Hi! I am building my own second ROV, and even though I can’t afford BlueRobotics parts yet (maybe I will for the next project), I have copied a lot of design solutions from BlueROV2, and I do mention it often when I present my project publicly, so even though I’m not a customer yet, I hope it is still okay to ask here for advice
Since I cannot afford Fathom ROV Tether, and I need 100 m. (I want to stay under $100 for the entire tether), I decided to do the PLC module hack, so I can use pretty much any 2 wire cable, even untwisted, and have 100 mbps data transfer with minimal jitter. So I have many cables to choose from - power cables, telephone cables, speaker cables. As far as I tested, anything works at 100 m. length. Trouble is, all cables sink. I want to achieve neutral buoyancy at any depth (disregarding temperature and salinity variations), but not with floats, since they would introduce a lot of drag and make tether prone to catching on obstacles. So I need to keep it smooth.
What I tried so far:
1. Pipe foam insulation, pool noodles - gets crushed under pressures of around 2-3 bars and loses buoyancy, also absorbs water over time.
2. EPDM foam - also gets crushed.
3. Neoprene tubing - way too expensive, also difficult to get in just the right size to have just the right amount of buoyancy needed for this cable.
4. Non-sinking polypropylene rope - turns out, it is mostly buoyant because of air bubbles trapped between rope strands, which get crushed under water pressure and lose buoyancy.
5. Threading cable through a garden hose / aquarium PVC tube - I tested many different hoses and tubes, and found that ones that have walls thick enough to withstand pressure without imploding are to heavy and stiff. To achieve enough buoyancy the final diameter becomes unacceptably large. Running cable alongside the tube instead of inside it provides extra buoyancy, but makes the tether even stiffer.
6. Using thin walled PVC tube or heat shrink tube over the cable, and replacing air with low-density oil to prevent implosion - requires even thicker tube to reach neutral buoyancy, so much that it becomes unacceptably stiff. Also, water pressure squeezes the parts of the tether which are deeper in the water, pushing the oil to the shallower part of the tether, where it bulges.
As I understand Fathom tether uses PUR foam jacket. I have done some PUR molding before, but I don’t think there is a way I can mold it evenly around the cable for a 100m length. Also, as far as I tested, only the densest and stiffest PUR doesn’t absorb water, which makes it heavy to begin with. Tether flexibility is very important to me, since my ROV is small and needs to maneuver freely.
I would very much love to hear your suggestions on a low-cost solution for tether buoyancy. Like I said, my budget for it is $100 for 100m length. Cable will set me back $30-60, depending on which one I buy (I can buy with thinner copper and jacket, expecting buoyancy material to provide strength, or with thicker copper and jacket so it is strong enough on it’s own). The most important thing is achieving neutral buoyancy at any depth, so the solution has to provide same amount of flotation regardless of water pressure.
Thank you in advance!