"Why? Because the torque drops off so quickly compared to the horsepower gain."
Kojac, first let me say I appreciate your thoughts and ideas on this topic. The more we think about this and the physics of what's happening the better. That said, your conclusion is wrong. How fast torque "drops off" compared to power gain has nothing to do with performance. Some have suggested an equation relating the ideal distance between peak torque and peak power for impeller matching. Also BS. Some have also suggested the rpm where torque and power cross is ideal...trying in vain to keep torque a perfomance player, only to discover that units (metric or english or whatever) drive the rpm crossing point not performance.
The only driver of performance is total power delivered to the impeller and the pump/impeller efficiency at that rpm.
Period.
Efficiency does make a difference but is related to power not torque. Efficiency is defined as power out over power in. Not torque out over torque in and for a good reason! FOR A GOOD REASON > Think about that. Performance is always related to power and efficency not torque no matter a jet boat, airplane, car or steam turbine.
The reason a small impeller matched to peak power on a high rpm engine doesn't perform is because that smaller impeller wastes power from flow losses. Less efficient. A less efficient impeller will burn more power to convert to the same thrust. And jet thrust equals performance. Not torque. Has nothing to do with the magnitude of torque applied or how fast the torque curve falls compared to the power curve. Only how much power applied and how effective the impeller/pump converts that power to thrust.
Those who insist on relating performance to torque are missing the truth and following a torque myth. Trying in vain to bend physics to a preconceived notion of how torque and how the torque curve behaves must in some way equal performance.
jer