Spring and Neap Tides

Tides have a considerable effect on merchant shipping, regulating available drafts and in some cases, periods of maximum and minimum port use. They are caused by the gravitational attraction of the sun and the moon and although the sun is much bigger than the moon, the moon is much closer and, as a result, the moon has over twice the tidal effect of the sun. At times of a new and a full moon, the gravitational attractions of the sun and moon add up to provide an extra-high tide called a Spring Tide whereas, at times of a half moon, the two gravitational pulls cancel out to give more nearly equal, Neap Tides. As the moon’s orbit is not quite circular, its distance and tide-forming capacity continually varies and tidal predictions are published to assist navigation at very many places. The following diagram shows Spring and Neap Tides. It is also useful to know how long reasonable tidal levels will remain in order that cargo can be discharged and vessels remain “always afloat”. In some ports it is impossible for the vessel to remain always afloat and here the term “NAABSA” may be negotiated which means “not always afloat but safe aground”. For additional protection the words may be added, “where it is customary for vessels of similar size to lie aground”.