132 EKMAN. ON DEAD-WATER. [noRW. pot. exp. 



The curves (1), (3) and (4) show that there is a marked, although not 

 very essential difference in the dead-water resistance, between vessels of such 

 different types. The sharp-bottomed boat (with triangular cross-section) ex- 

 periences comparatively greater resistance than the round-bottomed one, if the 

 surface-layer be thin, and smaller resistance if the surface-layer be thick. 

 In the case of the flat-bottomed boat (with rectangular cross-section), it is 

 vice versa. The reason is apparent: if the vessel reaches below the fresh- 

 water layer, that part of its body which moves in salt-water does not con- 

 tribute to the wave-making and to the dead-water resistance, but would rather 

 have the contrary effect. Other circumstances being equal, that vessel which 

 has comparatively the greatest part of its body in the fresh-water layer, will 

 therefore experience the greatest resistance. On the other hand, as long as 

 the vessel moves entirely in the fresh-water layer, it will generate more waves, 

 and consequently experience more resistance, the nearer its whole body is to 

 the fresh-water salt-water boundary where the waves are generated. 



The influence of the ratio between length, breadth and draught of the 

 vessel, was not examined, as it seemed likely that it would have brought the 

 investigation beyond reasonable limits. This omission is however of no im- 

 portance, as long as the dimensions of the vessel considered, are in approx- 

 imately the same ratio as in one of the models 1 — 4. 



The above results do not confirm the experience of seamen that a sharp- 

 built fast-sailing vessel should be worse in dead-water than one of bluff 

 design (Accounts Nos. 3, 7, and 9, Chap. I). But as a matter of fact, the 

 contrary is stated in Account No. 8. The more general experience may perhaps 

 be explained simply by the circumstance that the finely constructed vessel had 

 a relatively smaller propelling force compared to its displacement. 



The force necessary to free a vessel from dead-water under given 

 circumstances, may, according to the theory given above, be approximately 

 calculated as follows. 



1) In accordance with the particular shape of the midship-section of the 

 vessel, select one of the resistance-curves (1), (3), (4) Fig. 3 PI. XI, or if neces- 

 sary, draw a new curve by interpolation. 



2) If the mean draught of the vessel is 4*4 m., the depth of the fresh 

 (or brackish) surface-layer may be sought out along the horizontal axis in 

 Fig. 3 PI. XI. If her mean draught is D metres, the depth of the surface- 



