Sec. 69.9 



GENERAL DESIGN OF PROPULSION DEVICES 



575 



reserve may then be as much as 0.3, 0.4, or 0.5 

 of the maximum designed power. 



Notwithstanding that the word "designed" 

 implies that the machinery is able to develop its 

 maximum power continuously, a full measure of 

 reUable everyday operation, over most of its 

 life, is assured by limiting the maximum designed 

 speed to that which is developed by say 0.95 of 

 the maximum designed power [Burkhardt, J. E., 

 ME, 1942, Vol. I, p. 28]. It is usually assumed 

 that this speed is to be achieved at the full-load 

 or other specified draft, in smooth, deep water of 

 the given specific gravity, in fair weather (little 

 or no wind), and with a clean bottom. In other 

 words, it represents a trial speed at 0.95 of the 

 maximum designed power, with a so-called 

 machinery reserve of 5 per cent. 



Actually, a ship design starts with the designed 

 sea speed or service speed, determined from the 

 schedule which the ship is to maintain, or from 

 a study of economic and other reasons. For the 

 ABC design this operation was completed by 

 the owner and operator before the design require- 

 ments were formulated. To compensate for slowing 

 down in heavy weather a reserve of speed above 

 the designed sea speed is necessary. This is 

 achieved either by one or by a combination of the 

 following: 



(a) Specifying it as an increment of speed, 

 resulting in the 1.8-kt differential of the ABC 

 design (the difference between 20.5 and 18.7 kt) 



(b) CaUing for a 'percentage increase in speed 

 over the designed sea speed, varying from about 

 8 to 15 per cent 



(c) Requiring a percentage increase in power 

 over that necessary to drive the ship at the 

 designed sea speed under trial conditions, usually 

 from 20 to 30 per cent or more. 



When taking account of small percentages the 

 question arises as to the point in the ship at 

 which the maximum designed power of the 

 machinery is to be delivered; also as to the kind 

 of power represented by it, whether indicated, 

 brake, shaft, or propeller power. There are 

 different means employed to measure power, and 

 there is still some uncertainty as to just where 

 along the hne, from the heat-to-work conversion 

 point to the propeller, the power is to be measured. 

 It is most important, therefore, that the hull and 

 propeller designers know exactly where this 

 point is, and what is transmitted there. In fact, 

 there are many good reasons for rating the 



propeller, not in terms of the power absorbed 

 (in horses) but in terms of the torque required to 

 turn it and the thrust achieved at the thrust 

 bearing, all at a specified rate of rotation [Smith, 

 E. H., lESS, 1954-1955, Vol. 98, Part 3, pp. 

 127-128]. The losses encountered in transmission 

 between the thrust bearing and the propulsion 

 device are then to be estimated or predicted by 

 the propeller designer in cooperation with the 

 machinery designer. 



As has often been done in the past, the designer 

 may wish to add a reserve of power over and 

 above that necessary for the sustained speed to 

 be achieved under trial conditions in good 

 weather, following the method of (c) preceding. 

 He may add an average percentage to this power, 

 using a figure taken from good practice, or he 

 may wish to base his reserve on an analysis of 

 the particular situation involved. At least four, 

 and sometimes six or more factors enter into the 

 percentage increase applied to the power pre- 

 dicted for sustained speed in good weather and 

 with clean bottom. These factors, with their 

 customary percentages, are: 



(1) Weather, involving an increase in 



power to maintain speed against 

 head winds and seas or to make 

 up time lost by slowing in waves 8 to 15 



(2) Fouling by marine organisms or 6 to 20 



other roughness or more 



(3) Increase with age of structural and 



propeller roughness and of dis- 

 placement (in some vessels) 2 to 5 



(4) Machinery reserve, to care for 



minor casualties, inefficient 

 handling, fuel under standard 

 quality, normal wear and tear, 

 and slow deterioration in per- 

 formance with length of service 4 to 6 



(5) Still-air and normal wind resist- 



ance of ship 2 to 4 



(6) Scale effect between model and 



ship (may be plus or minus) 1 to 3 or 4 



(7) Cavitation loss in high-powered to 10 



vessels or more. 



All these factors, if taken into account, may 

 total from 23 to 54 per cent or more, depending 

 upon their individual signs and values. It is 

 customary to omit some and to emphasize others, 

 particularly the increase for fouUng. The total 

 increase in power, over that required to maintain 

 the sustained sea speed under trial conditions, is 



