74 EKMAN. ON DEAD-WATER. [norw. pol. EX P. 



The chief reason, for the latter fact, is, however, probably a much simpler 

 one. For as a rule, a sailing-vessel will be put in motion more gradually 

 than was the case with the boat-model in the experiments; and in the case 

 of a screw-steamer the propelling-force diminishes when the velocity is in- 

 creased, which circumstance prevents or reduces the velocity-oscillations. 



To the above investigation, one more remark may be added, which will 

 be of use later. The wave-making resistance depends, from one point of 

 view, on the work spent by the vessel in generating waves; and in this way, 

 formula (a) p. 72 is deduced, giving the wave-making resistance in the case of 

 approximately stationary motion. On the other hand, the immediate cause 

 of the resistance, is the water-pressure which is exercised on the vessel by 

 the waves. The instantaneous resistance therefore depends only on the shape 

 of the waves and their position relative to the vessel — whether they are ge- 

 nerated by the vessel or not. [In this case we disregard the frictional resi- 

 stance as well as the resistance to the vessel's acceleration]. A vessel might 

 consequently experience resistance even when at rest, if she be only sur- 

 rounded by waves. In this case, however, the waves will not have any per- 

 manent effect upon the vessel, for by passing the vessel, they will consequti- 

 vely take every position relative to her, and their only effect upon the vessel 

 will therefore be an oscillatory motion, insensible if the waves are short com- 

 pared to the vessel and of small time period. The dependance of the resi- 

 stance upon the waves only, seems to be in contradiction to formula (a) p. 72, 

 as the latter also contains terms depending on the vessel's velocity. The 

 latter formula, however, is based upon the supposition that the vessel's velo- 

 city is uniform, and that the waves follow her at the same velocity. In this 

 case, the waves obviously take such a position relative to the vessel, that the 

 work done in overcoming the sternward water-pressure, is equal to the work 

 required for the generation of new waves. 



The difficulties which the velocity-oscillations introduced into the determin- 

 ation of the connection between velocity and resistance, will be discussed ex- 

 plicitly in section D of this chapter. The most important one may, how- 

 ever, be shortly mentioned here. The maximum resistance in a given water- 

 layer, represented by the top-point of the resistance-curve, was determined 

 by a series of towing-experiments. A constant towing-force, each time greater 

 than in the preceding experiment, was applied to the boat-model, which was 



