25 percent greater than the power on trial at speed V g. This is in keeping with the general 

 design practice that the service speed should be attained under trial conditions at 80 percent 

 of the maximum continuous power. 



Troost defined the speeds as follows: 



'BP 



1.85 - 1.6 Cp for sustained sea speed 



and 



[2] 



V J. = 1.06 y J for trial speed. 



For the Series 60 models, these two formulae lead to the following speeds for ships 

 400 ft in length. 



TABLE 9 

 List of Alexander and Troost Speeds 









ALEXANDER SPEEDS 











TROOST SPEEDS 







Cb 



Cp 







(Equation (1)) 











(Equation (2)) 







SERVICE 



TRIAL 



SEA 



TRIAL 1 



V 



© 



V 

 knots 



V 



® 



V 

 knots 



V 



® 



V 

 knots 



V 



^^BP 



® 



V 

 knots 



"J^BP 



"J^BP 



^^BP 



0.60 



0.614 



0.90 



2.358 



18.0 



0.96 



2.515 



19.20 



0.869 



2.274 



17.38 



0.921 



2.410 



18.42 



0.65 



0.661 



0.80 



2.045 



16.0 



0.86 



2.198 



17.20 



0.792 



2.022 



15.84 



0.839 



2.142 



16.78 



0.70 



0.710 



0.70 



1.746 



14.0 



0.76 



1.895 



15.20 



0.714 



1.781 



14.28 



0.757 



1.889 



15.14 



0.75 



0.758 



0.60 



1.462 



12.0 



0.66 



1.609 



13.20 



0.637 



1.552 



12.74 



0.675 



1.645 



13.50 



0.80 



0.805 



0.50 



1.190 



10.0 



0.56 



1.334 



11.20 



0.552 



1.338 



11.24 



0.596 



1.419 



11.92 



A comparison of these speeds with modern American practice was made by Mr. H. 



de Luce in his discussion on the first series paper. ''^ He examined the (C) curves for a 



V* 

 number of ships and plotted the value of the speed-length ratio ^— against prismatic 



coefficient for the point on the (C) curve where there was a sharp "upturn" (Figure 20). 



V 

 He drew a mean curve through these points, designated as the mean "upturn" , — on 



Figure 20. This figure also shows the Alexander and Troost lines, and it is clear that the 

 latter craiform much more with the general trend of the points and the de Luce line. An 



examination of the Cj curves for the Series 60 parent models, as given in Figure 18, shows 



V 

 that the values of ~~r^ for the "upturn" points for these designs also lie very nearly on the 



Troost "sustained sea speed" line, and the latter would therefore seem to be a close guide 

 to modem design trends (Figure 20). 



*Mr. de Luce used L np for single-screw ships and Lmr for twin-screw ships. 



V-15 



