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



so will not disturb the boundary 1 . It is clear that a vessel getting free 

 from dead-water after having run aground (see p. 13) might owe a good deal 

 to the simple fact that she was there in very shallow water. 



It is obviously more difficult for a sailing vessel to get rid of the dead- 

 water. To stop and then to set all sail again suddenly, will hardly be possible 

 especially as the steering-power is lost. But by diminishing the effect of 

 the wind upon the sails to a minimum and then after a while increasing 

 it again as quickly as possible, it might nevertheless often be possible to get 

 free. If the wind-direction, the possibility of manoeuvring, and other circum- 

 stances allow, it would certainly be advantageous to steer inshore into as 

 shallow water as possible (see above). 



There may be cases in which neither of these methods proves success- 

 ful or is available, and in which the vessel cannot be freed from dead-water. 

 It will in this case be a great advantage to be able to manoeuvre the 

 vessel, even if her speed be reduced. If there are no strong currents in the 

 water, this would as a rule be possible simply by using a rudder so situated 

 that the water surrounding it has a sternward motion or at least, does not 

 move with the vessel. It is shown, that aft of the stern, to the side of the 

 vesseVs midline, the water is running sternwards at a considerable rate ; and 

 the chances therefore appear very favourable that with an auxiliary 

 rudder or simply with an oar, in this region, the vessel can be steered 

 even in dead-water. 



The occurrence of dead-water. 



It follows from the experiments and their discussion in the next section 

 of this chapter, that the resistance due to the generation of boundary-waves, 

 may be considerable if the difference of specific gravity between the surface- 

 water and the sea-water below, be not too small, and if the former have a 

 thickness comparable to the draught of the vessel (between half and twice 

 as great, say). It is not essential that the transition from salt to fresh water 



1 This is obvious, because the motion will then be symmetrical with regard to the 

 boundary; at any rate if the vessel has sides vertical from the water-line right to the 

 bottom. It would have been very interesting to have an experimental confirmation of 

 these points, but I did not think of this method of getting rid of the dead-water, 

 before the experiments were finished. 



