Negative buoyancy

[Kale]

Hi there,

My main goal with my ROV is to be able to get god stable footage of the marine life. Most of it on the bottom. To achieve that I want to park the ROV on the bottom. Does anyone here have any experience of using a ROV that is slightly negative buoyant?

/Kale

[a_shorething]

Hi there,

My main goal with my ROV is to be able to get god stable footage of the marine life. Most of it on the bottom. To achieve that I want to park the ROV on the bottom. Does anyone here have any experience of using a ROV that is slightly negative buoyant?

/Kale

Most of the designs I've seen call for intentionally having it slightly positively buoyant, for obvious reasons of recovery in the case of loss of communication or whatever.

There are a few designs I've seen that have a 'drop ballast' system of magnets or something else that could probably be set up with negative buoyancy so that if it loses communication with the controls for a certain amount of time it drops some lead weights and rises to the surface (hopefully not under your boat ).

I'd also be afraid that once you get down to depth a 'slightly' negatively buoyant ROV would become more than slightly like an anchor without sufficient power to return to the surface on it's own. As depth increases the motors have to work harder to move all that denser water around so what took only 3 amps to do at the surface might require a lot more power to do at depth, like overcoming the negative buoyancy of your ROV.

[Kale]

Thanks for your input. The idea of having a ballast that can be dropped in case of emergency has crossed my mind. I assume that the magnetic coupling is powered as long as everything is ok. Also if there is power failure, the ballast will drop automatically.

Besides that, will the ROV be harder to maneuver?

[a_shorething]

Thanks for your input. The idea of having a ballast that can be dropped in case of emergency has crossed my mind. I assume that the magnetic coupling is powered as long as everything is ok. Also if there is power failure, the ballast will drop automatically.

Besides that, will the ROV be harder to maneuver?

If you mean harder to maneuver when it's negatively buoyant vs. positive or neutral I would be guessing since I haven't launched mine yet , but in my mind I think it would be just as hard to control a positively buoyant ROV as one that is negatively buoyant in that you'd have to offset the tendency to float (or sink) in all of your maneuvering vs. just working with whatever depth you are currently at.

I'm a scuba diver and I know when my BC isn't adjusted it's like work the whole time. You have to work to maintain current depth rather than just 'hanging there' when there are no control inputs required otherwise. The trim in an airplane works the same way. Set it up to maintain altitude or (ideally and for the same reason) be in a slight climb and then there's very little work required to stay where you are. Additionally if you mess up, going up is safer than going down (in both situations).

I thought about this and the question of current in my design by adding in features similar to adding trim in an airplane. My interface will have an 'offset' or 'zero out' function that will allow you to dial in some thrust from the vertical thrusters or a combination of the lateral/horizontal thrusters in order to attempt to maintain position. From there, all control inputs will get this 'offset' added to them. This will use extra power if the ROV is not neutrally buoyant, but at least I'll be able to maintain depth and/or position without struggling to control it with the controller.

[rossrov]

Hi Kale. I really like the idea of having the ROV remain still for periods of time on the bottom when so desired. Just how to achieve this depends on how long it needs to stay in the one place. Thruster pushing down could suck up particles but you could put the thruster higher up on the ROV or some other approach such as intakes on the sides, not below. Then there is the power consumption.

Dump ballast is good but for me only as a last resort in emergency - seems a messy way to do it - and as stated is more mass to drag around from place to place. A while back I posted a diagram of a submarine type ballast arrangment. A little complicated, but the best way I reckon. An air filled bladder inside a housing similar to your metal one, and a pressure washer pump to pump water out. There seems no simple way to deal with the adjustable re-useable ballast problem
viewtopic.php?f=13&t=1156#p6077

Depending what terrain you are working in, another option may be simply a strong tether to lift with, maybe with the auto-dump ballast weights for peace of mind

cheers,
Ross

[kenl]

Most times mine ends up being slightly heavy, from my days diving for crayfish (lobster) I prefer to be a bit heavy when working on the bottom. But my tether is slightly buoyant so the deeper I go the more the tether tries to lift the ROV.

Certainly from my point of view being able to sit on the bottom with the rov is helpful, particularly when trying to film something else that is on or near the bottom.

[Kale]

Thanks I like your input on this matter!

My idea is that the ROV is as near neutral buoyancy as you can get but when you let go of the thruster it will drop very slow to the bottom.

...My interface will have an 'offset' or 'zero out' function that will allow you to dial in some thrust from the vertical thrusters or a combination of the lateral/horizontal thrusters in order to attempt to maintain position. From there, all control inputs will get this 'offset' added to them. This will use extra power if the ROV is not neutrally buoyant, but at least I'll be able to maintain depth and/or position without struggling to control it with the controller.

Thats a good way to stabilize when you do pelagic filming. In my case I have to deal with seabeds that are very silty or muddy. I think I have to park the ROV and wait a while to let the silt disappear before I even start to record the video.

...An air filled bladder inside a housing similar to your metal one, and a pressure washer pump to pump water out. There seems no simple way to deal with the adjustable re-useable ballast problem
viewtopic.php?f=13&t=1156#p6077

As you also mention, I think this is to complicated. But I will look in to it.

Depending what terrain you are working in, another option may be simply a strong tether to lift with, maybe with the auto-dump ballast weights for peace of mind

I do have another problem. The currents are sometimes strong so I will probably need to use a strong tether in which case.

[Kale]

...some other approach such as intakes on the sides, not below.
cheers,
Ross

Good point! That may work!

[a_shorething]

I was thinking about your posts and I'm reminded of the dive site where I got my C-card. The current in this part of a local river is extremely strong and when I went back to dive there one time the current was so strong we lost our dive flag (which we had tied to a submerged log). I wonder if you could put something on the bottom of your ROV to help you 'hold the bottom' in addition to being slightly negatively buoyant. Maybe some pointed legs that you cold kind of jam into the silt or mud using the down thruster and then wait for the silt to clear. You might even shape it so that any current moving across it would push it down into the silt more, rather than lifting it.

In this way you could stake out a spot, 'land', film for as long as you like, and then lift off when you're done.

[rossrov]

Hey a_shorething! Has merit I think. I nearly suggested something similar yesterday Would need motor(s) and gears. A miniature version of these: http://www.rapidanchor.com/The_Wombat.htm