huge error. On 6mm plate that's 0.3mm which is quite a lot on top of all the other variables like blacking, etc. A good tester should have an accuracy of no more than +/- 0.5%  

 

 

Edited November 30, 2020 by blackrose

[Atticus Finch]

Thank you for your valid and valuable contributions everyone, I am taking it in.

[Mad Harold]

12 hours ago, blackrose said:

As I suspected, the accuracy of different testing kit varies greatly. 

 

Go onto Amazon and you'll see for yourself. 

 

The one I've shown in the picture below says: ”When the given thickness over 20mm, the accuracy is ±1%; when the given thickness less than 20mm, the accuracy is ±5%"

 

+/- 5% is a huge error. On 6mm plate that's 0.3mm which is quite a lot on top of all the other variables like blacking, etc. A good tester should have an accuracy of no more than +/- 0.5%  

 

 

Even taking errors into account,It looks a worthwhile buy for when buying a boat you will get a rough idea if the boat is worth paying for a lift out/dry dock for a full survey.

I do realise that not every boat will have easy access to the inside of the baseplate,but if the probe will work underwater,running it just below the waterline would be helpful.

Edited December 1, 2020 by Mad Harold

[Mike Adams]

It could be argued that measurements from the inside are more accurate because you can clean to bare metal and take a reading from there. There is unlikely to be as much scale on the outside  of the boat as you can find inside on the base plate. I have seen several mm of scale on the internal side of the base plate and when taking a reading from outside you can get false readings. That's why proper surveyors have a very heavy hammer as you can tell by the noise if its metal or scale.

13 hours ago, blackrose said:

+/- 5% is a huge error. On 6mm plate that's 0.3mm which is quite a lot on top of all the other variables like blacking, etc. A good tester should have an accuracy of no more than +/- 0.5%  

I have the device shown, it was not in a fancy box but on test pieces of typical  boat i.e. bits I had cut from the side, it was to within 0.1mm on 6mm plate. I have a feeling the accuracy refers above to other materials. I think the OP needs to know if the boat is about to sink so well worth the investment in my opinion.

[Mad Harold]

36 minutes ago, Mike Adams said:

It could be argued that measurements from the inside are more accurate because you can clean to bare metal and take a reading from there. There is unlikely to be as much scale on the outside  of the boat as you can find inside on the base plate. I have seen several mm of scale on the internal side of the base plate and when taking a reading from outside you can get false readings. That's why proper surveyors have a very heavy hammer as you can tell by the noise if its metal or scale.

I have the device shown, it was not in a fancy box but on test pieces of typical  boat i.e. bits I had cut from the side, it was to within 0.1mm on 6mm plate. I have a feeling the accuracy refers above to other materials. I think the OP needs to know if the boat is about to sink so well worth the investment in my opinion.

Once saw an advert for a narrowboat which pictured one of these devices on the inside of the baseplate showing a reading of 11mm.

It didn't say what the original steel thickness was,but I was sceptical.

Some boats do have thick baseplates,there is one moored here,a Polish built one that the owner tells me was built with a 13mm baseplate.

Plenty of meat there for rust to go at.

 

[Alan de Enfield]

31 minutes ago, Mad Harold said:

Once saw an advert for a narrowboat which pictured one of these devices on the inside of the baseplate showing a reading of 11mm.

It didn't say what the original steel thickness was,but I was sceptical.

Some boats do have thick baseplates,there is one moored here,a Polish built one that the owner tells me was built with a 13mm baseplate.

Plenty of meat there for rust to go at.

 

 

My last "Reeves" NB had a thick base plate - it supposedly started life with 1/2"

After scraping, it was measured by the yard doing the blacking at 13mm and not a single pit was found.

[malp]

I have both a cheap Chinese one and a Tritex.  A key difference is that the Chinese one reads a single echo, whilst the Tritex reads multiple echoes.  The Chinese one will be cofused by paint or corrosion on the surface the probe is placed against, while the Tritex won't (usually) be.   So to use the single echo type effectively you must clean paint, corrosion or any other crud off down to clean metal, but if that is done they are reasonbly accurate, bearing in mind the stated error range (as mentioned in #6 above).  Also make sure the meter is calibrated correctly before use as the echo travels at different speeds through different metals.