BUCKINGHAM: BALANCING OF TURBINES 201 



pressure curve, turn the L.P. part end for end, and put in a sepa- 

 rate casing. In this way we may get as close an approximation 

 to exact balancing as is permitted by the accuracy of the data 

 available in designing. The writer is not aware that this expe- 

 dient has been used in marine turbine construction, but it is so 

 obvious that it seems highly probable that it will be used soon. 



The question next arises as to how nearly a balance attained for 

 one speed will be preserved at other speeds, and it turns out that 

 if the turbine is regulated in the ordinary way, a balance for full 

 speed will be fairly maintained at much lower speeds. The 

 amount of the residual thrust, forward or aft, at any speed, de- 

 pends on various elements; and to compute it exactly, data 

 corresponding to those already used for full speed and power 

 must be available, but it will suffice here to treat a single case 

 roughly. 



Let us assume that at half speed the effective horse power 

 needed to drive the ship is to of that at full speed, — as it will be 

 approximately. The propeller thrust is then | of that at full 

 speed, and if the thrust was balanced at full speed it will still 

 be so if all the steam pressures are reduced to I of their values at 

 full power. We have, therefore, to ask how nearly this condition 

 regarding the pressures is fulfilled. 



The water rate of the turbine at half speed will probably be 

 some 40 per cent greater than at full speed. Leaving out of 

 account the change in propulsive efficiency, which is not great, 

 we may conclude that at half speed the total amount of steam 

 flowing thru the turbine will be about 1.4 X rtj or t nearly. 

 How, then, is the pressure distribution in the turbine altered 

 when the steam flow is cut down to -f? The answer to this ques- 

 tion is most opportune : if the steam flow is cut down, by throt- 

 tling or by closing first stage nozzles, leaving all the subsequent 

 passages thru the turbine unchanged in area, the pressure 

 at every point in the turbine except the last few stages next the 

 condenser remains very nearly proportional to the total flow of 

 steam thru the turbine. 



In our present example, therefore, the steam thrust will be 

 reduced to f while the propeller thrust is reduced to \, leaving :ui 



