Just thinking outside the box here. Please be nice to me 
Would an air motor work for propulsion?
A LZL air motor will run up to 9000 rpm, so I guess no gearing will be needed.
Big battery pack will be replaced by a scuba diving tank.
ESC and some electronics are still needed to regulate the airflow to the motor.
I’ve no clue how long an air motor will run on a big scuba tank.
Again. Just thinking outside the box.
Some info (Pocket guide to Air motors) : https://bit.ly/2Dv0uw1
Hi Kian,
The motors seem good, but the problem is energy density of pressurised vessels, which is lower than old Lead-Acid batteries.
Efoil would not have worked out practically in the pre-Lithium battery era.
I believe this topic was discussed long ago. The biggest thing that was different is a compressor was discussed rather than using a tank. I will see if I can find it but this rings a 
Hmmm… I should have used the search better.
Thank you for pointing me to the post.
No biggie been studying the knowledge of the prophets for awhile now! 