RESISTANCE, PROPULSION AND SPEED OF SHIPS 



247 



Table 4 



/! = niinilxT of (ilit^crviiliiiMs 



B = mean wind force — Heaiifort iscale 



D = weeks oul i.il clr\il(ick 



-V = averafif KPM 

 V = speed in knots 

 DHP = delivered horsepower 



Table 5 Particulars of Ships Investigated by Aertssen ( 1 959) 



Ship's Xanu' .1 (idolvilk / Bniiitoii nirillr Elitiiihdh 



Type of vessel Single-screw Single-screw 



steam-turbine stcam-turliine 



passenger-cargo vessel tanker 



Lengtli IIP, ft 511.8 628.3 



Breadth molded, ft 69,9 82.7 



Depth molde<l to upi)er deck, ft 40.4 44.5 



Depth molded to second deck, ft 31 .8 



Mean draft molded, ft 27 . 4 33 . 2 



Displacement (plated ), tons 18970 3S000 



Block coefticient, i . 672 770 



Prismatic coeflicient, <p 0,690 0,776 



Half-angle entrance, deg 10 24,5 



Deadweight, tons 10200 29200 



Power of engine, cv.l / 9500" in service 13000 



1 pv = 986 hp. . . . / \12500 maximum 



Uevolutions per minute 100-110 120 



Nominal sea speed, knots 16,5 16 



Screw particulars: 



1 Diameter, ft 19.7 20.4 



Mean iiitch, ft 18.2 14.8 



Develo|)ed area, sc^ ft 156 170 



Numlier of lilades 4 4'' 



2 Diameter, ft 19.5 



Mean pitch, ft 14.5 



Developetl area, sq ft 184 



Numlier of blades 5' 



" In service. 



* The four-bhided propeller was used for the tank tests and for the measured-mile trials; 

 the five-bladetl jiropeller was used din-ing the trials of May and June 1956. 



asse.-^sniciit of the sea conditions. This is an iiiiporttiiit 

 point wlien selecting a \veather scale by which the per- 

 formance of a number of vessels of different types are to 

 be compared" . . . "When analysing service performance 

 data statistically taking into account the separate effects 

 of wind and sea, a high degree of correlation was found to 

 exist between the wind and sea scales, ..." "This led to 

 the conclu,sion, which has often been suggested, that the 

 performance of a vessel may be determined from a single 

 weather scale, the value of this scale being determined 

 from the relative wind force and direction".... "If 

 this scale is then fixed, the statistical analysis determines 

 the combined effect of wind and .sea on performance, in 

 this instance being taken as the percentage increase in 

 power over still-air, calm-sea conditions to maintain a 

 given speed". . . "Considering first the case where the 



wind and sea are from ahead, analysis of the 



showed that increa.se in power was roughly proportional 

 to wave height. Analysis of the weather data for these 



two vessels and other data indicated that the average 

 wave height, for a given Beaufort number, agreed reason- 

 ably well with that c|uoted by Kus.scl and Macmillan." 

 It was also noted, as shown in Table — '- that a good ap- 

 proximation to these wave heights could be obtained by 

 scjuaring the sea distiu-bance number'^ (given in the 

 Table — •-) and dividing by two. Thus, since the increase 

 of power tlue to .sea condition is proportional to wave 

 height, which can be related to sea-disturbance numl:>er, 

 which can in turn be related to Beaufort number, we can 

 relate increase of power due to sea condition to Beaufort 

 number." Clements demonstrated by a plot that the 

 derived relationship checks well with the data reported by 

 six ships for winds up to Beaufort 6. Continuing with 

 quotations: "...for winds from ahead a convenient 



" Reference IV of Chapter 1. 



'^ This table is not reproduced 



" I.e., Admiralty sea scale number. 



