Ultrasonic Thickness Guage measurements?

[lenseman]

Does anyone know whether an Ultrasonic Thickness Guage can be used on a steel yacht whilst she is afloat or does the yacht need to be out of the water?

Would there be different readings measuring the thickness of the hull below the water line and above the waterline for that matter if afloat compared to lifted out onto the hard?

 

[VicS]

Crumbs I used to use one ! I don't remember if we used it on things full of water or not. Mainly used it for boiler tube thickness surveys but the boiler would have been drained anyway. Also used to do air receiver thickness checks, but they are full of air!

Pretty sure that water on the other side will make no difference. Its not going to affect the speed of the signal through the steel and back again.

Perhaps there's an NDT guy around

what does its instruction manual say?

 

[TradewindSailor]

Crumbs I used to use one ! I don't remember if we used it on things full of water or not. Mainly used it for boiler tube thickness surveys but the boiler would have been drained anyway. Also used to do air receiver thickness checks, but they are full of air!

Pretty sure that water on the other side will make no difference. Its not going to affect the speed of the signal through the steel and back again.

Perhaps there's an NDT guy around

what does its instruction manual say?

Should be no problem at all. My unit records multiple echoes coming back from one blib ..... so bad echoes can be filtered out. Ultrasound travels through water and grease, but the strong echoes come from the faces of the metal sheet. If you start getting inconsistent readings, it's time to get the chipping hammer out to find out what's there.

 

[johnlilley]

Hi there, in my experience it makes no difference, the echo bounces off the other side of the plating if the plating is single thickness. Some gauges will register only the paint thickness on the gauge side, so ensure you get an accurate reading in this respect, you might have to remove thick paint on the test side. Some gauges might register the total thickness of paint & steel which might be misleading as the paint reading can cause inaccuracies in the actual thickness of the plating. Take several readings in the same area to isolate false readings then make your own judgement on where readings are obviously constantly indicating thinner plate.


John Lilley

 

[flipper]

Yes. I used to use them on sub sea pipelines when I was a diver. Seem to remember we had to take the pipe back to clean, bare metal first though.

You also might want to check it's a waterproof unit first

Patrick

 

[Mariner69]

I have used the things inside the ship for plate thickness to meet the Lloyds classification requirements.

You need to remove the surface paint first and apply some vaseline to the emitter head to ensure proper connectivity sound wise.

Also a good idea to test first on a steel test piece to check the meter is reading correctly.

Enjoy.

 

[NorthUp]

Another vote for use while the boat is in the water.
Wallpaper paste makes a good acoustic coupling gel as well!

 

[sail045]

No problems with testing the plate thickness with the vessel in the water. The compressional wave generated by the meter via the probe will reflect from the backwall thickness at the steel water interface. Grease, thick oil,vaseline, wallpaper paste even water will act as an air gap eliminator, U/S sound will not pass through air. Remove the paint to bare metal grid off the area and take a number of readings,to establish max'-min' readings. Main point is to calibrate the instrument before testing using an approved test block of the same material, in this case steel, possibliy medium to low carbon steel of known properties. Simple thickness meter gauges usally come with a calibration block. If testing from the outside you will need watertight probe & leads.
Hope this helps.

 

[Herald]

Not a problem at all, have done previously on bottom plating of an FSU with 6 feet of marine growth on the outside.....

Ensure the unit is correctly calibrated, check reading against a test piece of a known thickness. Clean back to bare metal and then away you go.

 

[TradewindSailor]

Check the type of meter ..... the better instruments do not require the paint removed as per my previous reply.

Some experience is necessary to interpret the readings, and indeed where to measure. I've seen a lot of surveyors measure mid-panel rather than along the horizontals and close to the welded edges where the corrosion tends to be.

 

[Bilgediver]

Does anyone know whether an Ultrasonic Thickness Guage can be used on a steel yacht whilst she is afloat or does the yacht need to be out of the water?

Would there be different readings measuring the thickness of the hull below the water line and above the waterline for that matter if afloat compared to lifted out onto the hard?

I am pretty sure we measured wall thickness on HP piping without drainiing.

Make sure you have a test piece to check the meter before you use it and don t use it blind. A piece of mild steel that you can measure with a vernier will be fine.

 

[BigART]

A couple of points

Just a point that we learnt the hard way. When we purchased our steel boat, we had a full survey including ultrasonic hull thickness measurements which were made on a 20cm grid pattern all over the underwater hull area. This was done out of the water from the outside (it sounds like you are checking the thickness from the inside). Looks very thorough and impressive - but is is literally a series of spot checks and unless the spot is on a problem area, it will not pick it up. 3 years after purchase, we found severe corrosion in localized areas that had obviously been there for some time. I have a picture of a gauge showing full hull thickness 4cm away from a rust hole.

Another point. The surveyor claimed that the gauge would only measure the metal thickness and ignore the paint layers. As we found out - he was wrong. His measurements included the paint. You may want make sure that the metal thickness claimed by the gauge is close to the real metal thickness.

You can fix stuff you can see but it was the hidden rusting parts that you cannot see that got us caused by errors in construction (poor drainage).

Angus

 

[TradewindSailor]

Just a point that we learnt the hard way. When we purchased our steel boat, we had a full survey including ultrasonic hull thickness measurements which were made on a 20cm grid pattern all over the underwater hull area. This was done out of the water from the outside (it sounds like you are checking the thickness from the inside). Looks very thorough and impressive - but is is literally a series of spot checks and unless the spot is on a problem area, it will not pick it up. 3 years after purchase, we found severe corrosion in localized areas that had obviously been there for some time. I have a picture of a gauge showing full hull thickness 4cm away from a rust hole.

Another point. The surveyor claimed that the gauge would only measure the metal thickness and ignore the paint layers. As we found out - he was wrong. His measurements included the paint. You may want make sure that the metal thickness claimed by the gauge is close to the real metal thickness.

You can fix stuff you can see but it was the hidden rusting parts that you cannot see that got us caused by errors in construction (poor drainage).

Angus

We had a surveyor check out a 80m steel hull .... from the outside. He declared all was well. We then invited him into the chain locker that had been stripped of the sacrificial wood ......... it looked like a clear moonless night .... so many stars! We ended up replacing about 2/3rds of the steel.

This surveyor was no amateur ..... but he had not enough experience with ultrasonic meters. He needed to go and play with it for a few days in order to discover what it was trying to tell him.

 

[aidanjames]

Multiple echo thickness meters will cut out the paint thickness. They cost about £1k. Single echo meters will not and cost about £250. If you are thinking of measuring from inside the boat whilst it is afloat, the problem will be that you can not get to the areas that are most likely to corrode beneath tanks and the like. You will also not be able to chip away any loose rust safely to get a good surface to hold the probe against - there's a risk of going straight through! If you really need to know the condition of the hull then it has to come ashore. Diving is possible but in murky UK waters and by the time you've paid someone with the right kit and qualifications it would probably be cheaper to have the boat slipped anyway.

 

[Sultans_Kapitein]

Ultrasonic thickness checking is not a 'Black Art' but some surveyors seem to think it is. When I bought my boat I specifically asked the surveyor to perform an u/t check. Waste of time, obviously the guy did not know what to do, or he was a chancer because I had informed him that I used to perform these on steel and if he needed anything to let me know. Any way after purchase I found how little steel was remaining in places.
But if it is done using a digital meter and the steel is cleaned of surface rust and/or paint then the results tell you how much steel remains. The photo in thi link shows how easy it is
http://www.geinspectiontechnologies.com/en/products/ut/thickness_gauges/index.html
It is true that you can have a professional survey with tests at 20cm centers or what ever you choose, but the price is PER TEST so the more you have the more you pay, mind you if you have them every 10cm then it may be cheaper to buy a NEW boat!!
This is a look at a professional report I had done
http://i52.photobucket.com/albums/g37/MalFogg/General/nortestinspectionreportcont.jpg

Mal

 

[sail045]

Ultrasonic compressional thickness checking unless carried over large areas including suspected corrosion areas is really as posters have said , pot luck! and all it tells you is thickness of tested area, scan a Sq meter, scan many meters! and 2mm outside your scan pattern, an area could be rotten and you have missed it. wallthickness scanning, or as it is known a compressional scan, is normally used in the areas of parent metal either side of a welded joint. These areas are refered to as the 1/2 & full skip distances where llaminer type defects would hinder or prevent the shear wave scan of the weld area itself. In a nut shell wall thickness checking using a thickness meter, is just a random spot check and nothing more, used to bulk check material possibly where actual edge thickness measurement may not be available. As a point of pure interest I wonder how many of these surveyors are full profesional members of the British Institute of N.D.T. (non-destructive testing) and properly trained to use these simple meters ?

 

[BigART]

Just a point that we learnt the hard way. When we purchased our steel boat, we had a full survey including ultrasonic hull thickness measurements which were made on a 20cm grid pattern all over the underwater hull area. This was done out of the water from the outside (it sounds like you are checking the thickness from the inside). Looks very thorough and impressive - but is is literally a series of spot checks and unless the spot is on a problem area, it will not pick it up. 3 years after purchase, we found severe corrosion in localized areas that had obviously been there for some time. I have a picture of a gauge showing full hull thickness 4cm away from a rust hole.

Angus

Another point about my earlier post. The corrosion (sounds less bad than 'rust') we had was mostly confined to the top of longitudinal stringers where water had been trapped due to poor drainage. You can just see where the stringers are on the outside of the hull by slight dimples where the hull plates have been tack welded to the stringers. If I had another steel boat, I would get the tester to test along the areas on top of the stringers.

To stop a re-occurrence, we cut more and bigger drain holes, and I have inspection ports on the inner woodwork where I can see the tops of the stringers at their low points to check if any rusty water is accumulating.

A mentally scarring experience as some of the really bad areas were below the waterline.

Angus