Manoeuvrability and Propulsion of Very Large Tankers 



reduction in propeller horsepower compared with a conventional stern arrange- 

 ment. From the basic concepts it appears that a reduction in propeller horse- 

 power is possible either by reducing ship resistance or by improving the pro- 

 pulsive efficiency. The present tests with a 90,000-TDW tanker model show 

 both these improvements, the resistance being reduced by 1 and 5 and the effi- 

 ciency improved by 5 and 4 percent at the loaded and light condition, respec- 

 tively. According to another well-known equation, the propulsive efficiency may 

 be expressed as the product of the propeller, hull, and relative rotative effi- 

 ciencies. Unfortunately the paper does not give any information as to the way in 

 which these propulsive coefficients are affected by the proposed nozzle propeller 

 arrangement, and I would like to ask the authors if any such information is avail- 

 able. It would also have been interesting to know what efficiency one may obtain 

 by using an unducted propeller in combination with the Hogner stern. From the 

 related work on a 39,000-TDW tanker model (Section 2 of the authors' paper), 

 one gets the impression that the Hogner stern permits the propulsive efficiency 

 to be improved also with a normal free propeller. 



The idea of using an ejector type of thruster is interesting as it seems to 

 offer a solution to the problem of combining a high-pressure cargo pump with 

 an effective low-speed transverse jet. However, I would like to draw your at- 

 tention to the fact that it might occasionally be desirable to have the possibility 

 of using the thrusters and the cargo pumps simultaneously, for instance when 

 loading or unloading at mooring, for trimming purposes when getting out of har- 

 bors, etc. Obviously there are several other aspects, mainly of a practical na- 

 ture, which should be regarded when considering a cargo-pump-driven thruster. 

 With respect to the human factor, one might expect for instance an increased 

 risk of accidental discharge of cargo oil. In my opinion, a cargo-pump thruster 

 should not be considered unless it shows marked economical benefits over con- 

 ventional thrusters. The present paper does not give any clear indication of 

 this point. The power figures given represent only the jet power. I would like 

 to know if the authors have made any estimates of frictional and bend losses in 

 the rather extensive pipeline they are proposing, and if a normally sized pump 

 equipment is capable of producing a sufficient thrust. It would also be interest- 

 ing to see a comparison between the costs of a cargo-pump thruster including 

 valves, nozzles, pipes, governors, etc., and a corresponding conventional 

 thruster. 



Among the possible merits of a thruster steering system that the authors 

 mention are the reduced tug assistance costs. To illustrate the rate of these 

 savings I would like to submit some statistical data given to us by a well-known 

 shipping company in Europe. The figures apply to two 12,000-TDW sister cargo 

 liners which have been in service between Europe and the Far East for about 

 two years. One of them is equipped with a 600 -hp bow thruster plus a 

 controllable -pitch main propeller; the other has a conventional fixed-pitch 

 propeller. It appears that for the ship with bow thruster, tug costs are reduced 

 by about 80 percent compared with her sister ship without bow thruster. For 

 some of the ports along the trade the following numbers of tugs are needed in- 

 cluding both arrival and departure: 



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