426 On the Various Modes of Propelling Vessels. 



forward at various times since the screw propeller first attracted 

 attention, but its excellent qualities were not completely utilized 

 until each screw was worked by separate engines. Even with 

 these, which may be comparatively light and inexpensive, it 

 secures great economy of space. A twin screw renders the 

 navigation of the most difficult channels easy, and it is invahv* 

 able with a turret ship. 



Of all the questions connected with the screw propeller, 

 that of the slip is the most interesting and important. The 

 screw propeller advances through the water — carrying the 

 vessel along with it — in the same way as a metallic screw 

 advances in a fixed solid nut. Were the nut of water in which 

 the propeller works, as immovable as the fixed solid nut, the 

 velocity of the ship and that of the screw would be equal. 

 But, as the water must, to a certain extent, give way, the 

 velocity of the ship must be less than that of the screw ; and the 

 difference of these velocities is termed the " positive slip." The 

 latter is easily accounted for, to its fall extent. Whatever may 

 be the apparatus used for propulsion, any motion imparted to 

 the water is so much lost, since none of it is communicated to 

 the vessel. Fortunately this loss may be diminished ; with 

 the paddle-wheel, it is lessened by enlarging the float-boards ; 

 with the screw, by increasing its size, which may easily be 

 made such as will render it superior to paddle-wheels. Care 

 must be taken, however, that the additional friction produced 

 does not counterbalance any advantage gained in this way. 



A large amount of positive slip may be due to centrifugal 

 force. When the vessel is retarded by adverse winds or cur- 

 rents, etc., the water may be so propelled centrifugally by the 

 screw, that there will be an empty space at its centre. This 

 may be prevented by deepening the screw in the water ; the 

 height of the column of fluid above it will then cause the par- 

 ticles to flow in, so as to rapidly fill the space which would 

 otherwise be vacant. 



The slip of the paddle-wheel cannot be decreased, like that 

 of the screw, since the more it is enlarged, the more disadvan- 

 tageous the angles at which the floats enter and leave the 

 water. A paddle-wheel of small diameter, or a screw of small 

 pitch, has a large power of traction, but with each the slip is 

 considerable. Increasing the diameter of the screw, without 

 altering the pitch, reduces the slip, without rendering it neces- 

 sary to change the velocity of revolution. If the slip is 

 judiciously decreased, the screw propeller becomes superior in 

 efficiency to the paddle-wheel. 



There is another variation between the velocity of the 

 screw and the vessel, which, though it would seem impossible, 

 undoubtedly exists. The vessel, in some cases, seems to go 



