| For ultrasonic thickness measurements of steel yacht structures i have available my Cygnus 4 and a Dakota Ultrasonics MMX-6. |
| Accurate triple echo measurement of a 15mm test block demonstrated through simulated coating.using Cygnus 4. |
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| The Cygnus 4 with my 2.25Mhz 13mm probe has a range in steel of 3mm - 250mm and works well with corroded materials and thick coatings (up to 6mm on normal mode, up to 20mm thick coatins on deep mode) provided the coating is adhered well to the steel. The triple echo method of the Cygnus means the readings are checked with the third echo and therefore are very reliable. The Cygnus 4 gauge model is used by many leading specialist ultrasonic companies. Cynus was the pioneer in through coating multiple echo technology. |
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| The Dakota Ultrasonics MMX-6 with 5Mhz 6mm probe does not work well with thick coatings or corroded material but has the possibility of working single echo from 0.63mm - 500mm and in multiple echo through coating mode from 2.54 to 25.4mm. |
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| I can work from the shell expansion plan of the yacht if the original thicknesses are noted on it. I can otherwise compare with a reasonably sound plate if possible, or I can just take measurements for general consideration. |
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| If I can see the welds between the plates then I would take 5 measurements of each plate (one in each corner and one in the center). But this is not always possible to see so otherwise I would take measurements equally spaced out over 300mm or 500mm. Patches of corrosion more severe than the overall general corrosion or pitting would be evaluated seperately. |
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| For pre-purchase survey of a steel yacht I believe it is essential to at least check the thickness of the underwater hull plating. It is likely anyway an insurance company will require an ultrasonic survey of a steel yacht. |
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| It is more scientific to work with ultrasonic thickness measurements than to make general visual assessment of the wastage. Generally 20-30% diminution (loss) of original thickness would be time to renew the steel. The minimum acceptable diminution would be determined within that range depending on how thin the original material was. It is not possible to estimate steelwork renewals accurately without ultrasonic survey. |
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| The work performed will be for the purposes of evaluation of a yacht by buyer or insurer. Also for owners information but will not be done for classification society purposes as that work is for the specialist ultrasonic companies which are licensed by the specific classification society. |
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| Past ultrasonic reports will generally be offered by the sellers. These may be recent or they may not. They may also fall short of being a good ultrasonic survey. Buyers should not rely on old ultrasonic reports which may have no relevance to the yacht's current condition. Additionally recent reports with only a few measurements per plate showing small diminution when the plate condition is obviously significantly wasted in particular areas, should not be relied on. Steelworks are expensive and are likely to require removals and reinstallation of the interior which may take equal or more man hours than the actual steelworks. |
