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



According to Mr. G. A. Larsen, the dead-water stripes may, in particular 

 cases, repeatedly disappear and recur again. This may possibly depend on 

 velocity-oscillations, and as a matter of fact, Mr. Larsen derives his experi- 

 ence chiefly from towing on a river, where owing to the narrowness of the 

 waterway, these oscillations would be greater than on the open sea. 



How to get free of the dead-water. 



The principal hindrance caused by the dead-water is, as far as steamers 

 and towed ships are concerned, the loss of speed; while in the case of sailing 

 vessels, the loss of steering is a still greater trouble. 



To get a steamer or a towed ship free from dead-water, the point to 

 aim at then, is to regain her speed. Practical experience seems to have 

 discovered just the best possible means of accomplishing this (see the bottom 

 of p. 7), although they have not all been so successfully explained. The 

 simplest way, which also has the advantage that it is practicable for steamers 

 as well as for towed vessels, is just to stop for a while and then after the 

 boundary-waves have disappeared, to suddenly go full speed ahead again. 

 It is shown (pp. 75 and 91 — 92) that a considerably smaller force may be 

 sufficient to get a vessel free from dead-water when she is starting from rest, 

 than when she is moving at a gradually increasing speed; and there is there- 

 fore a good chance of succeeding in this way. If necessary, the manoeuvre 

 may be repeated, at suitably chosen intervals. It is said that vessels 

 towed in dead-water have sometimes got free after having run aground 

 (p. 13). Very possibly the simple stopping is the explanation of this, with 

 perhaps the addition of the fact that the steam was at top pressure when 

 the vessel at length got off. But the desired result might have had another 

 important origin, which will be mentioned below. 



Several means tried with the object of getting free from the dead-water 

 seem calculated to mix the fresh surface-layer with the salt water below. 

 Tug-masters for example sometimes take their tug to and fro along the sides 

 of the towed ship and then make full speed again; or they will shorten the 

 tow-rope so that the screw can stir up the water around the vessel in tow. 

 It will however be shown in the next section of this chapter, that the reduction 

 of the dead-water resistance caused by a moderate mixing of the water-layers, 

 is very small; and to sufficiently affect the water layers in this matter of mixing, 



