Noob question about battery pods


#1

Hi Guys, First post so be gentle. I am looking at building a ROV and have what might be a stupid question:

Why are the batteries in 2 external pods with all that entails for complicating the pressure and water proofing problems? Why can't they be internal to the electronics section?


#2

HI Paul

I would imaging for recharging purposes it would be less problematic to open the pods as opposed to the main chassis, also possible centre of mass reasons for stability and buoyancy.

Perhaps we can look at modifications later,my main aim is first to get an operating prototype and the easiest way is to follow in the path others have chosen.Like you I am new to this too. Good luck.

James


#3

Paul:

Putting the batteries in the electronics section would shorten the distance between the center of mass and the center of buoyancy of the vehicle, degrading its stability.

-W


#4

I get this but you could use some other type of ballast in the pods for stability and keep the

batteries in the main compartment and maybe use some kind of inductance to charge the batteries. It seems to me the biggest problem for these ROV's is water leakage at depths so the less places you have to water proof the better IMHO.

No matter, this site has inspired me to make one so I might start with the openROV as it is and experiment a bit. Thanks all...


#5

Hi Paul:

If together with the batteries, a kind of ballast is added, buoyancy must be increased as well, and this, can only be achieved by adding volume.

If volume is increased while keeping the "single watertight compartment" concept, working pressure (depth) results drastically decreased unless wall thickness grows at the same time.

Some key submersible design starting points, that work one against the others, must be taken into account, and a choice must be made depending on desired features.

From a structural point of view:

Sphere is the best shape, but it is expensive and hard to be built. The second choice is a cylinder, where the balance between thickness, lenght and diameter have to be adjusted.

From the hydrodynamic point of view.

The higher L/B (Lenght/Beam), the better, but at the same time, the less wetted surface the better.

Minimum wetted surface for a given volume, is achieved by a sphere, but its L/B=1, and hence, shape dragg is quite big.

Best L/B is achieved by a very long, very narrow sape, but as a volume is required, beam is limited by practical concerns. Hence a big wetted surface results and surface dragg becomes big as well.

From experimental channel tests, the best practical L/B while minimizing wetted surface, resulted to be between 7 and 8.

At the same time, in a long shaped submersible, the Reynolds number grows very fast along the length, deriving in a quite turbulent flow astern, that badly affects propulsion and yaw-pitch stability.

What means long ? A body submerged in a fluid is short or long, not depending on its size, but on its Reynolds number that relates speed and length to fluid conditions.

For this kind of ROV device, speeds are very little and hence, the capacity for withstanding high presures prevails over dynamic considerations.

Hope to see your modifications to the ROV.

Kind regards.


#6

Hi Paul:

Every ROV reflects a set of design tradeoffs or compromises- the current OROV is just the set of tradeoffs that Eric happened to choose when he designed it. If you add ballast that means you have more weight to carry around.

That being said, if you want to build your own ROV differently, that's great! We heartily encourage experimentation when it comes to building. Try something new and then post the pictures here to show us all.

-W