FinalIy!! After yearzzz of reading and learning (most of the time, anyway) and being seriously amused at clever comments/retorts, I can post on a topic that I have a wee bit of experience with... In my former life, I was heavily involved with offshore ROV engineering/installations, specifically the launch and recovery of such. My first out of university project was designing a towfish and it's deployment package to be used to 'talk' with an AUV. Now, I'm a mommy, who has nothing better to do (I wish!) but hang out with you guys.
Anyhoo, I'm not sure of the Phoenix towfish's exact spec, but using a depressor wing serves to decrease the length of cable necessary to be payed out to achieve depth. Without a wing, the ratio is something like 4:1, ie 4x amount of cable for a desired depth vs. with a wing, ratio becomes something like 2:1 (not sure of exactness, I'm going off of memory). I was wondering the same, whether they were using a wing. I searched a bunch looking for broader/more installation/operations view(s) - I was actually curious in the installed winch - to see if a wing was present, but haven't seen anything in the sparse amount of ops pics from the Shield. Perhaps they're using one? If I had to guess, I'd say they would want to take advantage of lesser cable payout (and others mentioned below). It's probably just that noone's taken a shot of it as it is placed on tow cable after a specified length of payout, and at this point, the 'star of the show' TPL is long gone subsea.
I do know that a towfish's design is also an important factor in determining whether a wing is deemed necessary because some designs 'fly' better than others. The design I chose for my particular application looks similar to the Phoenix one out there, yet it is a larger version where the housing (hollow inside) was deeper to accommodate the mounting of the 'modems' (one was like 20" or so tall) inside. Point being that towfish designs modelled after nature, ie think sting rays, whose electromechanical termination placement on fish is engineered correctly (our fish flew nose down several degrees) serves to help it 'fly' more consistently at depth
. I agree with all points made earlier about benefits of using a wing re tow cable harmonics and isolating the termination (that's a biggie when you're talking limiting factors). Utilizing a bend restrictor (essentially a rubber 'tube' on tow cable located just after the electromech term, designed with incrementally larger diameter thicknesses along length of restrictor) serves to keep the cable from assuming too acute of an angle during operation, which is essential.
I'm thinking the reason for the chosen 2 naut speed is simply for truly zeroing in on ping location as to not 'miss it' and when ping's located, that they get enough recorded data in a specific region along path for analysis purposes. It also indeed could be to keep cable harmonics at bay, to keep lower forces on electromechanical term because higher speed equals increased drag equals higher forces at term connection (kevlar term is weaker than steel 'potted' type), and/or the tow cable's strength, ie the max design load of the cable may be encroached upon at deeper depths in cases like this at higher speeds. Kevlar's great and all, but a parted cable isn't pretty. And a lost tool is even less pretty haha.
A huge thanks to you guys for the immense knowledge and entertainment.
PS. Reason I believe a TPL AUV isn't readily available is there's rare demand for such a tool at exorbitant costs of design/manufacturing. Dolphins would definitely be cheaper to train haha.