Has anyone considered an air powered dpv?

[gunnutz]

Seems to me everyone is hooked on using electric power to build a dpv...but how bout this idea...we've all seen those little air-powered airplanes at wally world...well it seems to me if you run air backwards thru a compressor(maybe need some sligh modification), it will spin the shaft...like a motor....attach a prop...walla...a dpv....no worry about water leaks...batteries....valves instead of switches...no wait to recharge...hook up to a fresh tank say an 80...and away you go...as an aside..perhaps mount on the back of a double rig...one tank for you...one for the dpv..a hand switch/valve and you could fly like superman...or perhaps aquaman....just an idea...what do ya think???

 

[miketsp]

There are quite a few threads on this topic.
I think one of the more recent ones was:
http://www.scubaboard.com/showthread.php?t=114808

and
http://www.scubaboard.com/showthread.php?t=50362
http://www.scubaboard.com/showthread.php?t=92085

 

[pipedope]

I don't have time to review the existing threads right now but there are a couple of things to consider.

Bouyancy change of the DPV during use.
None with electric. About 6 pounds with an AL80.

Power and duration.
Air power doesn't get you very far.

Cost.
Compressed air is expensive.

Lots of times small jobs are done underwater using standard air tools and a scuba air tank for power. These are jobs that are small and short and don't justify the purchase or rental of hydraulic tools.

 

[gunnutz]

thought the idea over a little bit more.....the compressor idea was, well...kinda dumb....instead i would think adapting an airtool motor would work more easily...also considered power source...i know compressed air is'nt going to get ya very far..but, im not really lookin for that...just something to do some limited distance...also would need a pressure regulator to drop tank pressure of 3200 psi down to say 100 psi..suitable for an airtool...typically these use up approx 10-12 cfm per minute @ 100 psi..so i suppose w/ a 3200psi tank you would get somewhat useful range...now as far as bouyancy, i was considering mounting the unit to the divers main tank w/ perhaps par of 40's on either side....this way could use bc to adjust for bouyancy as is normal...as far as cost....tank fills only cost me 3 bucks...a trolling motor, speed control, batteries and housing alone coud cost over 500...not to mention if it leaks..which isn't a problem w. air power.

 

[gunnutz]

also considered corrosion...this would probably be a freshwater only unit..in heory this all works...one thing i haven't quite got past yet is noise...airtools are loud...underwater this would carry quite a ways

 

[miketsp]

also considered corrosion...this would probably be a freshwater only unit..in heory this all works...one thing i haven't quite got past yet is noise...airtools are loud...underwater this would carry quite a ways

The first link I posted above talks about noise and the use of ceramic pistons to avoid corrosion.

 

[gunnutz]

..been doing a little more research..looking for a cheap, suitable motor....found pneumatic cutter @ harbor freight for $19.99..uses 5 cfm of air @ 90 psi...not that bad on the air...only thing is it tuns @ about 18000rpm...so some sort of a reduction gear would be needed to spin prop @ about 1600 rpm max...this is the standard maximum for most trolling motor props....any faster efficiency drops fast due to cavitation, not to mention spinning a plastic prop that fast would seem like a lot of stress....or another option might be some sort of an impeller...as far as the noise...to silence it to some degree, i've considered encasing the motor in a pvc housing in foam...any thoughts