The leeway of 20-foot containers in typical distress conditions is
established through field experiments in a Norwegian fjord and in open-ocean
conditions off the coast of France with wind speed ranging from calm to 14 m/s.
The experimental setup is described in detail and certain recommendations given
for experiments on objects of this size. The results are compared with the
leeway of a scaled-down container before the full set of measured leeway
characteristics are compared with a semi-analytical model of immersed
containers. Our results are broadly consistent with the semi-analytical model,
but the model is found to be sensitive to choice of drag coefficient and makes
no estimate of the cross-wind leeway of containers. We extend the results from
the semi-analytical immersion model by extrapolating the observed leeway
divergence and estimates of the experimental uncertainty to various realistic
immersion levels. The sensitivity of these leeway estimates at different
immersion levels are tested using a stochastic trajectory model. Search areas
are found to be sensitive to the exact immersion levels, the choice of drag
coefficient and somewhat less sensitive to the inclusion of leeway divergence.
We further compare the search areas thus found with a range of trajectories
estimated using the semi-analytical model with only perturbations to the
immersion level. We find that the search areas calculated without estimates of
crosswind leeway and its uncertainty will grossly underestimate the rate of
expansion of the search areas. We recommend that stochastic trajectory models
of container drift should account for these uncertainties by generating search
areas for different immersion levels and with the uncertainties in crosswind
and downwind leeway reported from our field experiments.Comment: 25 pages, 11 figures and 5 tables; Ocean Dynamics, Special Issue on
Advances in Search and Rescue at Sea (2012
