The seaplane could be a possibility if the engine was ungeared, i.e. there was no gearing to reduce the prop RPM below the engine RPM, usually around 2:1 depending on the application.

Many engines in the inter-war period did not utilise reduction gearing, the prop being attached directly to the front of the crankshaft. Gearing allowed a bigger prop at a more efficient lower RPM. Basically the bigger the engine the more likely it would be geared down at the prop. 400hp is more in the larger engine category but it is still entirely possible it used direct drive – especially pre-WW2.

Anon.