ELECTRIC PROPULSION OF MERCHANT SHIPS. 289 



with the figures given in the paper. Then, without any changes in the test equipment, a 

 second test was run, this time with an oil temperature of 116° F., which decreased the vis- 

 cosity to slightly less than 300 seconds. (And, by the way, the gears will run at that tem- 

 perature without any possibility of scoring.) The results of this test indicated a friction 

 loss of 37.5 horse-power per gear, which corresponds with an efficiency of 97.5 per cent. 



The question of the pitting of gear teeth is one which is very much in the minds of 

 marine engineers of late, and numerous attempts have been made to explain its cause. 

 However, it is generally known that the major portion of the pitting occurs on or near 

 the pitch line on the teeth of the driven gear, and an analysis of the situation shows that 

 the metal at this point is under a peculiar combination of stresses. Sliding action occurs 

 in both directions toward the pitch line, alternating with which there is the pure rolling 

 pressure directly on the pitch line. This action is, perhaps in a way, similar to that which 

 takes place in a seamless tube rolling mill, where the billet is being squeezed first in one 

 direction and then in another. As a result, the center has a tendency to break up, so that 

 the piercing plug has comparatively little work to do. 



The action on the tooth face is for the sliding action to raise a slight ridge at the 

 pitch line ; and then, of course, there is the pressure of the pure rolling action right on that 

 ridge, which gives us the unusual combination of stresses. The result is that small particles 

 of the ridge flake off, thus relieving the pressure. If we could conveniently build a gear 

 with a groove cut at the pitch line for the full length of each tooth, perhaps we would have 

 the solution of the problem. Needness to say, such a procedure is impracticable. Neverthe- 

 less experience proves conclusively that a large portion of the pitting action occurs within 

 the first few weeks of service, and these flakes which come off seem to relieve the stress at 

 the pitch line, after which no further pitting takes place. At any rate, the pitting does not 

 seem to induce scoring or detrimentally affect the proper action of the tooth surfaces in any 

 way. 



Mr. Emmet has spoken about the heat generated in the ahead turbine, while rurming 

 astern. Unfortunately, the advance copies of his paper did not reach us in time to permit 

 us to work up the experimental data which we have available. Our recent experiments 

 along this line indicate that this point should not cause any serious alarm. 



In the diagram shown in the paper the astern speed is given as 3,600 revolutions per 

 minute, which we can assume is the same as for full speed ahead. As a matter of fact it 

 is impossible to attain this speed in the asteiin direction, because the astern turbine is much 

 less efficient than the ahead element ; and since the total amount of steam available is defi- 

 nitely limited by the boiler capacity, it is rarely possible to attain an astern speed greater 

 than about two-thirds of the full ahead speed. 



Experience has proven that, with a reasonably good vacuum, the turbines may be run 

 at their maximum astern speed for an indefinite time without sign of distress. Perhaps the 

 best example of this was the performance of the turbines on the steamship Westward Ho, 

 which are admitted to be of a type more prone to heating than some others. That vessel, 

 as you will remember, was torpedoed, with the result that practically her entire bow was 

 blown away, and the forward bulkhead being thus exposed it was not deemed safe to at- 

 tempt to drive the vessel in the ahead direction. Therefore she was driven astern for a 

 period of about three days, covering a distance of some 400 miles and arriving safely in a 

 French port. During all of that time there was no sign of any distress or excessive tem- 

 peratures inside of the turbine. 



