Why patrol aircraft could not replace cruisers in the 1920s

Could South Africa have relied on a substantial maritime patrol air service instead of cruisers and escorts?

Not in the 1920s: maritime aviation lacked the weapons, basing network, and anti-submarine follow-through to replace surface forces.

Maritime patrol required multiple squadrons, dispersed bases, maintenance systems, and trained support personnel rather than a few inexpensive aircraft. In the 1920s, patrol aircraft lacked a reliable way to destroy a maneuvering surface raider, while carrier torpedo attacks depended on massed specialist aircraft. Aircraft could locate submarines and force them to submerge, which reduced their speed and freedom of action, but could not then prosecute them underwater. Cruisers and escorts were therefore the practical foundation of South African maritime defense, with aviation becoming a stronger complement only in the late 1930s.