Hi @Guillaume , welcome to the forums!
I’m answering your (identical) enquiry here rather than via email, so the broader community can benefit.
- To reach more than 200m, you do indeed need to upgrade from acrylic to aluminum tubes.
- The slim tether is definitely best for deep dives, to minimize drag.
- WetLink Penetrators used on the BlueROV2 are rated for 1000m depth, so should be fine for your application!
- Longer battery life could be achieved with multiple batteries in parallel - easier still may be a topside power supply - several vendors have advertised options for this here on the forums, or there is the Outland solution on the Reef!
- The standard foam in the BlueROV2 should be good to 300m, if coated with cleary epoxy this is boosted to 488m! This is noted in the technical details.
- Using a tether-attached weight on the tether is key for reaching deep depths with a small ROV, especially if a strong water current is present and the system is not drifting along with it. Securing 3 to 20lbs 5 to 30m from the ROV on the tether, and lowering this weight will passively fight the drag the ROV would have to overcome. The only drawback is you must lower the weight close enough to your desired region of investigation!
- 200m is the limit for this with the Fathom-X, however some users have had success using other two-wire ethernet solutions to achieve good communications across longer tethers!