CHAPTER VI 



EFFECT ON RESISTANCE OF VARIATION 

 IN LCB POSITION 



In planning the Series 60 parents, a decision had to be made as to the longitudinal 

 distribution of displacement for each model. This distribution is conveniently described, 

 other tilings being equal, by the position of the LCB. 



This is an important parameter in ship design for more than one reason. So far as 

 resistance is concerned, the optimum position of the LCB depends very much on the speed- 

 length ratio at which the ship is to run. At high values of "^ , it is essential to keep the 



\/L 



bow fine to delay the onset of wavemaking resistance; at the same time, the stern cannot be 

 made too full or eddymaking resistance will increase. The result is a ship of overall low 

 block coefficient with the LCD aft of midships. For low ~ values, the stern must still be 

 kept reasonably fine to avoid excessive resistance, but the bow can be made much fuller, 

 since at such speed-length ratios the wavemaking resistance is only a small percentage of 

 the total. The result is a ship with a fine run and full entrance, witJi the LCB forward of 

 amidships. This trend is well illustrated in the Series 60 parents. The prismatic coeffi- 

 cients of the afterbody range only from 0.646 to 0.750 in going from the 0.60 to the 0.80 

 block coefficient designs, whereas the forebody prismatics go from 0.581 to 0.861. If the 

 efficiency is measured by the resistance per ton of displacement, the fuller ship is the more 



efficient at low spfeed-length ratios, and the advantage passes to finer and finer ships as 

 V 

 "y= is increased. 



The position of the LCB also affects propulsive efficiency for, in general, as it moves 

 forward for a given overall coefficient, wake and thrust deduction both decrease, but the 

 effect of the former usually predominates. Thus it is not unknown for a forward shift of LCB 

 to reduce both resistance and propulsive efficiency in such a way that the final shaft horse- 

 power* is increased. Insofar as hydrodynamic efficiency is concerned, the location of LCB 

 tJierefore rests finally on the delivered horsepower* required and not on the resistance, 

 although the latter is an important component of the former. 



There is also another feature in ship performance which depends on the LCB position, 

 and that is the behaviour in waves, both as regards ship motions and loss of speed. There 

 is little doubt that in the past ships have been built with too full bows, which may have given 

 excellent smooth-water results but have militated greatly against good seagoing qualities. 

 This question is one which should have an early priority in future methodical series testing. 



*Shaft horsepower is the power measured in the shafting, for example by torsionmeter. Delivered horsepower 

 is the power absorbed by the propeller. 



VI-1 



