NO. 15.] EXPLANATION OF THE DEAD-WATER PHENOMENON. 97 



once retarded, and the vessel is caught by dead-water. Independently of whether 

 it is the water-layers or the propelling-force which is changed, it will have 

 no peculiar effect on the speed of the vessel, until the propelling-force is 

 equal to the minimum or the maximum resistance; but then the vessel with 

 a sudden change of velocity, falls into dead-water or gets rid of it, as the 

 case may be. This obviously explains the fact lhat as a rule, only sailing 

 vessels and towed ships are liable to dead-water; the propelling force of a 

 steamer being in most cases sufficient to get over the maximum resistance. 

 It also explains the experience of sailors, that they (at a given place and at 

 a given season etc.) are safe from dead-water when going at above a certain 

 speed; because the vessel's speed is a measure of her force of propulsion. 



Keeping to our first example (Fig. 8, PL VI, curve 4), suppose the vessel 

 to move — under sail or steam -- at a speed of 14 dm. per second. If the 

 vessel- accidentally — for example by currents or dining a manoeuvre — has 

 her speed reduced to somewhat less than 9 dm. per second, oidy for a few 

 moments, the resistance becomes increased, the velocity is further decreased 

 to 4'6 dm. per second, and the vessel is then in dead-water. This explains 

 why vessels, according to the experience of several sailors, are most liable to 

 dead-water when making turns in their course or as a consequence of bad 

 steering. It also explains the capriciousness with which the dead-water some- 

 times seems to occur. Indeed, it is often a matter of mere chance whether 

 waves, manoeuvres, changes in the wind, etc., conspire together unfavourably 

 or favourably — in other words so as to bring a vessel into dead-water or 

 out of it. Currents in the surface-layer depending on boundary-waves 1 may 

 especially be one of the causes affecting the speed (see foot-note, p. 73). 

 One vessel may therefore fall into dead-water, while another quite similar, or 

 a worse vessel, goes past her a short distance away without experiencing any 

 trouble at all. 



The appearance of the sea-surface. 

 The various descriptions in chapter I, of the appearance of the sea-surface, 

 do not agree with oneanother, in all their details. A close agreement was 



These may have been created by other vessels in dead-water, or by various other 

 causes, as for example by the motion of one water-layer above another just as a wind 

 affects the water-surface, or by accidental or periodic motions at sea reaching the mouth 

 of a fjord covered with a layer of light surface-water. 



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