Steel and aluminium yacht and motor boat surveys with ultrasound thickness testing



Steel & aluminium surveys 
Survey Types
Pre-Survey Inspection & Report
Survey Costs
Terms and Conditions

Fay Marine Blog

Information Site

Fay Marine Yacht Plans



Steel and aluminium yacht surveys



Description and costs.

Click for time and costs       The Survey 


If you need a steel or aluminium boat survey then this is the place for information.

I have been working with steel and aluminium for many years. I survey all types of vessels including GRP, Wood and Ferro Cement but have specialised in metal since beginning to build steel yachts in 1981.

Surveying metal hulls is a specialist skill, it is something that takes years of experience and although the general surveyor will make a reasonable job of inspecting a metal hull, it really needs a specialist to carry out a detailed inspection.




The use of metal for small craft has changed over the years. Before the second world war there were very few steel and virtually no aluminium pleasure craft. Those that were built of steel had a plate thickness to allow for a good margin of corrosion which made them extremely heavy, even by the standards of that time.

During and just after the second world war, vast leaps forward were made in technology which began to allow small craft to successfully be built from steel. The standards of producing steel became better. In my early days dealing with steel for boats we would even check for flaking of the rolled steel caused by inadequate heat during the rolling process. All that seems to have been rectified, including steel coming from far eastern producers.

A major step forward during the post war era was the use of epoxy paints on sand blasted steel. These paints are so superior to all previous methods that the designers began to realise that yachts and motor boats could now be built without the need for a large margin for corrosion and the excessively thick plate needed to provide that.

This led to a quick growth of the production in small steel boats and there are many to be seen today of up to 50 years old. The Dutch really became the masters of the small steel boat and many are still produced in Holland.

During the last quarter of the 20th century aluminium boats began to come onto the scene. At first there could be problems with the material but technology quickly moved on and we now have the 5000 and 6000 series of aluminium for use in salt water which has largely cured most of the problems. There were horror stories of dropping a coin in the bilge and a hole being eaten in the alloy virtually overnight. Also there were and still are stories of stray current corrosion eating away the bottom of an alloy boat in hours. However with reasonable care none of this needs to be a problem, highlighted by the rapidly increasing number of alloy yachts to be seen today.

Aluminium of the correct grade is light, strong and corrosion resistant due to the formation of a layer of tough aluminium oxide. Aluminium is considerably lighter than steel but not as strong. The material used in the construction will therefore be considerably thicker than steel but will still represent an excellent weight saving. An aluminium hull will cost more in materials, needs slightly more care when welding and may be more difficult to repair in some out of the way place. However, it will generally require less maintenance and not need painting inside. It can produce a vessel which will be light and fast.


The Survey.                  Back to top of page      


When surveying a metal boat the first check will be visual. I will make a brief inspection of the whole vessel including bilges and inside lockers to identify any obvious areas of concern.

During the survey the areas I will be examining will include :-

Type or grade of material. Distortion. Paint quality. Flaws in the paint.

Thickness of the paint. Staining. Crevice corrosion. General visible corrosion. Galvanic corrosion and Cathodic (anode) protection. Pitting corrosion. Quality of welds. Fatigue. Signs of corrosion under fittings. 'White puss' corrosion. Thickness of the steel or aluminium. Use of doubler plates. Forming of the hull such as radiused corners, sharp edges etc. Deck drainage. Cockpit drainage. State of interior framing. How the stringers are finished. Condition of bilges. Depth of any corrosion. Whether corrosion is within acceptable limits or does it need re plating. Sea cock fittings / spigots etc. Type of sea cocks. Electrical insulation between hull and sea cocks. Electrical insulation between propeller, shaft and hull. Deck covering. Corrosion around ports.

During most surveys a surveyor will first have a general look around the outside of the hull and 'sound' or 'percussion test' the boat below the water line. This is where the surveyor uses a hammer along the hull and listens to the change in note which often identifies problem areas. I use a small plastic mallet. A skilled surveyor will quickly establish most areas of concern in any material with this method. Generally when sounding a hull the surveyor will tap the hull at around 75mm to 125mm centres depending on the surveyors personal preference. This highlights differences in the structure and a skilled surveyor will recognise where frames and stringers or internal fixings are present.

However, although this will generally highlight any flaws it is still possible for small areas of damage to remain hidden and not be found such as delamination of the bonding by a stringer or bulkhead on a GRP boat or a small area of corrosion next to a frame or stringer on a metal boat.

The next stage is the use of ultrasound (UTM) to measure the thickness of the metal. This is usually only necessary below the waterline where problems are suspected or where access is difficult and any other areas of concern that are noticed such as the edges of the decks and perhaps the web floors in the keel and any areas which are continually wet such as the anchor chain locker and built in tanks.

The ultra sound measuring tool sends ultrasound pulses through the metal which bounces back from the far side and the time taken for the sound to travel is measured and then gives the thickness.

There are two types of ultra sound tester. The first measures the thickness in one go and for this to work properly the paint must be removed. This is fine when the hull is being cleaned back to bare metal such as when being sand blasted for re painting but that is not the norm when carrying out a survey.

The second type takes two measurements. It first measures the thickness of the paint and then sends a second ultra sound which measures the thickness of both the paint and metal behind and subtracts one from the other to give the thickness of the metal. In general this is the preferred method as it is non destructive. However there are problems with this type measuring thinner metals through a coating and these normally will not measure the thickness of the metal if it is below 3mm thick.

As many yachts are now built of metals that start at 3 or 4mm thickness it can be seen that for thinner sections only a single pulse ultrasound will work accurately and for this the paint will need removing. However, a single pulse sounder can successfully be used through paint if comparing a suspect area to sound surrounding plate. This is similar to where an experienced surveyor will always take comparisons when taking moisture measurements on a GRP hull.

Obviously the through coating (paint) ultrasound measuring tool is also very expensive and is not carried by many surveyors, as most will rarely have cause to use it.

In order to take measurements a gel or oil must be used in-between the probe and the surface of the material. This is called couplant. The couplants that I use are a water based jelly, others are oil based. After the measurements are taken, spots of couplant will remain on the paint. With water based couplants, these will dry and can be washed off with water without any problem. Oil based couplants are more difficult to remove and are less preferable. As the couplant leaves a mark it is easy to see exactly where measurements have been taken.

In some small areas it may be necessary to resort to the use of a chipping hammer where UTM readings are not possible to identify plating with very severe corrosion.

As with the tap test although an ultrasound check will help highlight any flaws it is still possible for small areas of damage to remain hidden and not be found such as a small area of corrosion next to a frame or stringer on a metal boat. In fact a tap test will generally highlight the most flaws, as when metal is struck the vibrations (sound) travel out in the metal like waves when a pebble is dropped in water. An experienced surveyor will interpret what is heard and where there is a suspect area will increase the density of the taps to highlight and pinpoint any flaws. Compared to an ultrasound check this method actually covers more of the hull.

The surveyor must also assess factors like pitting corrosion. These are localised losses of material caused by galvanic corrosion resulting in pits. The depth and number of  pits over a plate must be assessed in order to determine if re plating is necessary.

The surveyor must also assess the quality of welds and if there was any defect when they were made or if they have become fatigued, cracked or are corroding.

Buckling of plates and localised structural damage to structural stiffening must also be assessed.

Galvanic corrosion and Cathodic protection (anodes) will also be checked to confirm there is not excessive galvanic corrosion occurring and poor cathodic protection present, requiring corrective action.

So there is much more than just the readings that are important. The readings must be interpreted in order to determine any repairs necessary.

Steel and aluminium is a specialist area of boat surveying and should be carried out by surveyors with particular experience in metals.


Time and costs              Back to top of page   

Find the cost of a survey here.

When making an ultrasound survey of a hull I will normally take readings at approximately every 75mm (3 inches). Where thin or suspect areas are detected I will start again at these areas and take multiple measurements at closer spacing to the original readings in order to determine the full extent of the thin areas.

I cannot see the back wall of the material while taking measurements so I am working partly 'blind'. It is likely that any plating which is heavily corroded on the interior surface is inaccessible in the interior of the boat due to furniture etc. being in the way. These are areas which may not have been painted properly when the vessel was new and will have had little maintenance over the years due to their inaccessibility and therefore have corroded due to condensation, leaks and standing bilge water.

It is not reasonable to take measurements so close that they cover the entire hull perfectly. This would be economically unreasonable as it would take too much time. Therefore I have to take sample readings and then further concentrate on areas where thin material is found.

It is not a small job to make proper measurements of a hull. Expect 1-2 days for the measurements of a 50ft hull and then I will need more time to make the report and diagrams if required.

In all my surveys I take many photographs and this is the same with a metal hull survey. Some photographs will be included in the report and all photographs taken during the survey will be supplied on a CD with the report.

In general the costs for a steel or aluminium hull only survey, will be approximately the same as the cost of a normal pre purchase survey of a vessel of the same size.

Therefore if ordering both a specialist hull survey and a normal pre purchase survey, expect the cost to be almost twice that of the normal pre purchase survey.


Back to top of page


Please read my terms and conditions if you require my services, as often there is no time for full exchange of written contracts when a survey is required. When employing Paul Fay to carry out a survey it is assumed that you have read and understood and accepted my terms and conditions.



For information or advice call or email.


 email    Telephone  (+ 44)   07715 102090



Paul Fay 2011

 Yacht Marine Survey Types | Terms and conditions Sitemap | Steel and Aluminium Boat Surveyor | Pre Survey Inspection | Walk Through Survey | Sail Boat Surveyor