In 1938, CMStP&P ordered six 4-6-4, Class F-7 Hudsons from the American Locomotive Company and assigned them road numbers 100 through 105. These locomotives were styled by the noted designer Otto Kuhler and were used to lead the "Twin Cities Hiawatha". They reduced the running time for the 412 mile trip from the Twin Cities to Chicago to 6.5 hours.
|Class||Qty||Road Numbers||Year Built||Builder|
Firebox heating surface included 110 sq ft in three thermic syphons and arch tubes. These engines replaced the A-class 4-4-2s in the Chicago-Minneapolis "Hiawatha" runs and the last 4-6-4s built for a US railroad.. They were similar to the C&NW's E-4s shown in Locobase 180 (including boiler pressure, driver diameter, combined evaporative and superheated surface areas) and were built by the same builder.
The F-7s had smaller-diameter cylinders supplied by 12" diameter piston valves, a longer stroke (resulting in a 9.4% decrease in volume per cylinder), and a 10% smaller 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. By 1945, EHS had been reduced slightly 4,133.5 sq ft (348 sq m) with the substitution of American Arch Company circulators totalling 77.5 sq ft (7.2 sq m) for the syphons.
As it was late-1930s superpower, the design featured all the typical accessories and enhancements including roller bearings all around (Timken on the engine truck and drivers, ASF on the trailing truck under the firebox and on all tender axles), Boxpok drivers, and multiple exhaust nozzle
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. The specs for the first 14 called for a maximum weight on the drivers of 189,000 lb (85,729 kg), but the diagram book shows that, at least in later years, actual weight exceeded desired weight by almost four short tons.
Firebox heating surface included 14 sq ft (1.3 sq m) in arch tubes, 72 sq ft (6.7 sq ft) in the combustion chamber, and 90 sq ft (8.35 sq m) in three thermic syphons (two in the firebox, one in the combustion chamber). They had 14" (356 mm) piston valves, Coffin feed water heaters, mechanical stokers, and mechanical and pressure lubrication.
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 (918 miles) in 30 days with no days out for servicing. The regular consist was 9-10 cars, but 11-14 cars wasn't unusual, and 15 or more might be seen occasionally.
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 lb. Such rates indicate the hard pressing that passenger engines received to tick off mile after mile of high-speed running - lots of wide-open throttle and late valve cut-offs.
A table in the Railway Age's 23 December 1933 story on this class (p. 876) provides a snapshot of what a large American locomotive could require in replenishment during one trip. Given that the locomotive was full of water and coal when leaving Minneapolis, it would stop 9 times for water and need most of a tenderful upon arriving in Harlowton, Mont. Five coal stops - most of which were not coincidental with the water stops - would take the engine to Harlowton with about 8 tons left over. In addition, the crew would clean the fire twice and dump the ash pan 5 times, lube the main pins 5 times and all other pins 2 times.
Ed Kehm emailed Locobase in January 2013 to point out that the F6s were delivered with 79" (2,007 mm) drivers. By the time VE Rennix of Baldwin's Chicago office detailed the F6's performance in 1934, however, his list of specifications included 80" drivers. The difference lay in the use of 4" (102 mm) tires on the wheels. The difference in starting tractive effort was less than 600 lb.
|Specifications by Steve Llanso of Sweat House Media|
|Railroad||Milwaukee Road (CMStP&P)||Milwaukee Road (CMStP&P)|
|Number in Class||6||22|
|Locomotive Length and Weight|
|Driver Wheelbase (ft)||14.67||14|
|Engine Wheelbase (ft)||42.33||40.75|
|Ratio of driving wheelbase to overall engine wheebase||0.35||0.34|
|Overall Wheelbase (engine & tender) (ft)||89.83||81.62|
|Axle Loading (Maximum Weight per Axle) (lbs)|
|Weight on Drivers (lbs)||216,000||196,550|
|Engine Weight (lbs)||415,000||380,220|
|Tender Light Weight (lbs)||375,000||287,780|
|Total Engine and Tender Weight (lbs)||790,000||668,000|
|Tender Water Capacity (gals)||20,000||15,000|
|Tender Fuel Capacity (oil/coal) (gals/tons)||25||20|
|Minimum weight of rail (calculated) (lb/yd)||120||109|
|Geometry Relating to Tractive Effort|
|Driver Diameter (in)||84||80|
|Boiler Pressure (psi)||300||225|
|High Pressure Cylinders (dia x stroke) (in)||23.5" x 30"||26" x 28"|
|Tractive Effort (lbs)||50,294||45,250|
|Factor of Adhesion (Weight on Drivers/Tractive Effort)||4.29||4.34|
|Firebox Area (sq ft)||458||411|
|Grate Area (sq ft)||96.50||80|
|Evaporative Heating Surface (sq ft)||4166||4205|
|Superheating Surface (sq ft)||1695||1815|
|Combined Heating Surface (sq ft)||5861||6020|
|Evaporative Heating Surface/Cylinder Volume||276.62||244.39|
|Computations Relating to Power Output (More Information)|
|Robert LeMassena's Power Computation||28,950||18,000|
|Same as above plus superheater percentage||37,346||23,400|
|Same as above but substitute firebox area for grate area||177,246||120,218|