Tides may have an impact on the safety of a berth. There is, for example, nothing unsafe about a berth to which the access is restricted (such as by a sand bar) at certain states of the tide, so long as the berth itself allows the ship to be afloat at all times. Some charters even allow for ‘not always afloat but safe aground’ (NAABSA) but unless the charter is negotiated in such a way to ensure time counts while waiting for the tide, then the owner suffers that delay. Theoretically this could be several days because during Neap tides when there is the smallest variation, the depth may not increase enough to allow access and the ship will have to wait for Spring tides which could be as much as a week or more. Where ships are engaged in short voyages, knowledge of tides can be even more important. There is no point in ordering overtime to discharge more quickly or rushing at full speed to loading port if the ship has to wait several hours for the tide to ‘make’ before she can enter the berth. Several areas in the world which are important to shipping can have navigation impeded or even made quite impossible during winter months. Take a look at a map of North America. It is far more economical for the farmers in the Canadian prairies to deliver their grain for export to Churchill in the Hudson Bay but navigation is only possible there for the four months July/October. The St. Lawrence Seaway linking the Atlantic to the Great Lakes allows ocean-going ships to reach deep into the heart of the North American continent but ice closes this route from about the middle of December to the beginning of May. Fortunately weather forecasting is fairly reliable and, so far, there have not been any cases of large numbers of ships being trapped in the Lakes by an unexpected early freeze-up.