FV Trident inquiry – variable factors and conditional probabilities

In the Aberdeen Press and Journal of 5 March 2010 we read about the dialogue that took place between Ms Ailsa Wilson QC (representing the Advocate General) and Professor MacFarlane (expert witness for the inquiry) concerning the stability of the FV Trident:

"Ms Wilson said: "The feeling among the group [i.e. the relatives of those lost on the Trident] is that Trident had survived for 18 months and sailed in much worse conditions to those during the loss, so she must have been a risk.
Mr MacFarlane said he did not see the logic in this, adding: "If she survived much worse sea conditions and more difficult conditions then I don’t see how it could be said that she had a stability problem. That doesn’t seem logical to me.""

If the testimony mentioned above is accurately reported, then we have to accept that the concerns expressed thereafter by Mrs Jeannie Ritchie, one of the Trident widows -“We seem to be going round in circles and being baffled by science,” she said - may be well grounded.
For it appears that, in response to one of the relatives’ reasoned arguments as to what may have caused the vessel to capsize, Professor MacFarlane chose to give a rather superficial and possibly misleading response, neglecting to explain that a vessel’s stability - that gives a vessel its resistance to capsize - is not a constant, but a variable property, which depends on a number of variable factors, such as the disposition and weight of fuel, water, ice, fish etc., and that his answer would only be true, if the Trident’s stability had been exactly the same during her last as well as her earlier trips.

The underlying logic behind the proposition put forward by the relatives is really quite simple: if we assumed that the probability of any given vessel capsizing is dependent upon two principal factors:

• The intact stability of the ship – where the probability of capsize is inversely proportional to the ship’s stability reserves – which is, as mentioned above, a variable

• The sea conditions in which the ship is sailing – where the probability of capsizing is relative to the size of the waves

then, for the Trident’s trips, which she had successfully completed earlier and where the sea conditions were worse, we would have to conclude that the stability of the vessel was better.
By virtue of the same logic, we would also have to conclude that, on the Trident’s last voyage, taken in more benign sea conditions, but ending with the capsize of the vessel, it must have been the vessel’s stability that was worse. The suggestion, therefore, is that it was the stability rather than the wave height, which led to the capsize and loss of the vessel.

The obvious question that follows from here is - what level of intact stability did the Trident have at the time of her loss?