Turning the Tide

The speeds of the currents in the English Channel and southern North Sea depend on the position and the time within a tide cycle (semidiurnal, so approximately 12 hours and 50 minutes); they range from 0 to more than 4 knots (nautical miles per hour). Also, the speed is different for different time cycles. This is because of the fact that the relative positions of the sun and the moon change in time.

There are times when one of the bodies magnifies the pulling effect on the others’ water. Other times one decreases the effect of the other. The first case is called spring tide, where the currents are stronger, and the latter is called neap tide, where the currents are less strong. Both occur approximately twice a month.

As a ship sails from the entrance of the English Channel to the harbour of Rotterdam, it covers more than 400 nautical miles (nm). If a ship sails at a constant speed over ground of 20 knots, this voyage takes approximately 20 hours, which is longer than a tide cycle. The ship encounters different speed levels and directions of currents because of its movement through time and space. As a result that the ship will have different speed levels at different times and places itself, which in turn has an effect on the position of a ship at a certain time.

The question arises how to optimally make use of the tidal currents on this voyage. Optimally can have different meanings. It is very interesting to know how a ship should sail to need as little time as possible to get to the destination. On the other hand, it could be decided that the ship should sail the route consuming as little fuel as possible within certain time limits. Both questions could lead to a different use of the tidal currents. Since each ship is different concerning maximum speed, sea keeping characteristics and fuel consumption, the questions have a different answer for each ship.

Royal Dirkzwager investigated how to optimally take advantage of the tidal currents during the voyage through the English channel and Southern North sea. The results of the investigation led to the conclusion that for higher sailing speeds (> 17  knots) an average fuel consumption reduction, depending on the moment of arrival at the start of the English Channel, of 1.8% is feasible without any consequences on ETA. To achieve this, the vessel has to adjust speed only once during the passage. Vessels travelling at lower speeds will have less possible advantages of the tidal effects.