South African Class 21 2-10-4

South African Class 21 2-10-4
No. 2551 with smoke deflectors
Type and origin Power type Steam Designer South African Railways (A.G. Watson) Builder North British Locomotive Company Serial number 24379 Model Class 21 Build date 1937 Total produced 1
Specifications Configuration: ​  • Whyte 2-10-4 (Texas)  • UIC 1'E2'h2 Driver 3rd coupled axle Gauge 3 ft 6 in (1,067 mm) Cape gauge Leading dia. 30 in (762 mm) Coupled dia. 54 in (1,372 mm) Trailing dia. 30 in (762 mm) Tender wheels 34 in (864 mm) Minimum curve 300 ft (91 m) Wheelbase 68 ft 4 1⁄4 in (20,834 mm)  • Engine 36 ft 9 in (11,201 mm)  • Coupled 18 ft 9 in (5,715 mm)  • Trailing 4 ft 6 in (1,372 mm)  • Tender 22 ft 3 in (6,782 mm)  • Tender bogie 12 ft 3 in (3,734 mm) rigid Wheel spacing (Asymmetrical) 1-2: 4 ft 9 in (1,448 mm) 2-3: 4 ft 8 in (1,422 mm) 3-4: 4 ft 8 in (1,422 mm) 4-5: 4 ft 8 in (1,422 mm) Length: ​  • Over couplers 76 ft 10 1⁄4 in (23,425 mm) Height 13 ft (3,962 mm) Frame type Bar Axle load 14 LT 19 cwt (15,190 kg)  • Leading 11 LT 7 cwt (11,530 kg)  • 1st coupled 14 LT (14,220 kg)  • 2nd coupled 14 LT 11 cwt (14,780 kg)  • 3rd coupled 14 LT 19 cwt (15,190 kg)  • 4th coupled 14 LT 12 cwt (14,830 kg)  • 5th coupled 14 LT 15 cwt (14,990 kg)  • Trailing 12 LT (12,190 kg) leading 10 LT 8 cwt (10,570 kg) trailing  • Tender axle Axle 1: 7 LT 18 cwt (8,027 kg) Axle 2: 12 LT 10 cwt (12,700 kg) Axle 3: 12 LT 10 cwt (12,700 kg) Axle 4: 12 LT 10 cwt (12,700 kg) Axle 5: 12 LT 10 cwt (12,700 kg) Axle 6: 7 LT 18 cwt (8,027 kg) Adhesive weight 72 LT 17 cwt (74,020 kg) Loco weight 106 LT 12 cwt (108,300 kg) Tender weight 65 LT 16 cwt (66,860 kg) Total weight 172 LT 8 cwt (175,200 kg) Tender type FT (2-8-2 arrangement) Fuel type Coal Fuel capacity 10 LT (10.2 t) Water cap 5,587 imp gal (25,400 l) Firebox type Round-top  • Firegrate area 63 sq ft (5.9 m2) Boiler: ​  • Model Watson Standard no. 3B  • Type Domeless  • Pitch 9 ft (2,743 mm)  • Diameter 6 ft 2 1⁄4 in (1,886 mm)  • Tube plates 22 ft 6 in (6,858 mm)  • Small tubes 136: 2 1⁄2 in (64 mm)  • Large tubes 36: 5 1⁄2 in (140 mm) Boiler pressure 210 psi (1,448 kPa) Safety valve Pop Heating surface 3,400 sq ft (320 m2)  • Tubes 3,168 sq ft (294.3 m2)  • Arch tubes 26 sq ft (2.4 m2)  • Firebox 206 sq ft (19.1 m2) Superheater: ​  • Heating area 676 sq ft (62.8 m2) Cylinders Two Cylinder size 24 in (610 mm) bore 26 in (660 mm) stroke Valve gear Walschaerts Valve type Piston Valve travel 7 1⁄2 in (191 mm) Couplers AAR knuckle
Performance figures Tractive effort 43,700 lbf (194 kN) @ 75%
Career Operators South African Railways Class Class 21 Number in class 1 Numbers 2551 Delivered 1937 First run 1937 Withdrawn 1952
The 3rd & 4th coupled axles had flangeless wheels

The South African Railways Class 21 2-10-4 of 1937 was a steam locomotive.

In 1937, the South African Railways placed a single Class 21 steam locomotive with a 2-10-4 Texas type wheel arrangement in service, designed as a mixed traffic locomotive suitable for use on light rail. A simultaneously proposed heavier mainline version Class 22 2-10-4 was never built.

The Class 21 2-10-4 Texas type locomotive was designed by A.G. Watson, Chief Mechanical Engineer (CME) of the South African Railways (SAR) from 1929 to 1936. It was built by the North British Locomotive Company in Glasgow and delivered in 1937. Only one locomotive was built, numbered 2551. At the time, the design represented the maximum power obtainable from a ten-coupled non-articulated locomotive which was limited to a 15 long tons (15.2 tonnes) axle load on 60 pounds per yard (30 kilograms per metre) rail.

Watson disliked articulated locomotives and his aim was to develop a powerful non-articulated mixed traffic branch line locomotive with an axle load suitable for light rail. To enable it to negotiate tight curves of 300 feet (91 metres) radius, the third and fourth coupled wheelsets were flangeless. In addition, a total side play of 1 ³⁄₄ inches (44 millimetres) was provided on the leading coupled wheels and spherical bearings were fitted to the leading coupling rods. Its 5-inch thick (127-millimetre) bar frames extended from the front to the hind buffer beams and were stayed by cross stretchers of a light fabricated construction. It used a Watson Standard no. 3B boiler, the same as that used in the Classes 15E, 15F and 23.

The end result could be considered as Watson's answer to the Class GF Garratt locomotive, the two types having very similar axle loads. The Class 21 carried more water than the Garratt and was about 23 long tons (23.4 tonnes) heavier with 43,700 pounds-force (194.4 kilonewtons) tractive effort, compared to the 34,200 pounds-force (152.1 kilonewtons) of the Class GF. Even so, only one Class 21 was produced and the design was not repeated.