204 MACHINERY AND TRIALS OF THE PASSENGER SHIPS 
kind of material, are superior to the built gears. They are lighter and are free from in- 
ternal stresses because they are twice annealed during the process of manufacture and are 
made with split hubs. The bore of the hubs is finished after the second rough machining and 
just previous to the finishing cut over the rim. Experience shows that these wheels keep 
their form and continue to run true. 
With reference to the remarks of Mr. E.H. Rigg and Admiral Taylor, the following 
shows the dates for docking and trial trips on the American Legion, President McKinley 
and President Pierce: 
Date of coming Date of standardi- 
Vessel. out of dry dock. zation on trial trip. 
Americansleecioneae ener ene 16 June, 1921 8 July, 1921 
President Mickanleyi seeder eh ee ar 18 April, 1921 26 May, 1921 
IPigeSOGme JENGKe! Gedo nonedaooecnoeeds 5 January, 1921 17 January, 1921 
In the case of the President Pierce, the vessel lay in the stream for two days previous 
to the trial for the purpose of taking on ballast. 
As pointed out, the difference between the shaft horse-power as estimated from the 
model test and the actual obtained on the trials is quite large, and it seems rather difficult to 
explain away the difference. The shafting with torsion meters in place were carefully cali- 
brated, and a survey of the records both at the time of the trials and since fails to show any- 
thing wrong with the shaft horse-power as calculated from the meter readings. The torsion 
meters were on the line shaft next to the gears, and as the shafting is fairly long there 
would be a loss between the torsion meter and the propeller. This has been estimated and 
the percentage of power delivered at the propeller would be 96.37 per cent at 12.31 knots, 
97.06 per cent at 14.31 knots, 97.48 per cent at 16.07 knots, and 98.2 per cent at 19 knots. 
These figures should be regarded as a maximum, and the normal loss might be expected to 
be half of these amounts. 
The self-propelled model was tried with all appendages, but the hull would be smooth, 
whereas the hull of the ship has projections for the laps and joints in the plating, rivet heads, 
under water fittings, etc., and probably some 2% per cent should be added to the horse- 
power as found from the model test to allow for this additional resistance. Also, as Mr. 
Rigg points out, there would be some air resistance, which it is difficult to estimate. 
We also have the knowledge, as Mr. Alfred J. C. Robertson pointed out in his re- 
marks, that these ships are making excellent time at sea, at displacements equal to or greater 
than that used on trial, which would confirm the idea that there was some interference due 
to the depth of water on the mile. In the case of the President Pierce the economy trials 
were run in varying depths of water, but mostly in shallow water near the Virginia Capes. 
Admiral C. W. Dyson has supplied an interesting discussion on the propellers of these 
vessels, but has apparently assumed that all the ships were run at the same draught. Plate 62 
shows curves for the President Pierce, President McKinley and American Legion in the 
form of a correction factor as applied to the self-propelled model results as against the 
actual power on trials. The shaft horse-power for the self-propelled model has been obtained 
for the corresponding displacements of the three ships by plotting cross curves. ' This would 
indicate that the President Pierce propeller shows a better efficiency than the other two and 
points the advantage of the solid propeller with machine driving surfaces. 
Mr. William W. Smith calls attention to the discrepancy between the temperature re- 
