102 EKMAN. ON DEAD- WATER. [norw. POL. EXP. 



of the vessel (see Figs. 12, 14, 9, 13, PI. IV). Such irregularities are, however, 

 very easily explained, and the explanation is, in most cases, already given 

 in the accounts. The stripe to the windward side, might possibly be invisible 

 owing to the effect of rather strong wind-ripples (compare account No. 8), 

 although under different circumstances, if for instance the boundary-waves 

 are larger, the ripples may only have the effect of making the stripes more 

 apparent owing to the contrast between the smooth area inside the stripe and 

 the ripples outside it. If the vessel is sailing in a shallow fresh-water surface- 

 current and reaches right down into the still salt-water below, the surface-water 

 will run past the vessel with a certain velocity. (That such shallow surface- 

 currents exist in the Kattegat, was directly proved in the case reported by Count 

 Hamilton, account No. 14). The motion of the stripes (boundary-waves) relative 

 to the vessel, is then a resultant of their own velocity through the surface-water, 

 and the velocity of the latter past the vessel; and in consequence, the stripes 

 will be bent in the direction of the surface-current. This was apparently the 

 case in the accident described by Mr. Sidner (account No. 13). It might 

 perhaps be questioned whether this accident was not an effect of the surface- 

 current only, but the two stripes ("wakes") issuing from the stern, and 

 indicating the diverging waves, are so characteristic of dead-water that there 

 is really little doubt about it. Neither should the current alone have had the 

 effect of retarding the speed of the vessel. When there is a surface-layer of 

 lighter water on the top of the heavier sea-water, the former is evidently very 

 often acted upon by other forces than the latter; it will in consequence run 

 as a distinct surface-current on the top of the sea-water, and it must there- 

 fore be expected that the dead-water phenomenon will often be modified by 

 surface-currents. When a vessel is sailing with the wind from the side, her 

 leeway-drift might sometimes have a similar effect, so as to give to the stripes 

 a direction more to windward (compare account No. 6 and Fig. 12 PI. IV). 



The motion of the water around the vessel. 



It is commonly observed that the water surrounding the stern of a vessel 

 in dead-water seems to follow the vessel; and this is apparently confirmed 

 by the fact, that if a small boat is in tow astern, it is sucked up to the 

 vessel and cannot be kept clear. These observations may, however, be ex- 

 plained as merely an effect of the above-mentioned boundary-waves; and the 



