You can attach a mag
clutch to the compressor drive on the
engine, an overrun
clutch on the compressor itself and when you want to
pump air, flip the switch and the mag clutch engages, like an A/C compressor has, and when you're done, flip it off and the over-run clutch on the
scuba air compressor spins freely.
Helped install one on my friends
boat. Incredible. I have the Max Air 35 in the Honda version and use it quite alot. I pump Nitrox (many different percents up to 40% as necessary for a specific dive) and mixed gases up to 35/30 trimix.
I paid $3500 for a brand new version 4 years ago.
I buy air filter cartridges and change it after every 30 tank fulls of plain air, less if I pump the other stuff.