The F-7s had narrower cylinders, a longer stroke (resulting in a 9.4% decrease in volume per cylinder), and a 6.4% larger grate. Also, the F7s had greater volume in their tubes and flues and the ratio of tubes to flues was much closer to the average.
Jim Scribbins (in Drury, 1993) notes that the design was really only suitable for the elite passenger service. Once the Milwaukee dieselized the Hiawathas, the F-7s were relegated to the Arrow (Chicago-Omaha) and Chicago-Milwaukee service. Other locomotives could perform as well in this service at less cost, however, so the F-7s were the first Hudsons to be scrapped in 1949-1951.
Known on the CMStP&P as a "Baltic". The differences between the two classes were the broken running board line over an air reservoir on the first 14 (F6) and the straight running board of the last 8 (F6a). Data similar for both, but some heating surface areas (F6a in the specifications) are slightly different. Firebox heating surface included 100 sq ft of arch tubes and syphons.
A large example of American orthodoxy in steam passenger locomotives, an engine of this class set a flying-average mark of 92.3 mph (148.5 km/h) over 65.6 miles (105.6 km) of level track in 1934. Another completed 10 round trips between Minneapolis and Harlowton, Mont in 30 days with no days out for servicing. Tests showed an average of 6.4 lb of water evaporated per pound of coal fired and coal consumption per thousand gross ton-miles between 90.32 to 123.2. They had mechanical stokers and mechanical and pressure lubrication.
| Specifications | ||
|---|---|---|
| Class | F-7 | F6/F6a |
| Locobase ID | 183 | 182 |
| Railroad | Milwaukee Road (CMStP&P) | Milwaukee Road (CMStP&P) |
| Whyte | 4-6-4 | 4-6-4 |
| Road Numbers | 100-105 | 6400-6421 |
| Gauge | Std | Std |
| Builder | Alco | Baldwin |
| Year | 1938 | 1930 |
| Valve Gear | Walschaert | Baker |
| Locomotive Length and Weight | ||
| Driver Wheelbase | 14.67' | 14' |
| Engine Wheelbase | 42.33' | 40.75' |
| Ratio of driving wheelbase to overall engine wheebase | 0.35 | 0.34 |
| Overall Wheelbase (engine & tender) | 89.83' | 81.62' |
| Axle Loading (Maximum Weight per Axle) | ||
| Weight on Drivers | 216000 lbs | 196550 lbs |
| Engine Weight | 365500 lbs | 380220 lbs |
| Tender Light Weight | 302960 lbs | 287780 lbs |
| Total Engine and Tender Weight | 668460 lbs | 668000 lbs |
| Tender Water Capacity | 20000 gals | 15000 gals |
| Tender Fuel Capacity (oil/coal) | 25 tons | 20 tons |
| Minimum weight per yard of rail on which locomotive could run | 120 lb rail | 109.19 lb rail |
| Geometry Relating to Tractive Effort | ||
| Driver Diameter | 84" | 80" |
| Boiler Pressure | 300 psi | 225 psi |
| Cylinders (dia x stroke) | 23.5" x 30" | 26" x 28" |
| Tractive Effort | 50294 lbs | 45250 lbs |
| Factor of Adhesion (Weight on Drivers/Tractive Effort) | 4.29 | 4.34 |
| Heating Ability | ||
| Firebox Area | 458 sq. ft | 407 sq. ft |
| Grate Area | 96.50 sq. ft | 80 sq. ft |
| Evaporative Heating Surface | 4166 | 4201 |
| Superheating Surface | 1695 | 1815 |
| Combined Heating Surface | 5861 | 6016 |
| Evaporative Heating Surface/Cylinder Volume | 276.62 | 244.16 |
| Computations Relating to Power Output (More Information) | ||
| Robert LeMassena's Power Computation | 28950 | 18000 |
| Same as above plus superheater percentage | 37322.33 | 23430.52 |
| Same as above but substitute firebox area for grate area | 177136.05 | 119202.76 |
| Power L1 | 60547.47 | 39354.21 |
| Power MT | 1853.95 | 1324.26 |
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