RESISTANCE, PROPULSION AND SPEED OF SHIPS 



251 



>l^ 



2S0 



(L/lOO) 



Fig. 14 



Aertssen's modification of Lewis' chart for ship speeds in a rough irregular sea 

 (from Aertssen, 1959 NSMB Symp.) 



factory parameter to whii'h to refer the performance of a 

 ship. The use of the wa\'e cHmeiisioiis appears to lie 

 preferable. The existence of several separate scales for 

 wave description is, however, confusing and it is sug- 

 gested that wave sizes in feet be used. Ships' log data 

 appear to be unsatisfactory in this connection because it 

 is often not clear whether a wind-wave or a swell code 

 numbers are used. 



4 Concluding Remarks 



The subject of resistance increase and speed loss of 

 ships in waves is subdivided into 



(a) Speed of ships as limited by the available power. 



(h) Speed of ships as limited by the severity of 

 motions, regardless of the power. 



In the past work, that of Kent, Kempf, and Mockel, 

 only the first of these subdivisions received attention. 

 The ships considered by these investigators were of full 

 form and low power. In a rough sea they rapidly lost 

 the speed, and dangerous amplitudes of motion tlid not 

 develop. These conditions still exist at present in the 

 case of tankers and bulk cargo carriers. The tanker in- 

 vestigated by Bonebakker, with 0.16 hp per ton, had to 

 reduce slightly the engine power in head seas when wind 

 exceeded Beaufort 5. This occurred, howe\'er, on only 

 16 observations out of 1)90. The primary objective of 

 research in this case is to evaluate the reserve power 



needed to maintain the normal ship speed in face of the 

 moderately ach'crse weather. 



The second consideration, {b), applies to all modern 

 ships except tankers and bulk cargo carriers. The lines 

 of these ships are sufficiently fine and the power is 

 sufficiently high to permit reaching dangerous speed in 

 wa\-es. In ad\'erse weather it becomes necessary to 

 reduce the propeller RPM and the engine power. The 

 level of weather severity at which this is done depends 

 on the qualities of a ship and on the judgment of her 

 master. With clean decks and without deck cargo a 

 fully loaded ship may be driven faster. The danger of 

 damage to the deck cargo is one of the main causes of 

 speed reduction. A dry ship will certainly be dri\-eii 

 faster than a wet one. 



The type of the chart developed by E. V. Lewis, as 

 shown in Figs. 12 and 18, appears to be an excellent 

 instrument for defining the permissible ship speed and 

 for judging relative seakindliness of ships. In its present 

 state of completion it permits only a rough judgment of 

 the first objective and none of the second. It must be 

 de\'eloped further by inclusion of a large number of 

 reliable sea observations. The initial steps in this direc- 

 tion were taken by Aertssen, but much more remains to 

 be done. It is suggested that log-book abstracts for a 

 numbei- of ships be plotted as this was done liy Lewis 

 and Morrison in Fig. 10. It is preferable to use the wind 

 strength for abscissa because the wave-height obseiva- 



