Monday, July 05, 2010

FV Trident - Centres of gravity

During the Original Formal Investigation (OFI - 1975) into the loss of the fishing vessel Trident, matters pertaining to her stability were examined in great depth; however, when it came to writing the final report of the investigation, the Court felt unable to make a pronouncement on whether the Trident had met IMCO’s minimum stability standards or not:

It is impossible to assert categorically that Trident did or did not comply with the IMCO recommendations, to which it was intended that she should be built

Apart from the uncertainty arising from the absence of an inclining experiment, one of the principal reasons cited for this unfortunate lack of assurance was the fact that there were known discrepancies between the designer’s original drawings and the as-built hull shape of the Trident and her sister vessel, the Silver Lining.

In the OFI’s final report, this point was emphasised by including two stability calculations as an Annex, one based upon the dimensions taken from the designer’s drawings (Bute lines) and one based upon the dimensions lifted from her sister vessel, the Silver Lining (Napier lines).

These two sets of stability calculations show that the Trident’s stability passed the IMCO recommendations when the calculations were carried out based upon the ‘Bute lines’ dimensions, but failed the IMCO recommendations when the same calculations were carried out based upon the ‘Napier Lines’ dimensions:

Partial copies from OFI final report plus amendments

This all seems to be quite straightforward and, apparently, justifies the Court’s uncertainty on this matter.

However, what is not immediately apparent, both from the 1976 OFI report and from the transcripts of Court evidence, is the fact that the real reason why the Bute hulled version of Trident ‘passed’ and the Napier hulled version of Trident ‘failed’ was that the vertical centres of gravity (VCG) that were used for these two separate stability calculations were different:
  • Bute hull lightship VCG = 10.072 feet above the keel (a value of unspecified provenance)
  • Napier hull lightship VCG = 10.487 feet above the keel (value derived from the inclining experiment carried out on Silver Lining and subsequently used in the 1976 NMI research)
If the same vertical centre of gravity figures had been used in both of these two stability calculations, they would have indicated either a double failure (when the VCGs = 10.487 ft) or a double pass (when the VCGs = 10.072 ft)
i.e The Trident’s stability calculations would show a pass or a fail (with respect to the IMCO stability standard) dependent upon the value of VCG used and not, as the OFI implied, due to differences between the Bute and Napier hull geometries, which were not, by themselves, sufficient to influence the results of the stability calculations.

What is more, it would appear that this was not just an oversight, but more of a deliberate ‘smoke and mirrors’ exercise by the DOT who, when they drafted conditions A1 and A2, also forgot to include the lightship VCG figures in the tables that are contained in the final OFI report:

If one reads through the transcripts of evidence for the 1975 inquiry, it is notable yet again [link to previous FV Gaul post] that Council for the DOT (who in 1975 carried out similar functions to those performed by the AG in the 2010 inquiry) was quite anxious to put forward the notion that Trident had complied substantially with the IMCO minimum standards.

(More to come)

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