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Towing the BlueROV2?


(Scott Marion) #1

Hi folks,

Does anyone have experience towing the BlueROV2? Or better yet, implementing a system where an ROV can be alternately towed and free-piloted on one dive?

Why? We are trying to conduct benthic video transects, but without position telemetry, we can’t precisely match the support boat’s heading, and before long the ROV gets pulled by the umbilical. We have found it difficult to recover proper orientation after getting pulled. We deploy the ROV at depth using a “clump weight”, a heavy weight hanging from a rope ~10 m off the bottom. The ROV umbilical is clipped to the rope at intervals, and there is a short section of free umbilical between the clump weight and the ROV.

So, the vision is to attach a tow-point to the front of the BlueROV2, and add a length of tow rope from the clump weight. We would deploy the ROV this way, and then once our heading is set, we would fly the ROV forward and down with the heading locked, and conduct a straight survey transect until eventually the boat’s heading diverges from the ROV’s heading and the ROV gets pulled.

I’m picturing a rigid semi-circular “bail” tow-point (sort of like the wire handle of a bucket) attached near the front of the ROV that can rotate over the ROV. When towing, the bail would be forward, and when free-flying it would rotate back to place the drag toward the rear. This way, when the ROV eventually diverges from the boat’s course, the bail will rotate forward and pull the ROV from the front, in the direction of the clump weight, and we will simply lock the new heading (using Stablilize mode) and resume piloting.

Any feedback? Particularly relative to exactly what position on the BlueROV2 might be optimal for attaching the tow point, such that it maintains maneuverability during free-piloting but still tows OK?

Thanks,
Scott


(Tim Pierce) #2

As part of my tether attachment point I’m adding a McMaster carr sourced 2 foot long braided stainless wire rope sling https://www.mcmaster.com/#catalog/123/1540/=17cqunb attached via clevises to the rear mount points. I have one of the blue robotics tether thimbles in the middle to make a center pull point. Adding similar front mount points under the float covers would be pretty easy. From there I attach the tether pull to using one of these things https://www.mcmaster.com/#69675k23/=17cquv2 and a locking clevis

It seems solid, and it seems like it will tie into the strength member of the tether in an emergency or allow me to attach a stronger cable as needed. Something like that would probably work? no idea how much force there is at the frame in a tow configuration. Losing the ROV is probably a real risk.


(shujun wang) #3

What you expect is a professional ROV equipped with TMS


(Mark Langille) #4

How did the McMaster tether sleeve work for you?


(Tim Pierce) #5

I haven’t started diving with them yet because my ROV has been on hold due to the worlds longest ever move process (long story, had to replace the roof before I could sell house, ROV has been in a box) But they seem to grip the cable great, and have not slipped on the test pieces I subjected to impromptou load testing.


(Mark Langille) #6

Great thanks. I think I’ll order one to try as well since we are added the bulkhead connector anyway, so need to rework the tether and would be good time to do it.


(Luke) #7

Hi Scott,
We have attempted the exact same thing from our 6m workboat. Whilst we didn’t master it yet, here are my thoughts which may allow you to achieve your benthic video transects without towing.

Your description is exactly what happened to us, except it was the wind that was blowing us off course. We had a ‘clump weight’ attached to the tether a couple of meters off the bottom. The aim was to fly the BlueROV 10-15m in front of this point and on a stable heading. As you describe this quickly leads to divergence. We actually have a tow video so we quickly resorted to that to do the job, but after thinking about it some more… why didn’t I just follow closely ‘behind’ the clump weight using the ROV video feed, with the boat going ahead at a very slow speed? This has several benefits in the absence of a waterlinked GPS (which we now have):
1. Can fly ROV whilst following a fixed reference
2. The boat can record a track of the exact GPS position, depth, and the ROV is essentially under the boat.
3. weighting the base of the tether and fixing it off ‘topside’ reduces risk of tether getting into motor.
4. the boat direction and speed can be controlled based on commentary from ROV pilot, or vice versa.

Anyway, saw your comment and it was a bit of deja vu. Hope you have some success. Let us know how you go.
Cheers, Luke


(Scott Marion) #8

Hi Luke,

Thanks for the feedback. It sounds like you are talking about following the clump weight by keeping it in the field of view of the ROV? This would not work for us, because: 1. we often have poor viz; 2. we are sometimes doing fish surveys, and having the weight right in front of the ROV would disturb the fish; 3. we often have several meters of rocky relief, so having the weight very close to the bottom (in visible range of the ROV) would risk snagging the bottom.

We did create a system that allows towing or free-flying. It has a rail along the top, with an attachment ring that slides forward or back along the rail as needed, so that in tow-mode the ROV is pulled from the front, but in free-fly mode the umbilical pull comes from the top. This did worsen the ROV’s significant porpoising motion. But we are now moving to the heavy configuration in order to stabilize the camera.

How useful/accurate are you finding the waterlinked GPS? We often operate in 25 to 35 m depths- do you think its performance there would allow long straight transects?

-Scott


#9

See this thread Drop Camera/ Tow Sled Frame

We have good experience with our small and simple modification.

Only downside to our wing is making it unstable in reverse.