CHAPTER 4 



Resistance, Propulsion and 

 Speed of Ships in Waves 



1 Introduction 



The resistance, propulsion and speed of ships will be 

 considered only inasmuch as they are affected by wind 

 and waves. The evaluation of these characteristics was 

 the main objective of Kent's work (see numerous refer- 

 ences to Kent in Chapter 3). In particular, Kent 

 (3-193G/37; 8-1957)' can be considered as a summary. 

 Kempf (3-193G) also presented a thorough treatment of 

 this subject. The data given by these investigators 

 apply, however, to ships built between 1913 and 1934. 

 These ships were characterized Yiy higli block coeJlicients 

 and low horsepower per ton of displacement and they 

 lost the speed in head seas so rapidly that severe motions 

 did not develop. The loss of speed in waves at the full 

 engine power was, therefore, the primary olijective of 

 investigation. These data are still \'alid for tankers and 

 bulk cargo carriers the speeds of which are limited 

 primarily by the available power. In other types of 

 ships the violence of motions makes it necessary to reduce 

 the engine power in iiigh waves, ^rherefore, ship motions, 

 and not the available power, limit the attainable speed. 



The speed loss of a general cargo ship in waves is shown 

 in Fig. 1. Quoting from Lewis (1955a): "Speed reduc- 

 tion in hea\'v weather results from two types of influence. 

 The first is the direct effect of the added resistance to 

 forward motion caused by the action of wind and wa\'es. 

 The second may be termed indirect and refers to the 

 necessity of \'oluntary reduction of power — and hence 

 speed — to reduce the violence of the secondary motions. 



"...We have recently analyzed some log data for 

 typical cargo ships in winter North Atlantic service — 

 north of the British Isles to Scandinavia — undoubtedly 

 the most severe of the important trade routes of the 

 world. The graph shows first the expected trend of ship 

 speed with increasing average wave height, if full power 

 were maintained and only the added resistance effects 

 were taken into account .... The other steeper line shows 

 the actual trend of speed found by plotting average daily 

 speeds from log records. ..." 



Fig. 1 discloses that apparently engine power had to be 

 reduced in waves over 5 ft in height. This low height is 

 explained by the high power (8500 hp) and smooth water 

 speed of Victory ships for which the analysis was made. 



' The numl)er 3 preceding the year indicates tliat the reference 

 is made to the Bibliography at the end of Chapter 3. All other 

 references appear in the Bibliography at the end of this chapter. 



10 15 20 



Average Wave Height -Ft 



Fig. 1 Effect of head seas on ship speed. Victory Ship, No. Atl., 

 West B. (from Lewis, 195 5«) 



Aertssen (1957 NSMB Symp.) considers that 5000 hp 

 would have been normal for a ship of the size of Victorys. , 

 The maximum speed in smooth water then would have 

 been lower and a further reduction of power would have 

 been required only in higher waves. The basic con- 

 clusion would remain, however, unchanged; namely, the 

 speed of a ship at sea must be considered under two 

 conditions: 



1 In a moderate sea in which ship motions are not 

 critical and the speed is determined by the available 

 power. 



2 In a higher sea in which the \'iolence of ship motions 

 limits the speed which can be maintained safely. 



These two conditions will be separately considered in 

 the following. 



2 Ship Speed and Power in a Moderate Sea 



The resistanccof a ship in a moderate sea can be further 

 subdivided into: 



(a) Waves so short that there is no appreciable 



237 



