244 



THEORY OF SEAKEEPING 



160 



160 



140 



120 



p 100 



o - 



80 



s 60 _ 



40 _ 



20 



IT) 



Figures near Spots indicate 

 Murnber of Observations on 

 which the Average is based 



/ 



/ 



/ 



Motor+anker 



1) Allowance Percentage due to 

 Wind, Waves and S+eerlng-Resistance 

 by Angles of Incidence I-V 



2) Wave Heights 



3 4 



Beaufort - 



Fig. 8 



Increase of shaft horsepower in rough weather (from 

 Bonebakker, 1954) 



Havelock and Hanaoka, Section 2.2 showed that ship 

 resistance is proportional to the scjuare of the wave height. 

 This fact precludes the application of the linear super- 

 position theory, Section 3-3, to resistance estimates. 



Since the superposition of the spectral components is 

 not possible, I'ershin and ^^oznessensky (3-1957) 

 solved the problem by summing the effects of 

 apparent waves. This solution was made possible by 

 adopting rather crude assumptions, such, for instance, 

 as neglecting the effect of phase-lag angles, which were 

 found to be important by Havelock and Hanaoka. The 

 authors justified these assumptions by showing that the 

 part of the resistance affected by assumptions formed a 

 rather small percentage of the total resistance at sea. 



The total resistance was defined by Per.shin and 

 Voznessensky as composed of: 



1 Change in effective resistance due to ship motions 

 and waves. 



2 Change of effective resistance due to wind acting 

 on .ship's hull and .superstructures. 



3 Change in propeller working conditions. 



4 Surface current induced by wind. 



5 Horizontal displacement of water participating in 

 water motion. 



6 Yawing. 



2.6 Observotions on Ships at Sea. RIacdonald and 

 Telfer (3-1938) gave a general review of .seakindliness of 

 .ships, which was based on seagoing experience. They 

 mentioned that a ship can be expected to face ordinary 

 heavy weather characterized by the apparent propeller 

 slip up to 30 per cent. They expressed the indicated 

 horsepower, needed in this connection, as 16\/.^, where 

 A is the displacement in tons. 



A good summary of ship resistance and powering in 

 heavy weather will be found in Kent (3-1958).^ Table 1, 



Table 1 Percentages of Total Ship Resistance With Weather 

 Head On 



Wind speed, knots 30 40 50 60 70 



Hull resistance in smooth %vater 56 46 36 28 21 

 Extra hull resistance in rough 



water .33 36 37 36 34 



Wind resistance 10 16 24 32 39 



Rudder resistance 1 2 3 4 6 



taken from Chapter 15 of that book, shows the distribu- 

 tion of the resistance for a low-powered ship in head seas. 

 2.61 Mockel's summary. Meckel (1944) summarized 

 the results of observations at sea collected by Kent 

 (3-1936/37) and by the "Sammelstelle fur Fahrtergeb- 

 ni.s.se der Hamburgischen Schiffbau-Versuchsanstalt." 

 Ship data, which satisfied the following conditions, were 

 chosen : 



1 The ship was fully loaded. 



2 The relative direction of wa\-es (wind-sea plus 

 swell) was within 30 deg off the bow. 



3 The swell was not higher than scale 4 when wind 

 did not exceed strength 5 on the Beaufort scale. The 

 swell exceeded scale number 4 when the wind exceeded 

 Beaufort 5. These conditions were defined on the basis 

 of the frequency distribution of meteorological observa- 

 tions. 



The wind strength on the Beaufort scale was chosen 

 as the parameter, to which all influences of weather on 

 ships were referred. 



The results of the investigation are summarized in 

 Figs. 6 and 7. These plots demonstrate vividly that 

 losses of a ship's speed increase rapidly with increase of a 

 ship's fullness and decrease of the engine power. 



Figs. 6 and 7 describe ship's operations in moderate 

 sea, as indicated l)y the A/L ratio not exceeding 0.75. 

 Under these conditions the ships ha\'ing block coefficients 

 in excess of 0.72 lose speed rapidly, while the ships with 

 lesser block coefficients are affected to a lesser extent. 

 It should be emphasized that with X/L ratio not exceed- 

 ing 0.75 there can be very little pitching and that the 



' Based on observations at sea and with emphasis on low- 

 powered ships. 



