WAGR P and Pr Classes - Details of Design

Details of Design

While initial plans called for a round-top firebox (such as featured on the New Zealand Ab class), the P class was eventually designed and delivered with Belpaire fireboxes, which improve steam production over the more traditional round-top types, but are harder to fit. The P class locomotives featured a wide firebox located behind the coupled wheels and supported by a trailing-wheel. The large firebox aided with the use of poor-grade local coal from the Collie coalfields, south of Perth. This low quality coal had frequently resulted in poor steaming in earlier locomotives, but the P class design largely avoided this problem, resulting in a locomotive 30% more economical than the earlier F class engines of similar tractive effort. The P and Pr class also featured innovations to alter the weight-distribution between the driving and trailing wheels, improving adhesive traction.

Two types of tender were used by the P and Pr class locomotives. The original tender (as designed) had a water capacity of 2,800 imperial gallons and a coal capacity of 8 tons. These were built with the initial builds of the 10 P and 10 Pr class locomotives. The remaining 15 P class locomotives (all locally constructed) were fitted with modified R Class tenders which were shorter, and had been upgraded to have a water capacity of 2,440 gallons and 7 tons of coal. These short tenders were distinctive and referred to as the 'bob-tailed' tenders.