RESISTANCE, PROPULSION AND SPEED OF SHIPS 



245 



Table 2 Particulars of Two Ships Investigated by Bone- 

 bakker (1954) 



speeti loss indicatecl is primarily due to direct iiction of 

 waves. A fine and powerful ship is shown to lose 10 

 per cent in speed under these conditions. The X/L 

 ratio of 1.0 was used earlier in an estimate of speed loss 

 of a Mariner which was made on the basis of the towing- 

 tank for series ()0, 0.(iO l)lock coefficient. Pitching and 

 heaving motions reach large amplitudes at this X/L ratio 

 and the speed loss of 29 per cent was indicated. 



Figs. 6 and 7 are of particular interest in connection 

 with full and low-powered ships. Under now pre\'ailing 

 design trends this applies to tankers and hulk cargo 

 carriers. Modern general cargo, passenger, and naval 

 ships are relatively little affected by short waves to which 

 these figures are limited. 



2.62 Bonebakker's dafo. Alockel's data were col- 

 lected from numi'ious ship oliservations and were pre- 

 sented directly in the form of speed loss versus wind 

 strength. It was impo.ssible to obtain detailed data on 

 ships' powering. Bonebakker, (1954) on the other hand, 

 made a detailed in\-estigation of the power consumed by 

 two ships under different weather conditions. The 

 properties of two ships are shown in Table 2. The in- 

 crease of the power caused by waves is plotted in Figs. 

 8 and 9. It will be observed that "pas.senger steamer" 

 is a relatively small and very slow .ship in ccjmparison 

 with a modern general cargo ship. 



Nine hundred and ninety sets of observations at sea 



P = Motortantter 

 SB = Passenger Steamer 



I ■= Weather Head On 

 U«Wea-fhen Following 



Figures near Spo+s indi'ica-Ve 

 Number of Observotions on 

 which the Average is based 



Following 



Fig. 9 Increase of shaft horsepower in rough weather (from 

 Bonebakker, 1954) 



were made on the tanker, but only eleven power records 

 were taken. It is a rather tedious process to obtain the 

 shaft horsepower of a ship at sea and ship's records often 

 do not contain sufficiently complete and accurate data 

 for this purpose. On the other hand, engineer's log 

 usually gives the data on the daily average apparent slip 

 Sa, defined as 



(propeller pitch) X total number of revolutions 

 distance by ob.servation 



(20) 



In a previous work Bonebakker (1951) established an 

 equation connecting horsepower with the apparent slip, 



(DHP)/(0.1 N)' = as, + b (21) 



where DHP is the delivered horsepower and is assumed 

 to be 0.97 of t he shaft horsepower. The coefficients a and 

 b u.sually have tiifferent \alues for a model and a sliip 

 and they also depend on the degree of fouling of a ship's 

 bottom. Once the coefficients a and b are e\aluated 

 from a few detailed power records, eciuation (21) can be 



