356 Prof. Adams on the Application of Screw-Blades 



in a minute nearly, and n the number of revolutions a minute, 

 then 27rft = ft>and 



If, as in the case of the ' Elfin/ the radius of the wheel is 5J 

 feet nearly, then 



therefore 



1 , 07 3000 90 , 

 1 + 2k= — — = — nearly : 

 Wirn n J 



therefore if 



rc=45, Jc=i, 



showing that, if the number of revolutions a minute be 45, 

 then all that portion of the blade should be cut away which is 

 within a distance of the axis of the wheel equal to two-thirds of 

 the radius ; i. e., half the blade should be cut away. 



In the trials with the l Elfin ' (see the report in the f Times ' of 

 April 5th) the area of immersed screw-blades when complete was 

 72 feet ; hence theoretically, with forty-six revolutions a minute, 

 rather more than half the blade should be cut away, leaving an 

 area of about 35 feet; practically it was found that the speed 

 was the greatest when the area was reduced to 32*7 feet, show- 

 ing a very close agreement between the theoretical and experi- 

 mental results. 



(4) If the radius of the paddle-wheel be 9 feet, then r =6 feet, 

 and 



J=8(l+»). 



If we again suppose the velocity to be 10 knots an hour or 1000 

 feet in a minute, and the number of revolutions a minute to be 

 33, then 



v_1000 



»- 200 ' 



and # = £, or one-third of the float should be cut away. If the 

 number of revolutions a minute be only 28, then 



v 1000 e . 

 ^ = l6^ =6near1 ^ 



and k = %, showing that in this case one half of the blade should 



be removed, or that no part of it should be at a less distance 



from the centre than two-thirds of the radius of the wheel. 



v 

 If — = 4 J, then ^ = |-, and for a velocity of 10 knots an hour 



