This is a subject that I believe warrants a lively discussion, but in the end it's really going to come down to what your end application is, and testing, testing, testing......
I believe they're in the process of setting up a pool at OpenROV HQ, and that will be a great tool for testing not only tethers but all aspects of the vehicle performance. But as for tethers, we need to be aware that, while the HomePlug stuff is working great on OpenROV so far, I don't think anybody has tested it in the water yet. I think it's going to do just fine, since there are lots of anecdotes of other groups using HomePlug on one-off ROV designs, but there's no substitute for actual in-water testing.
There are a lot of personal opinions and ideas about what type of wire is best, but I don't feel there's enough real data to make any kind of definitive statement. One of the best things about HomePlug is that it adapts itself (through an auto-equalization process) to whatever kind of cable you're using, so we can swap out different kinds of tethers rapidly for testing purposes. In the field this is an ever bigger advantage. I know Eric is a big fan of inexpensive tethers, so that when a tether breaks or corrodes or whatever you can just change it out. But it also opens up the possibility of having multiple tethers, and picking a tether for any given mission that best suits that mission. So, to summarize my thoughts and answer some of your questions:
- The need for finding a workable connector scheme for the tether on the outside of the ROV should be bumped up in everybody's mental priority queue. I think that once we have a way for doing fast swapout of tethers, folks are going to be amazed at the capability that it brings to the ROV.
- I don't think twisted vs. untwisted makes any difference for HomePlug.
- In my mind, the outer jacket is just one more place to trap saltwater and add tether drag. If I'm going to add mass to my tether, it's going to be either in more copper, or, for exotic applications, a Kevlar strength member.
- Eric is a big fan of using the thinnest possible tether possible, to minimize water drag. One downside is tether tensile strength, so you've got to be real careful when you're driving around objects with a thin tether. I'm also concerned that the really thin tethers are going to end up being invisible in the water, so it will be tough to get yourself unstuck if you get caught somewhere. Note that once we get real HD video working properly, the tether visibility issue will be eased somewhat.
-I'm a big fan of sending power over the tether, since I'm looking at missions that need a longer run length than ~1 hr. This is going to require a thicker tether. For the OpenROV kits I'm getting in the next batch, I'm thinking 22AWG tether without an external jacket. Again, if we find a proper connector solution, then I'm not going to have to choose during construction.
- For power over tether, the amount of power you can squeeze over the tether goes up as the square of the voltage you apply to it. There is the temptation to apply as much voltage as possible, but you end up with safety issues with people working around the water. This is not a new problem, and there's a standard kicking around somewhere that defines how to safely send power through a tether where people are nearby. I haven't had the time to look this up, and it would be nice if somebody could find this and maybe get it placed on this web site as a reference document.
- To get an idea of how much power you can get over the tether, measure the resistance of one side (one conductor) of your tether. If we apply a voltage V, and the conductor resistance is R, then the maximum power you can send through is:
(V * V) / (8 * R)
Figuring out why this is so makes for an interesting Matlab simulation or scribbling on the back of an envelope. Enjoy.