246 



THEORY OF SEAKEEPING 



used to e\'aluate the mean horsepower in all other ol)ser\-a- 

 tions.' The assumption is made here that wake fraction 

 and thrust deduction remain unchanged in waves. 

 Bonehakker defined s„ on the basis of the effective 

 propeller pitch: that is the propeller advance at zero 

 thrust. 



The use of ship's log data is often criticized because of 

 the alleged low accuracy. In this connection it is of 

 interest to quote from Burrill (1951):^ "The analysis of 

 .service or trial data is to me the correct means of assessing 

 propulsive performance, and my experience has been that 

 if sufficient information is available and is carefully 

 analyzed, a great deal can be learned from these ship 

 data. Those who would reject ship data on the ground 

 that they are not sufiicientl.v accurate are, I think, reject- 

 ing the criterion by which they are ultimately judged." 



Quoting from Bonebakker (1954): "First of all, the 

 relatiiin lietweeu wind forces and wave height was in- 

 vestigated. In Table 3 frequencies are tabulated, the 

 ab-scissa being wind forces, the ordinates wave heights in 

 meters. In the majority of cases the directions of wind 

 [uid waves coincided; so we may speak of the direction 

 of the weather. The direction of the weather relative 

 to the ship's cour.se is grouped according to Fig. 8." 



The discussion will be limited here to head seas; i.e., 

 Condition I or the uppermo.st curve of Fig. 8. Each 

 point on this curve is a mean of an indicated number of 

 ob.servations. It shows the increase of the delivered 

 horsepower o\'er the horsepower used at the same speed in 

 smooth water. The speed, however, is reduced from 

 that in smooth water because of the lack of engine power 

 to overcome the added resistance in waves. The data 

 incorporated in Fig. 8 are given in Table 4. 



The tanker, to which Table 4 applies, had a l)lock 

 coefficient of 0.744 and horsepower per ton of only 0.16. 

 Nevertheless, it was not practical to maintain the full 

 power beyond the wind force of 5. At the mean wind 

 force of 5.3 the RPM were reduced by 4.9 and the power 

 l)y 8 per cent. It was possible to maintain the full power 

 in two observations at the wind force 7 only because 

 the ship resistance was increased by the bottom fouling 

 (o9 weeks out of dock). The combined action of waves 

 and of increased fouling kept the ship at a safe speed of 

 9 knots: i.e. at 75 per cent of the initially low speed of 

 11.9 knots. 



The information given by Bonebakker (1954) on the 

 slow passenger ship is limited to Fig. 9 and material for 

 further discussion is not available. 



Clements (1956-57) extended Bonebakker's work by 

 explicitly expressing the effect of the bottom fouling, 

 the effect of the weather, and by basing the analysis on 

 the voyage data obtained from eight ships of different 

 commercial types. He assumed the relationshiji of the 

 form 



Table 3 Relation Between Wind Force and Wave Height 

 (from Bonebakker, 1954) 



' The reader is referred to Brard and Jourdain (.3-195:5 ) and Retail 

 and Birdel (:5-U).'j5) for description of another method of estimating 

 shaft horsepower on basis of propeller properties. 



'Discussion of Bonebakker's (1951) paper, Trans. XECI, 

 1051, pages Dl 15. References to figures and tables were changed 

 to correspond to the present monograph. 



Selected observations are F 



SHP/iV^ = aT, + bW + c 



(22) 



where T,i is time out of dock in weeks, and W the weather 

 factor (to be defined later). The linear assumption for 

 the increase of SHP/jV^ with the time out of dock is the 

 best approximation to cover most eventualities. In case 

 of tankers having short periods in port, the variation 

 of SHP/iV^ is well defined and the term in T/ can be 

 added. 



E. \. Telfer pointed out in the discussion of the 

 Clements' paper that the greater the fouling the less is 

 the propeller efficiency and hence the greater must be the 

 6-value for any particular weather. He suggested that a 

 more rational basic equation than (22) would be the 

 following: 



SUP/N' = aT, + (b + /37'Jir -f c 



(23) 



Telfer also pointed out that his own publications'" can be 

 considered as precursory to the work of Bonebakker and 

 Clements. 



The "weather factor," IF, is an empirically evaluated 

 function relating the increase of ship resistance to the 

 Beaufort wind force. A few sentences can be quoted 

 from Clements (1957) in order to outline the general 

 apiM'oach: "It was also considered after the analy.sis of 

 published data. . . , that the estimation of prevailing wind 

 conditions ^\•as far more uniform and accurate from man 

 to man and from ship to ship than the corresponding 



'" See entries inidi-r Telfer in the bibliography at the end of this 

 chapter. 



