141 REPORT — 1875. 



the stern of the boat in a direction opposite to that in which the tips of its 

 lower blades are moving. This is undoubtedly the case when the screw is 

 racing or acting in broken water (i. e. water mixed with air), also when the 

 screw is not completely covered with water. When, however, the screw is 

 properly immersed and is working in unbroken or continuous water, and is 

 not aifected by dead water, it has not the least tendency to move itself 

 laterally whatever it ma}- have on the ship. Under these circumstances the 

 screw-shaft can exert no lateral pressure on its bearings ; and in ships with 

 fine runs this is the case. 



Owing to the effect of the dead water, however, it may happen that even 

 when the screw is properly immersed it will tend to move laterally. If the 

 water be following the ship faster above than below (which it often is), the 

 upper blades of the screw will have more work to do than the lower, and con- 

 sequently they will have to meet with greater lateral resistance ; and hence 

 upper and lower resistances will not balance, but there wUl be a lateral 

 thrust transmitted to the bearings. 



Besides the lateral pressure which may be transmitted through the bear- 

 ings, the screw may also tend to turn the ship by the lateral motion which 

 it imparts to the water, which is again communicated to the ship or the 

 rudder. If the form of the ship and the rudder were symmetrical above and 

 below the screw-shaft, then the effect of the lateral motion which the screw 

 imparts to the water below would exactly balance the effect above the screw- 

 shaft ; but owing to the fact that the surface both of the ship and the rudder 

 is in general much greater above than below, the water which is driven 

 laterally by the upper blades has much more surface to act upon than that 

 which is driven in the contrary direction by the lower blades, and therefore 

 drives the stern of the ship laterally or tends to turn the ship. This effect 

 is in the opposite direction to that which arises from the unequal rate at 

 which the water is following the ship as long as both the ship and the screw 

 are going ahead ; and consequently these two effects tend to counteract each 

 other. When, however, the screw is reversed, and the vessel is still moving 

 forwards, the two effects are in '^conjunction ; and conseqi;ently they are more 

 likely to become apparent and important. This was the case in the experi- 

 ments with the spring model. When screwing ahead she went straight 

 enough, but when towed ahead with the screw reversed she turned to the 

 left. In this case the effect was smaU ; and I imagine that it must always be 

 so, particularly when the ship has a fine ruu. In the steam model, of which 

 the run is very fine, the screw-way very large, and the screw small (being 

 only three inches while the boat draws five), the effect of the screw to turn 

 the boat when not racing was altogether imperceptible. I conclude, there- 

 fore, that these effects may be left out of consideration with reference to 

 steering ; and in opposition to a popular notion I derive law 4. 



4. That when not breaking the surface the screw has no considerable 

 tendency to turn the ship so long as the rudder is straight. 



The effect of racing. — Although the direct effect of the screw is insig- 

 nificant when it is not racing or breaking the surface, this is not the case 

 when it is racing. It then exerts a very decided and important effect ; and it 

 is doubtless experience of this which has given rise to the popular notion 

 above referred to. 



In the experiments with the spring model when the screw was drawing 

 air down, the stern always showed a tendency to move in the opposite 

 direction to that in which the tips of the lower blades were moving, even 

 when the boat was going ahead at full speed and the quantity of air very 



