NO. 15.] EXPLANATION OF THE DEAD-WATER PHENOMENON. 105 



the water around the vessel's stern actually follows her, and because the 

 water in front of the "bulk" — just behind the two rips, which evidently 

 indicate wave-crests — has actually a forward-motion against the sea-water 

 outside, and thereby causes a roar. When it is said that the bulk of water, 

 which follows the vessel, comes nearer to or draws farther off from the vessel, 

 according as the wind slackens or increases, the real cause of this observa- 

 tion is obviously to be found in the corresponding changes of position of the 

 dead-water stripes. 



When treating of the motion of the water behind the vessel, the oscillatory 

 movements observed by Mr. Larsen (see p. 14) should also be taken into account. 

 As has already been mentioned on p. 100, they are, however, a so obvious 

 consequence of the "dead-water waves", that no further explanation of them 

 is needed. Mr. Larsen's statement, that the moving effects of the dead-water 

 waves upon another vessel at her moorings, are greatest in the case of a ship 

 being towed down the river, can also be explained. The moving effect will 

 as a rule be greatest at the wave-crests, where the fresh-water is thickest, 

 because at the wave-hollows, a vessel of not too small a draught dips into 

 the salt-water below, which has a motion against that of the fresh-water and 

 consequently diminishes its effect. And the velocity of the fresh-water in a 

 wave-crest will be increased if the waves are travelling down river, and 

 vice versa if they are travelling up river. 



The loss of steering. 



The very common fact, that a vessel, when in dead-water, loses her 

 steering-power, finds its simple explanation in the above described motion of 

 the water around the stern. The fresh-water just around the rudder, moves 

 forward with the same velocity as the vessel, or even faster; the salt-water 

 just below, has perhaps a sternward velocity, but since the thickness of the 

 fresh-water is increased just around the rudder (i. e. it is a wave-crest there), 

 only a comparatively small part of the rudder is in the salt-water. A large 

 part of the rudder is consequently working in forward-moving water, 

 and that thoroughly explains why the power of the helm is lost, or at any 

 rate largely reduced. 



The degree and the manner, in which the steering is affected in dead- 

 water, vary infinitely ; it is hardly necessary, and it would indeed require too 



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