Mini ROV on a 20 meter cable

[87TurboIroc]

I'm working on a larger ROV project but it is getting expensive and heavy quick. To motivate myself and to assist with thruster testing, I decided to throw together a very simple tri thruster ROV based around my existing Color Sharp CCD underwater camera. It came with a 20 meter cable and I will be using it for a specific task in 30 feet of water. Because the cable length is so short, and after doing testing with 20 meters of Cat 5 cable, I'm going to simply send the voltage down through the cat 5. No onboard relays or dry spaces required.

One thing that is interesting about ROV design is bouyancy calculations. When computing the displacement needed, you must also factor in the weight of the displacement vessel. So as you add displacement, you actually increase the amount of displacement required for a net effect. Also it is important to weigh components such as thrusters and then calculate their displacement. My thrusters weigh 220 grams but displace 88 grams of water, for a net submerged weight of 132 grams. I was at a 500 gram bouyancy deficit until I factored in the water displaced by the thrusters and camera housing. I ended up with a net positive boyouncy of 28 grams.

So the specs on this design are as follows:

Three Rule 500 GPM cartridges ($75)
Three 80 mm tail rotor props (Blade EFLH1324) perfect slip fit to the Rule shaft ($6)
Four PVC tubes 1 1/4" (1.75 OD) X 20.5 inches (had some here)
One Color Sharp CCD Fishing Underwater Video Camera w/60 ft cable) ($110)
Ten stainless hose clamps 2 inch ($10)
Assorted other stainless hose clamps ($10)
Eight PVC endcaps ($7)
(more coming)
Switches..
Project Box..
Alligator Clips..

LOA 20.5 inches
Weight (dry) 3038 grams / 6.7 lbs
Max depth (limited by cable) 50 feet

[87TurboIroc]

It ended up being longer than I wanted. My other choice would have been to purchase larger tubing, but I wanted to keep expenses low and use what I had on hand.


This is a mockup of the basic layout. Positions for everything will be adjustable by loosening hose clamps, so balancing the vertical thruster performance should be pretty easy.


For the horizontal thruster mounts, I simply spot tig welded two hose clamps together. This is very solid and you can pick the whole ROV up and shake it by the thruster without any play. That's with only two spots. Four would be nice but it is easy to burn through and ruin a clamp so I will stick with two for this project.


Here you can see the camera housing and the thruster with the Blade EFLH1324 tail blade. Of course this is a directional prop and provides very good thrust in one direction. I'm going to buy some extras and try to heat one up to flatten the blade's airfoil design. If this works, it will balance the thrust better between forward and reverse. If not, I've only lost $2!

[KR2_Diving]

welding the hose clamps together is a simple and brilliant idea!
well done! keep us posted!

[87TurboIroc]

The Mini ROV is done! First dive was tonight in my pool, which is unfortunately green for now so there wasn't much visibility. Hopefully tomorrow evening I will get to the marina and do the first salt water dive. It moves surprisingly quick. I bought the wrong kind of switch for the vertical thruster, so right now I only have dive thrust on that one. Also the project box was too small so I need a new box and a different switch and then this thing will be 100%. Oh and I have to make a new bracket for the vertical thruster when I get some more hose clamps. This one didn't line up the way I planned and the thruster is at a slight angle for now.



I posted a series of three videos showing the design and build process starting with this one:

http://www.youtube.com/watch?v=4E0Y-RW4BCc

[87TurboIroc]

Two dives so far at the marina. This thing is lots of fun!


http://youtu.be/6COcM-ba8go
First dive!


http://youtu.be/4SRy3fO8yG4
Second dive.

[fryslan76]

Extreem fun to watch.

[87TurboIroc]

First 'production' dive was successful in that the ROV functioned and we were able to bring it back to port. It was fail in terms of locating the two target objects below. Basically the short (50 foot useable) tether is a real limitation with the boat drifting back and forth on the anchor in 30 feet of water. However the biggest enemy was simply not knowing which way the ROV was going. We could figure out where it was at between visibility (12-14 feet from the surface) and tether direction. However working at 30 feet we only had the tether direction to go by. A compass would have worked wonders! So I now have one to mount on it for tomorrow's dive. We did master the process of getting over the target. Basically dropped a marker bouy on the target, then free drift to establish an average direction. Then we achor upwind about 100 feet and pay out line until we're on it.

The current is persistent and I will keep a better eye on the tides tomorrow. Hopefully I can catch it right near the peak. Winds today also kicked up more than forecast but tomorrow is looking better.

[87TurboIroc]

The compass worked fantastic, however we once again ended up being off-target by some 80-90 feet. Relying on the marker bouy is certainly not the best plan. Why I didn't bother to check the GPS once we were in position is beyond me, but it is very easy to be pre-occupied with the operation of the ROV... especially on this 'ropey' target that is sure to have at least a 50% chance of snagging the ROV. This morning I drove by the spot and the bay was smooth as glass. Next time I get on this spot I will make sure it is during calm conditions like that.

For anyone considering building an ROV with plans to use it in open water, make sure you plan for all of these conditions I have experienced this far. All currents in the water are significant. If your ROV goes really fast in a pool, you have enough to deal with light currents in a bay. It's not just the ROV you are dragging across those currents, but the whole of the tether as well. If launching from a boat, keep in mind the swing radius of the boat on the end of the anchor line. Tether length requirements are significant to account for this movement, and then you have that much more tether in the water to catch the current. In 30 feet of water depth, we were swinging about 40 feet in a steady breeze. A short tether like mine means you'll be dragging the ROV backwards - possibly right into a snag.

I am now in the process of revision 2 of the ROV: moving the camera to the front and mounting it in such a way that it can be pointed up or down (180 degrees of motion). This will give me the ability to have a look down for searching and a look up for hull inspection, along with a forward vew. Moving the camera up reduces the size of the ROV vertically and hopefully reduces frontal drag slightly.

Here is the compass test and the two bay dives:

First bay dive: https://www.youtube.com/watch?v=WAzyFZT-jAI

Compass test dive + lots of fish: https://www.youtube.com/watch?v=5zUOMGOZ1Z0

Second dive and fishing trip: https://www.youtube.com/watch?v=-w9m5dSKwiQ

[87TurboIroc]

Brought the ROV back home from the boat and made version 1.1! This involved removing the lead weights from the top and putting new ones on the bottom of the ROV at the four corners. This moves the mass further out and lower, which hopefully will provide more stability. Next I cut the two weights off of the camera housing and attached them directly to the lower PVC with hose clamps. Then I mounted the camera to the very front using a single hose clamp. Now I can mount the camera at various angles - straight ahead, 45 degrees up or 45 degrees down. Also the camera is right near the surface now and I can see things above the surface when floating. The overall height of the ROV is now half what it was and much easier to transport. Also the compass was mounted to a dowel that is attached via hoseclamp. This allows easy positioning and can be retracted all the way in for transporting.

Hoping to get on the debris again today and perhaps finally see what wrecked my boat.

[87TurboIroc]

Conclusion: the tether is too short on this ROV. I've learned a lot about currents and drag with this ROV and it has made me reconsider the larger 'PAROV-1' plans. I want to downsize that one by at least 50% or I'm not going to build it. I just won't be able to afford the power system on it because of drag on all of the surface area. I'm even thinking about going back to my earlier idea of a wheeled ROV for my next build and hold off on the larger one for a while. Also I will try to redo this mini ROV with a 100 foot tether.