STEAM NAVIGATION. 



such a propeller will be nearly the same, whatever position may 

 be given to it in the water. However the ship may pitch or roll, 

 or however unequal the surface of the sea may be, such a pro- 

 peller will always produce the same backward current without 

 any variation of effect. 



The circumstances which prevent the co-operation of the power 

 of steam with that of the sails in steam- vessels propelled by the 

 common paddle-wheels, will not operate with submerged pro- 

 pellers, inasmuch as tneir effect is altogether independent of the 

 careejiing of the ship. 



63. But though this defect is remedied, the submerged pro- 

 pellers in general are still subject to objections, to which even the 

 common paddle-wheel is not obnoxious. Being permanently 

 submerged and liable to accident, fracture, and derangement from 

 various causes, they are inaccessible, and cannot be repaired at 

 sea. But, besides this, when the object in view is to take full 

 advantage of the power of the sails at times when it is expedient 

 to suspend the action of the machinery, the submerged propeller 

 becomes an obstruction, more or less considerable, to the progress 

 of the vessel. Various expedients have been contrived, and in 

 some instances practically applied, by which the propeller can be 

 lifted out of the water when it is not in operation, but hitherto 

 this has not been found practically convenient, at least for com- 

 mercial vessels, though sometimes adopted for vessels of war. 



64. The screw-propeller is similar in form and mechanical prin- 

 ciple to the hydraulic machine known as the screw of Archimedes. 

 A cylinder placed at the bottom of the vessel, and in the direction 

 of the keel, is surrounded by a spiral blade similar, precisely, to 

 the thread of a common screw, but projecting from it instead of 

 being cut into its surface. If such a screw were turned in a 

 solid, it would move forward through a space equal to the distance 

 between two contiguous threads in each revolution ; but the water, 

 not being solid, yields more or less to the re-action of the screw, 

 and consequently the screw moves forward through a space in each 

 revolution less than the distance between two contiguous threads. 



65. The distance between two contiguous threads is technically 

 called the pitch of the screw ; a term, however, which is some- 

 times also used to express the angle formed by the blade of the 

 screw with its axis, such angle supplying the means of calculating 

 the distance between such contiguous threads. We shall here, how- 

 ever, use the term pitch in the former sense. The difference between 

 the pitch of the screw and the space through which the screw actu- 

 ally progresses in the water in one revolution is called the slip. 



In the first vessels to which screw-propellers were applied, the 

 screw consisted of a single spiral blade, which made one convo- 

 153 



