DC motors and efficiency

I have a general question about DC electric motors. I have found a servo motor with the right size and price to meet my application.
However, it's best efficiency is within the continuous torque range that places the rpm higher than what I want (2700). However, this is an underwater propulsion application and so the motor will have plenty of cooling. 1) Can I operate this motor at a lower rpm (and thus higher torque for the same power out) and still maintain efficiency being I am providing cooling? 2) Basically, how does complete cooling effect the efficiency and continuous torgue ratings of these motors?
Thanks for any insight. Bailey--
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MBS wrote:

What about a gear?
Sjur
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