Boat propellers are curved for a reason, my guess would be it’s a more efficient design.
^^^ And there's a reason why a water turbine has the shape it does.
Ditto for a torque-converter pump, and turbine, and stator...
and a jet engine's compressor and turbine stages, and...
and...
I'm pretty sure that the proof has been in the pudding for many, many years.
Pumps are designed as they are, for a reason.
I'm thinking laminar flow is one of the objectives, among the many.
Ditto, the characteristics of the medium, e.g., density, compressible or incompressible,...
@L31MaxExpress observations possibly (probably?) coincide with pumps that were not optimal to begin with, e.g., the impellers weren't enveloped by an appropriate housing and/or the impellers were of fundamentally poor design (functionally, not perhaps economically $).
I'm reminded of those $18 bathroom ventilation fans that consist of a straight-bladed plastic "fan"...
Below: Illustration of a Francis water turbine
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