ON THE STEERING OF SCREW-STEAMERS. 145 



small ; and -when the screw regularly raced, frothing the water, its effect to 

 turn the stern of the boat was very great. 



The screw of the steam model was so deeply immersed that it would not 

 race ; but if the stern of the boat was raised by a string it then raced, and the 

 effect of the screw to turn the stem of the boat was the same as with the 

 spring model. 



The screw of the spring model showed a much greater tendency to draw 

 air when reversed (the boat being towed) than when it was driving the 

 boat ahead; but its greatest tendency to race was when the boat was 

 stationary, or nearly so. This latter tendency I have observed in large 

 steamers ; in fact I have never seen a large steamer start or reverse her 

 screw when moving but slowly without frothing the water. It appears, 

 therefore, that the effect of racing on the steering may be stated in the fol- 

 lowing laws : — 



5. That when the screw is frothing the water, or only partially immersed, 

 it will have a tendency to turn the stern in the opposite direction to that in 

 which the tips of the lower blades are moving. 



6. That when the boat is going ahead its effect wiU be easily counter- 

 acted by the rudder ; but when starting suddenly, either forward or back- 

 ward, at first the effect of the screw will be greater than that of the rudder, 

 and the ship -^vill turn accordingly. 



7. That if when the boat is going fast ahead the screw is reversed, at first 

 it almost destroys the action of the rudder, what little effect it has being in 

 the reverse direction to that in which it usually acts. If, then, the screw 

 draws air or breaks the surface, it will exert a powerful influence to turn 

 the ship. 



In accounts of collisions it may be frequently noticed that there is con- 

 trary evidence given of the steering of one or both of the ships (if they both 

 happen to be steamers). In the instance of the collision between the 

 ' ynie du Havre ' and the ' Loch Earn,' the captain of the ' Loch Earn ' 

 stated that the steamer altered her course almost at the last moment, thus 

 rendering the collision inevitable. The officers of the steamer asserted that 

 such was not the case; they state, however, that the screw was reversed just 

 before the collision. In this case, therefore, the evidence is to show that the 

 reversal of the screw caused the steamer to change her course, either by its 

 direct effect or by its action on the rudder. The latter effect would be 

 sufficient to explain the facts ; and my experiments leave no doubt but that 

 this must have taken place. "With regard to the former I have no evidence ; 

 although, considering that the ship was moving rapidly at the time, it seems 

 probable that the screw may have raced on being reversed, and added its 

 direct effect to turn the ship to its effect on her rudder. In this case, 

 therefore, the reports of what took place are strictly in accordance with what 

 was to be expected from my experiments ; and I think that fi.-om the light 

 these throw upon the subject in many cases, the accounts may be less con- 

 tradictory than they have hitherto appeared ; and I am in hopes that in the 

 future these experiments may assist not only in the discovery of the causes 

 of accidents, but, as these become recognized, in the prevention of the acci- 

 dents themselves. 



As an illustation of how important a clear conception of the whole cir- 

 cumstances of the effect of the screw on the rudder may be, I will read an 

 account with which I have been kindly furnished by Mr. Henry Deacon ; 

 from which account it appears that a ship was saved by a combination of 

 accidents, which led to her being handled in tlic vcrv manner in which she 



1875. ' L 



