‘The shortest distance between two points is a straight line’. This rule does not work at sea.  The first and most obvious example of this is governed by the fact that the world is a sphere and that therefore the ship has to travel round the surface of the sphere, following a course (a Great Circle) which on an ordinary chart using the Mercator Projection would show as a curve.  To this has to be added the effect of tides, currents, prevailing winds and the occasional storm. Both tides and currents can have significant effects on a vessel’s performance.  A prudent Master will plan his voyage so that he can take the full benefit, where possible, of these two factors, even to the extent of deviating from what would appear to be the normal course.  It is as easy to imagine the result of having the welcome assistance of an ocean current moving at a speed of, say, 4 knots, as it is to imagine the effect of having to spend several days punching into a contrary current moving at a similar speed.  A storm can equally well affect a vessel’s performance not only forcing it to slow down, even to the point of zero progress, but also increasing the risk of damage to the ship and to the cargo.  A prudent Master will, if possible, alter course well in advance to avoid sailing too close to a storm. In order to assist the Master in planning his voyage so as to take maximum benefit from tides and currents while at the same time avoiding areas of seriously adverse weather, there are now a number of weather routing services.  These are specialist companies, staffed with experts in meteorology and navigation, who have a constant stream of weather information being fed into sophisticated computer systems.