ON THE STEERING OF SCREW-STEAMEUS. 143 



The mdder Tvas then set so as to turn the boat to the left, and the opera- 

 tion was repeated with very nearly corresponding results so long as the 

 screw did not race ; but the action of the reversed screw on the rudder when 

 set to the left was not so great as when set to the right. This ditference 

 led me to suppose that the screw itself might exert an influence to turn the 

 boat to the left when it was reversed, although it had been found to exert no 

 such influence when going ahead. This was at once shown to be the case 

 by setting the rudderj^straight and starting the screw reversed ; the boat 

 immediately turned to the left, but not fast unless the screw raced, then she 

 turned very rapidly. 



These direct effects of the screw to turn the ship appear to me to account 

 for several of the anomalies which have hitherto beset the subject ; and further 

 on in the paper I shall discuss them at length. 



The steam model was provided with paddles as weU as screw, and the 

 screw could be reversed without reversing the paddles, in which case the 

 l^addles overpowered the screw, and the boat moved forward somewhat 

 slowly. In this boat the screw was so deeply immersed that it would not 

 race, and it had no direct effect to turn the boat when reversed like that of 

 the spring model. 



When the screw was reversed and the boat drawn slowly forward by the 

 paddles, the effect on the rudder was almost to destroy its action, it having 

 only a slight power to turn the boat in the opposite direction to that in 

 which it would have turned the boat had the screw been going ahead. 

 Practically the boat had lost aU power of steering. Coupled in this way 

 with the paddles the screw turned but slowly, the engine being held up by 

 the opposing actions. On releasing the paddles and allowing them to turn 

 freely, and applying the whole power of the engine to the screw, the model 

 behaved almost exactly as the spring model had done, showing when towed 

 against the screw a strong tendency to turn in the opposite direction to that 

 in which the rudder was set. 



The screw was then set full speed ahead ; and when the boat had acquired 

 way the rudder was set, so that she began to turn rapidly to the right ; the 

 screw was then reversed, and by the time the boat had lost all forward way 

 she had turned to the left through an angle of 30°, so great was the effect of 

 the screw on the rudder when stopping the boat. 



This completed the list of the experiments, which, however, were repeated 

 over and over again with exactly the same results. 



Conclusions to be drawn from tlie exjjeriments.^The general conclusion is 

 that in screw-steamers the effect of the riidder depends on the direction of 

 motion of the screw rather than on the direction of motion of the boat. Or 

 we have the three following laws : — 



1. That when the screw is going ahead the steamer wiU turn as if she 

 were going ahead, whether she have stern-way on or not. 



2. That when the screw is reversed the rudder will act as if the vessel 

 wore going astern although she may be moving ahead. 



3. That the more rapidly the boat is moving in the opposite direction to 

 that in which the screw is acting to drive it, the more nearly will the two 

 effects on the rudder neutralize each other, and the less powerful will be its 

 action. It would appear reasonable to suppose that a boat may move fast 

 enough to overcome the effect of the reversal of the screw ; but this was not 

 the case with the models. 



Thd effect of the screw to turn the boat independently of the rudder. — 

 It seems to be supposed by some that a screw necessarily tends to force 



