94 EKMAN. ON DEAD-WATER. [NORW. POL. EXP. 



This conclusion would be correct if the resistance against a vessel were directed along 

 the same line as her velocity relative to the water. But just because the water offers a 

 greater resistance to the athwart motion than to the headway motion of the vessel, the 

 resistance will have a more athwart direction, if the vessel be moving obliquely through 

 the water; that component of the resistance which acts in the direction of the keel-line, 

 and opposes the vessel's head-way motion, will obviously be independent (or nearly inde- 

 pendent) of her athwart-motion or of a current impelling the vessel sideways. If Kapt. 

 Meyer's deduction were correct, it would also appear, that a sailing-vessel should be unable 

 to make head-way in a strong side-wind because the water makes a great resistance against 

 her leeway-drift. Likewise, the "otter-board" used for keeping a fishing-line with hooks 

 strained sideways from a moving boat, would not be able, in spite of the strain in the line, 

 to run up almost in a line with the vessel, as it actually does. 



One more mistake in the same paper, may be pointed out, although it is not of the 

 same definite importance in estimating the value of the explanation given. Kapt. Meyer 

 distinguishes distinctly between two cases; viz. when the two water-layers move with dif- 

 ferent velocities but in the same direction, and when they move in different directions. 

 In the former case a vessel should not be essentially influenced by the currents when 

 moving in their common direction of velocity, but in the latter case the vessel must always 

 be impelled from the side, by one of the currents at least, in whatever direction she might 

 move. The latter is, however, not true, since there is always one direction in which both 

 currents have the same velocity component; and if heading perpendicularly to this direction, 

 the vessel will apparently be carried sideways with a velocity equal to that velocity-com- 

 ponent, and will cut the water at all levels in the direction of her keel-line. That a vessel 

 is able to cut both currents in the direction of her keel-line, is also clear, for the simple 

 reason that the case considered may be reduced to the case of a current moving on the 

 top of still water, by simply superposing on the whole system a velocity equal and opposite 

 to the velocity of the under-current. 



The loss of speed; the sudden appearance ami disappearance of the 

 dead-water ; its apparent capriciousness. 



The generation of boundary-waves and the consequent loss of speed proved 

 by the experiments, has been repeatedly pointed out above, and it is therefore 

 not necessary to further enter into this subject very closely. It is common 

 knowledge that a vessel when moving, under ordinary circumstances at a good 

 speed, creates appreciable waves in the surface, which contribute to the re- 

 sistance; and when she attains a certain higher speed depending on her di- 

 mensions, these waves and the resistance caused by them, increase so forcibly 

 as to practically restrict the vessel's speed to within this limit. In shallow 

 water there will be a corresponding limit, which as shown by Scott Russell 

 (see pp. 38 seq.), is comparatively low, and is determined by the depth of the 

 water. If the vessel's velocity is raised above this limit (which is the maxi- 

 mum wave-velocity) the waves practically disappear and the resistance corres- 

 pondingly diminishes. The case of dead-water is quite analogous. But the 

 boundary-waves causing it, arise at a much slower velocity than do the 



