134 



EKMAN. ON DEAD-WATER. 



[nORW. POL. EXP. 



is available for propelling the ship, the ratio between the useful work of the 

 propeller and the indicated work of the engine being generally called the 

 "propulsive coefficient". For a single-screw ship with thick wood stern- and 

 rudder-posts, the propulsive coefficient is estimated at about 40 per cent 1 . 

 The useful work should then be 84 H. P. = 6 300 kgr. m. per second, when 

 the ship had a speed of 3'1 m. per second; which gives a force of propulsion 

 = 6300/3'l = 2 032 kilogrammes. This value however, when compared 

 with the results of Froude's famous and excellent towing-experiments with 

 H. M. S. Greyhound 2 , seems rather too high. The dimensions of the Grey- 

 hound compared with those of the Fram are: 



The resistance of the Greyhound, when towed at a rate of 6 knots, was 

 1'4 english tons or l - 42 meter-tons. Supposing that the resistances of the two 

 ships are to each other as the areas of their wetted surfaces, the resistance 

 of the Fram at the same speed should be 0'92 ton, only. This supposition 

 is certainly approximately true; for at such slow speeds, by far the greatest 

 part of the resistance is due to friction against the ship's hull (see White's 

 "manual"). It is true that the Fram was proportionately shorter than the 

 Greyhound, and as Froude has proved, this circumstance slightly increases the 

 frictional resistance per unit of wetted area. But on the other hand, the dimen- 

 sions of the boats, and their displacements, show that the Greyhound cannot 

 have had such fine lines as the Fram. Finally, the Fram's resistance i. e. 

 her force of propulsion at the speed considered, has probably not been more 

 than 1 ton at most. This corresponds to a propulsive coefficient of 20 per 

 cent, which is not improbable, as the Fram's screw was made extraordinarily 

 thick to stand the shocks of the ice, and its effect was hindered by very 



1 See White, 1. c. p. 33, p. 543. 



2 White, 1. c, p. 475-476. 



