That "deltic" engine just for the water pumping is incredible, I'd never seen that cylinder layout before. > https://en.wikipedia.org/wiki/Napier_Deltic
Piston engines got pretty wild before turbines eventually took over the world. The most efficient ones were more efficient than today's turbines in terms of BSFC[0]. One of the most interesting to me was the Napier Nomad[1], which used turbo- and super-charging. However, the turbo had secondary fuel injection and effectively ran as a turbine to drive the compressor. [0] https://en.wikipedia.org/wiki/Brake-specific_fu…
- Power-to-weight ratios. Critical in aerospace applications.
- Long duty cycles. Everything spins, reducing wear-and-tear relative to reciprocating designs. Maintenance on piston engined aircraft during WWII was a major logistical concern.
- Raw speed. Supersonic flight requires high rotary speeds, and the few propeller-driven aircraft which achieved this had ... issues. Ground crews and pilots suffered health effects from the noise alone, and notoriously often flat refused to work with the XF-84H "Thunderscreech": https://en.wikipedia.org/wiki/Republic_XF-84H_Thunderscreech...>. At near-supersonic speeds and above, propeller blade tips themselves break the sound barrier, losing aerodynamic flow over the blades, making quite a racket, and greatly reducing efficiency.
Propeller-driven planes remain more efficient than jets in many instances, though last I checked US military forces rely on turboprops over reciprocating engines in virtually all instances, possibly excepting some civilian-based (e.g., Cessna / Piper, etc.) trainer or observer variants.