Hydrodynamic Problems of Ships in Restricted Waters

In contrast to land-based vehicles, ships and airplanes normally move in a relatively unrestricted fluid environment, except when approaching or leaving ports. Hence, in the early periods of scientific study of the dynamics of these vehicles, only minor importance was attached to the finite extent of the fluid in which they move. This was especially so for ships, whose only necessary close approach to solid earth ideally takes place at extremely low speed. This comfortable situation hardly applies today. The explosion in size (Phillips­ Birt 1970) of supers hips over the past three decades (a factor of about 15 in displace­ ment, or about 2t in each of length, beam, and draft), has created new problems. Open water that was formerly considered unrestricted in depth now becomes shallow for some of these ships. The biggest of them can hardly enter any port at all when fully loaded. In order to make use of facilities such as oil refineries already built in harbors with restricted depth, supertankers must sometimes enter partly unloaded and move very slowly in a situation of some hazard. Hazard is indeed ever-present for large ships in restricted water, and the consequences of the slightest error of judgement can be multimillion dollar damage and loss of life; a typical example is the Hobart bridge disaster of 1975 (Australian Department of Transport 1975). The task of a pilot is a very difficult one. Hydro­ dynamic effects on the ship's dynamics of restrictions of available water are poorly understood and sometimes contrary to intuition. Furthermore, they can be accom­ panied by loss of rudder effectiveness. This article concentrates on theoretical studies of hydrodynamic problems for ships in water restricted in depth and/or lateral extent. Experimental work via modcls is very difficult and expensive in shallow water. The unfortunate tendency of the practical naval architect has always been not to trust theory, but instead to place perhaps undue trust in extrapolation of model experiments. This tendency is justifiable in some situations where water is unrestricted, but the problems of scaling are so much greater in restricted water that the role of model experiments should ultimately be simply to verify the accuracy of theoretical predictions.