Tether buoyancy very positive in deep dives

Hi All,

The tether of the ROV is pretty positive in the water and when i dive deep it tends to lift the back of the ROV and make it very unstable. I am thinking to add few weights along the tether to keep it a bit more close to a neutral situation. Do someone here face the same problem ? Any suggestion ?

Thank you


Most likely this is an issue of tether drag rather than increased buoyancy - I suggest you have a read of that thread :slight_smile:

If the problem is indeed the drag, then you can reduce the drag with a “clump weight” and/or a slimmer tether.

Hi Eliot,

is not tether drag. I am using the ROV in a lake where current is 0 and i am flying it from the shore. Use a clump weight is not suitable for what i do and my idea is to add small leads along the tether to make it a little bit more neutral.
I’ve read the clump weight topics but there is no one that is describing my situation.
Do we know what is the buoyant rate per meter of our tether ? In the specifications it says that it should be neutral in fresh water but mine is completely positive, it stays in surface all the way and it pull the rov up when i dive, please see the photo attached.

Thank you
Best regards

If your tether does not match the specification conditions then either

  1. it does not match the design specification
    • in which case we don’t know what buoyancy it has, and
    • it could be any density that is less than that of fresh water, or
  2. your conditions do not match those in the specification being compared to
    • in which case your lake water may be more dense than “normal” fresh water, which could be due to
      • salt / mineral content
      • temperature
      • abundance of deuterium (“heavy” water - quite unlikely)

An overly buoyant tether is not something we’ve had people mention or complain about before, but as far as I’m aware it’s also not something we have super tight control over from our tether manufacturer, and it’s not impossible that a tether could have slightly more foam than specified.

I’m not sure what the manufacturing tolerances are for the tether, but if you’re concerned that yours is not what was advertised then I recommend you contact support@bluerobotics.com to try to resolve the issue (in which case please include a link to this thread, and your order number / order date for our tracking).

In your photo the ROV is at the surface, so it’s expected that the tether should also be at or near the surface, especially since there is seemingly not much slack. If the tether is positively buoyant then there should be some proportion of the tether floating above the surface in fresh water, and any slack should stay at the surface even when the ROV dives.

It’s possible that’s the case here, but the photo doesn’t seem to be obviously showing that.

Hi Eliot,

the photo was just an example, when the ROV is diving the tether is at the exact same position.
I am not concerned and mine is not a complaint, i was just trying to find a solution.
The lake where i dive is a completely normal lake where i also scuba dive. I can tell you for sure that the water is not the problem here also because i had the same issue in another lake.
No worries I will find a solution,

Thank you

Hi Andrea:

Have you tried moving the mechanical attach point of the tether to the top center of the ROV? I don’t have a picture handy, but I seem to recall that other people have done this and have posted photos here.

If there is a buoyancy or drag issue with the tether it will still affect the ROV, but it will move the ROV up and down, as opposed to causing pitching movements. The depth-hold mode should be able to easily counter this.

The only drawback of having a top attach point is that there will be a small pitching motion of the ROV when you drive forwards, due to the tether drag acting on the top of the vehicle rather than the centerline.

It’s worth a try- if you do decide to do this, please report back your experiences with the modification.


Hello Walt,

yeah i was thinking about moving the attach point the the center of the ROV but first of all i want to try to make the tether a bit more negative and see what happen. I will perform few tests over the next days let’s see if it works :wink:
Thank you for the advice

Mine has always been the same way. It starts to become slightly neutrally buoyant if it has been used in the last day or two and absorbs a little water.

You can see parts of mine being above the water:

The positive buoyancy will pull the ROV backwards if it gets deep enough. It gets hard to stay in position since a constant horizontal thruster input away from the spool is required, but it’s always obvious which direction the home position is. :wink:

I have always assumed getting dragged backwards was a normal part of “Tether Drag”

like 34:47 here when I took my hands off the controls to adjust the Ping360 settings

and around 17:40 here:

Hello @EliotBR,

i’ve performed few test in the last few days.
I think @btrue is right about the fact that the tether becomes more heavy when you use it. I’ve noticed the same thing and i can see that the part that is less used it is slightly more positive.
I’ve added 4 grams along the tether. 4 grams the 1st meter, then 8 grams the 2nd meter and the same all along till the 100 mt. (4 gr then 8gr then 4 then 8 etc) Yesterday i performed 2 dives from the boat, one was at 60 mt with no clump weight and the tether was working very well and the other one was at 114 mt with the clump weight.
Especially considering that the dive was from the boat, i am very happy about the result and i can see that the ROV is pretty stable. Would be nice to do the same test with boat that is stable and not drifting but i think it is quite hard to find this kind of situation.