The tracks are those made good by two similar 20 knot container ships both crossing the Bering Sea from North America to Japan.  Ship B in fact left a day earlier (11th January) than Ship A but the latter took the advice of the routing service to steer a more northerly course to avoid an area of low pressure.  Ship B, although following a shorter route in miles, encountered strong winds almost head on, with swells as high as 7.5 metres. It will be seen from the lines connecting the two ships’ positions as reported every 12 hours that by the 14th January, Ship A had already drawn level with Ship B and by the 16th January A had overtaken B by a whole day and so had completed the voyage in two days less by taking the advice of a routing service.  When one thinks of the daily running costs of a modern container ship one can easily see how quickly taking the advice of a weather routing service can pay.  It is not always as clear cut as this example and the routing services themselves admit that they do not get it right every time.  Meteorology has such an infinite number of variables that despite using some of the fastest computers in the world one may still be excused for contending that weather forecasting is an art rather than a science. A ship’s loadline – the greatest depth to which she may be loaded, varies depending upon the density of the water, the time of the year and the part of the world in which the ship is trading. The question of density of water is simply that if the ship is loading in fresh water she can load to a greater depth because when she reaches the sea, the greater density of salt water will lift her up to her salt water loadline.