Creating a neutrally buoyant tether that is strong yet flexible can involve considerable research of past efforts and development.
The object of this discussion thread is to examine the various ways that a flexible, neutrally buoyant, cost effective tether can be either purchased or constructed at a reasonable cost with “reasonable” being considered a relative descriptor.
I presently have 75 meters of the BR Fathom Tether which is of outstanding quality, however the level of its flexibility has prevented me from being able to conveniently store it on a drum/reel of reasonable size and weight and can require a support person to help with the logistics. For those builders using the Fathom-S Interface this tether is the ideal solution unless one is inclined to build their own multi twisted pair tether.
On the other hand there is either the OpenROV single twisted pair tether or their neutrally buoyant twisted pair tether.
These two OpenROV twisted pair tethers are designed for bidirectional communication using a Fathom-X like interface. The biggest drawback to the unjacketed single twisted pair tether is that it is not physically robust and is not neutrally buoyant. The neutrally buoyant jacketed twisted pair tether has a Kevlar center thread which makes it physically robust, but very pricey. Both of these tethers are small in diameter and flexible, though they both vary significantly in cost.
To overcome the physical limitations of the unjacketed single twisted pair tether it can be threaded up the center of a 1/4 inch diameter poly rope. Several discussion threads (Tether Management 1 / Tether Management 2) have documented this process. Not only do they document the threading process, but offer tether storage management solutions. Use of the poly rope can approach the neutral buoyancy requirement, but it depends on the characteristics of the poly rope in use.
The OpenROV neutrally buoyant twisted pair tether is the ideal solution as it does not require the cable to be threaded up a poly rope and can be easily stored on a standard cable reel. However this tether is pricey at around $5 per meter and obviously does not allow for additional signal paths.
Not many options out there I’m afraid. I think what makes the Blue Robotics tether so stiff is how tightly the wires are twisted and all the layers of insulation / fill. The big benefit is that it should hold up pretty well for a long time and to much greater depths than you need.
One option that costs about half the price, but isn’t neutrally buoyant (and wouldn’t survive a leak very well), is “tactical” or “rugged” Cat5e. You can find it from lots of different sources. Here’s one on Amazon that includes a reel:
I’ve also used the single pair tether threaded through poly core rope with my OpenROV project. I have mixed feelings and results with it.
I’ve found it does the following:
Provides additional abrasion resistance for the single pair tether
Provides much needed pulling strength to the tether(my ROV became entangle in an underwater building foundation, and I had to resort to pulling on the tether "rope" to successfully recover the ROV.
I have found it gets caught or "hung up" quite easily on pilings, encrustations, jelly fish tentacles, etc...
It's buoyancy is a result of the fact it holds air bubbles, which means that it may be neutral initially, then at depth, the bubbles escape or "hose" up through the tether and then the lower part of the tether becomes negatively buoyant and sinks, then gets hung up on the bottom and/or drags the ROV.
I found it to be a PITA to do the initial threading and would rather plunk money on a commercial tether than do it again. lolz;-)
The commercial tethers usually provide the functionality of the first two bullet points, alleviate the second two, and solves the last, albeit with a higher price tag.
I have seen 3d printed like clips that you can spread out evenly on the tether. Personally I have used zip ties and pool noodle chunk every few feet if depth is pretty large might move to like Pink insulation foam or the blue robotics buoyancy foam for it. Not pretty but easy.