90 EKMAN. ON DEAD-WATER. [norw. pol. EXP. 



The necessary arrangements were made very simply, and worked satisfactorily ; 

 nevertheless, the method was not successful. The oscillations were somewhat 

 diminished, but they were on the other hand less regular than before, and 

 the estimation of the mean velocity was therefore more difficult and less 

 reliable (see diagrams 62—64 and 90—94, PI. VII-VIII). This method 

 was therefore soon given up and a constant towing-force used during the 

 whole experiment. Only when the towing-force was very small, was a 

 small extra starting-force used. 



Special attention was paid to the determination of the maximum resi- 

 stance. As was pointed out on p. 75, this may be defined in two different 

 ways: either as the greatest resistance which the boat would experience at 

 any steady speed (below the maximum wave-velocity) ; or as the greatest 

 constant towing-force not able to bring the boat beyond that velocity, when 

 starting from rest. Theoretically, the former definition would offer the greatest 

 interest; but it would be very difficult to measure this maximum resistance. 

 A towing apparatus arranged for different uniform velocities and combined 

 with a sensitive dynamometer, would be required for that purpose; and further 

 a much longer tank, than that I had, would be a necessity, because the resi- 

 stance in the neighbourhood of the maximum, increases very slowly up to its 

 final value (see p. 73). The maximum resistance defined in the latter way, 

 is comparatively easy to measure; and it is also of more practical interest 

 than the former. The maximum resistance according to the former definition, 

 is equal to the propelling force which a vessel must have, so as to be never 

 troubled by dead-water; according to the latter definition, on the other hand, 

 it gives the propelling force which a vessel must have to get rid of the 

 dead-water (by simply stopping for a while and then making head-way again), 

 if she be accidentally caught by it. In either case, the maximum resistance 

 is physically a just as strictly defined quantity, and the rules in Chap. II, E, 

 are obviously applicable to it. For all these reasons, I have desisted from 

 determining the maximum resistance at steady speed and have kept to the 

 latter of the above mentioned definitions. 



The maximum resistance was therefore determined by towing the boat 

 repeatedly with gradually increased towing-forces. Take as an example the 

 experiments 77—84 (spec. gr. of salt-water 1'032; depth of fresh-water 5 cm.). 

 With a towing force of 2'2 gr. (exp. 82) the boat was held in dead-water. 



