These are absolutely 'off the cuff' observations and are offered solely as food for thought. i have more than a passing interest in almost anything biological, particularly biological/tech mimicry.
The first thing that came to mind was the recent innovation with 'props' (mainly helicopter propellers ?) that benefited from improved performance based on the tubercles present on the trailing edges of the fins and tails of certain whale species.
Apparently these small bumps helped to disrupt or enhance the small vertices created by the thrust of the cetacean's swimming movements and added to the efficiency of of their forward propulsion ? Eons of R&D at work.
I recently read that 'DARPA" or like agency had successfully capitalized on this natural design feature by the fairly simple addition of similar small 'bumps' on the trailing edges of helicopter blades ?
This minor modification (if memory serves?) added measurably to the efficiency and stability of the helicopters' propulsion and lift.
I'm wondering if something mimicing that same action might add not only to the efficiency of any ROV propellers but perhaps decrease any water resistance in general with their judicious placement on the surface of the body of the ROV itself ?
Along similar lines, during deep dives cetaceans such as sperm whales. rather than resist the enormous pressures their bodies are subjected to at depth with a rigid outer counter force. Instead 'go with the flow ' as it were, by allowing their highly flexible bodies to actually "collapse" and compress with the pressure, including their large air filled lungs (while at the surface).
This adaptation allows for far greater flexibility and freedom of movement. Though probably not of great importance as such here ? It also helps to dissipate oxygen more evenly and effectively to their core organs. This helps prevent the well known effects of the 'bends' when resurfacing by preventing the reformation of large 'bubbles' as the gasses decompress from solution as the pressure quickly starts to decrease.
What it does do is bring me to the propylene glycol (or similar non toxic, non conductive, non compressible fluid) aspect of my title.
Many of you may have probably witnessed the "science experiment" where someone has taken a TV, computer or other sensitive electronic component and submerged it fully operational into a clear tank of such a liquid without any evidence of ill effects to the electronics ?
I've stretched this simple demonstration enormously with the supposition of "What if" the outer 'shell(s) ' of an ROV's component(s) were encased in some sort of semi-flexible or flexible shell, either single or double walled (mesh encased silicone ?) which was then filled with a similarly propertied liquid ?
Since liquid doesn't compress the addition of an appropriately sized bladder/reservoir to contain the displaced liquid at depth may serve a the same purpose as the cetacean's tissue does with its' displaced gasses when submerged ?
Liquid is more viscous than gas and the flexible outer shell allows the ROV's interior to 'cope' a bit better with the external pressures with a controlled collapse instead of trying to resist the considerable pressures, it also seems less likely to 'leak' for the same reason ?
The non conductive nature of the liquid would also appear to help protect the electronics more effectively from any salt water 'creep' better than an air barrier, and, in the case of shaft/gasket interfaces may also help reduced wear and additional likelihood for leaks with its' intrinsic lubricating action and reduced friction ?
The increased viscosity would not doubt cause some impediment to the motors' rotations so a larger motor and increased attendant power consumption would be needed as an offset and may negate its' use for all but the more extreme conditions ? The current motors being fairly inexpensive and easily replaced.
the liquid is either neutral or slightly buoyant, certainly much less buoyant than the same volume of air, so shouldn't have too adverse an effect in that regard besides a little added weight ?
The liquid's viscosity vs. gas should, in my mind anyway, be lass prone to find tiny areas to cause leaks. Although its' lubricating properties may cause other unseen issues along those same lines ?
admittedly I'm in no way well versed in any of these disciplines so there may be myriad reasons all or little of this missive has any application whatsoever ?
I had just come across your amazing "Kickstarter" project and these among other "outside the box" thoughts immediately found their way from the "box" into my head and then quickly onto your blog for digestion, comment or derision !
Thanks for reading and any thoughts it may provoke. The "box" itself is still pretty full, its' contents' value however, remain open pending critical review.