Distances d'arrêt des grands navires en canal Etude de l'extension du canal de Suez : essais sur modèle

Stopping distances for large ships in a canal

¹ Ingénieur Expert, Etudes Maritimes SOGREAH
² Ingénieur d'Etudes, Service Maritime SOGREAH

Abstract

The stopping distances of large ships in a canal are a major factor in determining the volume of traffic it can convey. This is the case in particular for the Suez Canal. In carrying out the design studies for the enlargement of this canal, which will make it navigable by 150 to 250 000 dwt tankers, it was necessary to determine the intervals to be left between ships in the convoy. The straightforward extrapolation of present values was considered to be hazardous, and the Suez Canal Authority consequently resorted to model tests, effected in part at SOGREAH's Laboratory, using a ship model operated by a pilot on board. The model represented 160 000 dwt and 260000 dwt tankers, at scales of 1/39 and 1/44 (Figs. 1 and 2). Essentially, the aim of the tests was to define a stopping procedure which would allow steerage-way to be maintained throughout the manoeuvre. This requires the very progressive reduction, by steps, of the speed of rotation of the propellor, with especial care being taken at the helm to ensure that the ship returns to the centre-line of the canal after each partial deceleration. It is possible, in this way, to reduce speed to 8 km/h. Below this speed it is necessary to use, in fairly rapid alternation, engine bursts forward and astern, with wide rudder angles, allowing the ship's speed to be reduced to 4 km/h without her touching the banks. Once this speed has been reached, the way can be checked by all possible means : engine full astern, anchors dropped and running along the bank. The deceleration procedure is represented in diagrammatic form in Figure 4. Apart from the actual procedure, the main parameters which determine the stopping distances are the displacement of the ship, the speed of transit and the cross-section of the canal. Tests were run to determine the influence of these factors for the 160000 and 260 000 dwt ships, fully laden, with various canal cross-sections defined by the cross-section ratio n = S/s and by the width coefficient t = canal width at keel/ship's beam, and with speeds ranging from 9 ta 15 km/h (Fig. 3). The main results are as follows : - the stopping distances, as shown in table 2, reach 3300 m for the 160 000 dwt ship and 4300 m for the 260 000 dwt ship, at 15 km/h, - they are more or less proportional to the linear dimensions of the ships, - they increase with the square of the transit speed (figs. 7 and 8). - they are shorter with "wide" cross-sections (r = 3) than with cross-sections that are deeper and narrower, - cross-section ratios of n = 4.5 to 5 are satisfactory. In determining the regulation intervals to be respected by ships in a convoy, account has to be taken of the distance covered before the pilot realises that the way of the preceding ship has been checked, of the margin of uncertainty in assessing ship speeds, distances, etc.. , with the result that intervals of 25 and 30 minutes respectively were recommended for the two ships considered. With a current from the stern, the stopping manoeuvre can only be effected with tug assistance. The stated intervals could be reduced, with control stations every 2 or 5 km. In general terms, the study highlighted the correlation which exists between the stopping distances required by large ships and the width available on either side of the route to be followed by the ship.