RESISTANCE, PROPULSION AND SPEED OF SHIPS 



241 



500 



400 





300 



■H 200 



(V 



a. 



100 _ 



Legend 

 -— \/L= 1.75 



V/L= 1.50 



- — \/L= I.2S 



X/L= i.o; 



X/L = 0.75 



S+lll Water 



--■■^ 





e.l5' Model 



0.2 



0.4 0.6 0.8 



Model Speed »- m/sec 



.0 



1.2 



Fig. 3 Resistance of Series 60, 0.60 block coefficient model 6.15 ft long in regular 

 waves 1.53 in. high (from Gerritsma, 1957 NSMB Symp.) 



0.40 



0.32 



^0.24 

 E 



-SO.IG 



0.08 



0.2 0.4 O.e 08 1.0 1.2 



Wave Length to Model Length Ratio 



1.4 



1.6 



Fig. 4 Speed reduction in waves of constant height at constant 



tow force — 5 and 10 ft models (from Szebehely, Bledsoe and 



Stefun, 3-1956) 



Several cur\'es result from the use of several towing 

 weights. An example of such a plot is given in Fig. 4. 

 This figure, as well as Fig. 3, gives the data on DTMB 

 Series 60, 0.60 block coefficient hull. One of these two 

 forms of plotting can be converted to the other by a 

 suitable interpolation. 



2.32 Typical data on resistance in waves. The 



uppermost curve in I'ig. 4 corresjioiKls to the towing 



weight which ga\'e the model speed in smooth water 

 defined by the Froude number of 0.28. For a Mariner 

 type ship this corresponds approximately to the trial 

 speed of 21 knots. The wave conditions giving the 

 maximum reduction of speed are similar to a swell 12 

 ft high. It may be assumed that in such a swell the 

 normal propeller RPM can still be maintained. The 

 towing-tank te.st shows that ship's speed has dropped to 

 about F = 0.20 or 1.5 knots. This is 71 per cent of the 

 smooth-water speed. Were propeller RPM maintained 

 corresponding to the third curve from the top, i.e., to 

 F = 0.185, the .smooth water speed of 15 knots would 

 have dropped to 6.7 knots. This is only 45 per cent of 

 the smooth water speed. This example demonstrates 

 the well-known fact that ships of low horsepower per ton 

 of displacement (i.e., slow ships) lose speed in waves 

 more rapidly than high-powered, fast ships. In this 

 example, howe-\'er, high and low powers were a]Dplied to 

 the same hull form. In practice low-powered slow ship 

 would have fuller lines and the lo.ss of speed in waves 

 would be further increased. 



The increase of the resistance in waves can be found in 

 Fig. 3. At the Froude number of 0.20 corresponding to 

 ship's speed of 15 knots, the model resistance is shown to 

 be about 110 grams in smooth water and it is increased 

 to 200 grams in waves of X/L = 1.01. This is an in- 

 crease of 82 per cent. The increase of resistance is about 



