Manoeuvrability and Propulsion of Very Large Tankers 



Table 7 



Hogner-Type Stern Better (+) or Worse (-) Than Conventional 



Stern Arrangement at Light Condition 



Finally, it may be concluded that an afterbody arrangement as suggested in 

 the introduction offers a means of improving the propulsive efficiency for large 

 tankers in addition to minimizing vibration and cavitation problems. Economi- 

 cal considerations with respect to the increase in building costs and the de- 

 crease in costs as a consequence of the smaller required SHP and probably 

 lower costs of maintenance will give the final answer to the shipowner on the 

 question in how far application of this advanced propeller arrangement for high- 

 powered tankers is not only a dream of the hydrodynamist. 



3. A COMPARATIVE STUDY OF THE STEERING OF 

 TANKERS WITH LATERAL JETS AND WITH 

 A RUDDER 



As stated in the introduction of this paper, the technical merits of steering 

 a tanker with lateral thrust units instead of with a rudder are twofold. Firstly 

 by eliminating the rudder a more favourable afterbody shape -propulsor combi- 

 nation can be designed resulting in a substantial decrease in required shaft 

 horsepower and improved vibration and cavitation characteristics; secondly a 

 better manoeuvrability at low speeds in confined areas, harbours, and river 

 estuaries can be expected resulting in partly or completely eliminating tug boat 

 assistance, the economic benefit of which will be important to tanker owners. 

 The main problem of the jet steering units are their horsepower requirements 

 at the higher ship speeds. 



English (11) and Chislett-Bjorheden (13) have measured the side force cre- 

 ated by a lateral thrust unit as a function of the ratio ship speed to jet speed VU. 

 English found a 50 percent decrease in side force for a typical screw-driven 

 lateral thrust unit when the ship speed was increased from to 2 knots. This 



255 



