For ultrasonic thickness measurements of steel yacht structures i use Cygnus equipment.

Simulation of through coating measurement of a 15mm test block

Cygnus 4 for Multiple Echo Through Coating Measurement

The Cygnus 4 is a Multiple Echo meter. This means it can exclude the coatings from the measurement and antifoul removal is not required.

Cygnus is a UK brand, one of the best and their products are class approved for use by surveyors making measurements for classification societies.

Probes Used


2.25Mhz 13mm : This probe has a range in steel of 3mm – 250mm and works well with corroded materials and thick coatings. It is very successful on hulls through antifouling and enables me to work fast. Actually it is more successful than anything else I have tried. Surveyors using other meters and probes may struggle to do a good job on yacht hulls.


5Mhz 13mm : This probe has a range in steel of 1mm – 50mm therefore can measure thinner sections than the 2.25Mhz but does not have as strong penetrating power. I make general inspection with the 2.25 and then use this one where I cannot get readings to check for thinner areas.

Through Coating Limits

The Cygnus gauges generally go through up to 6mm of coating on normal mode and up to 20mm of coating on deep mode. There is no problem with coating thickness. It is very successful. Where the hull is faired though it is usually not possible except from the inside surface.

Pre-Purchase Survey of Steel Yachts

An ultrasonic survey is recommended in a pre-purchase survey of Steel Yacht. To get a good opinion of the hull I would propose to divide the underwater hull into 1m x 1m squares and do minimum 9 measurements per square and more in an area where excessive diminution found. It takes an additional day for a good ultrasonic survey. Steelworks required will be proposed.

Insurance Survey of Steel Yachts

The service is the same as above. The reports have been accepted by many major international insurers.

Owner’s Consultancy of Steel Yachts

For Owner’s work I would be working to 500mm x 500mm squares and making 9 measurements per square or more in areas where excessive diminution found. The survey would take one full day for the measurements. Then on the second day morning I would be making re-assessment and determining the steelworks required if any. Periodic Ultrasonic inspection every few years is recommended for steel yachts.

What other techniques are used?

Apart from visual inspection externally and internally where access permits, I use hammer sounding to listen to the change in note of the plating and identify thin areas. I also employ brutal use of a chipping hammer where I cannot get readings which will penetrate the unacceptably thin plating if it there. This will not damage sound plating but will just remove the coating which can be touched up.

Making holes in steel boats

How can we discuss the findings at site?

For ease of discussion I always use the method of writing the readings taken on the hull in chalk as I am taking them. This makes discussion at site very easy.


Does the couplant damage the paint?

The spots you see in the above photo are indeed couplant. This is needed for the probe to make proper contact with the surface and transmit the ultrasonic sound through the material.

I do not use oil based or grease couplants. I use a water based couplant that is specifically designed for this work and recommended by Cygnus. It will dry after the survey and can be washed off with water before any recoating. If recoating is not going to be done it would be fine to leave it.


How is the report presented?

The report will include text, photographs and tables of the readings but also CAD drawings showing the readings taken and the steelworks required. The drawings take a while to produce but are essential.

Do you do ultrasonic survey for classification societies?

No. I am not interested in that work so why go to the great expense of being class approved. I can recommend specialists companies for that work and it is very expensive.

Where is the Corroded plating going to be on Steel Yachts?

The hull surfaces may look good with coatings maintained over the years, no pitting or diminution externally but in the interior coatings may not have been maintained in some areas. There may have been leaks or condensation in these areas leading to severe diminution of plating. There may also have been wastage of frames at interface with the hull, in the bilges. If the boat is 30-40 years old it is very likely there will be areas of unacceptable diminution or potential holes if steelwork renewals have not been kept up to date. I think it is guaranteed for steelworks to be required on that age of boat.

What about Inaccessible Areas?

Areas inaccessible for survey are also the areas on the interior where coating has not been maintained. They may also be the areas where steelworks were going to be very difficult or expensive because of removals in-way of the interior required for steelworks to proceed. So steelworks in these areas may have been ignored. At the topsides where the exterior surface is faired and the interior surface lined with wood these again are areas where diminution may have occurred but it is impossible to inspect without removals.

This is the problem we face, the worst areas may not be inspectable.

Can ultrasonic inspection find the small wasted areas?

On external hull if the areas are tiny or small they will not be found by ultrasonic inspection. They may be found by hammering or visual inspection. It is not guaranteed however.

What is unacceptable diminution?

Diminution is loss of thickness when compared to the original thickness. Generally 20-30% diminution is where steelworks are required. Depending on how thick the plating was originally.

How can I help prepare for the survey?

The hull must be strongly pressure washed and may be scraped too after hauling so there is no marine growth. Any constructional drawings and any other documentation which notes original thickness of plating and indeed any previous ultrasonic survey or steelworks plans should be provided.

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