Lifting capacity of the ROV?


#1

I was reading one of the discussions here on the forum about how to attach a robotic claw to the ROV and it got me wondering about how much weight the ROV can lift and be able to resurface? Any ideas? I'm thinking about a kilo.

Anyone ever try this is a test?


#2

This is a good question! I'll test this out in the lab this week, but i don't think it's going to be very much weight.

A few people have talked about using an ROV to attach a hook to something underwater, then pulling it up using the hook/line.


#3

You could make it lift a lot of weight by attaching to the ROV a lead weight with compensating flotation. Once the object to be lifted has been secured, drop the weight and the ROV will become positively buoyant equal to the weight lost.


#4

in an ideal world one could use the figures from this test: http://openrov.com/profiles/blogs/preliminary-propeller-efficiency-testing


#5

Well, this test shows the efficiency of the Graupner props when run in forward direction (i. e. downward for the vertical thruster).

I recently switched from the Deltas to the Graupners for the port and starboard thruster and it shows that they are less effective when operated reversely (see a link David Murphy posted some time ago here), which is the case for the vertical thruster when the ROV is ascending.

I'm still hesitating to switch for a Graupner prop for the vertical thruster, so I would be very happy if sombody has some test results here!

-Stefan


#6

Hello, I done a test for some time ago. Result is that there is lower thrust in revers. I do not know whether this is because of the propeller designs or engine which can prevent flow of water.



#7

Hi all ยก

Propellers work as an open turbine with no stator. Hence the blades design is critical.

That's not only a matter of diameter/surface/pitch, but a matter of blade hydrodynamic profile properties and their distribution along the radius.

The blade design tries to search for the best performance at each blade section, from the hub to the tip, taking into account the crossed interference between stations. It means looking for the best Lift/Dragg relation along the whole blade, while holding low pressure areas under control. Blades shape must also take into account the blades interference that depends upon all variables above.

It means changing the profile, angle of attack, chord lenght and the fixing point(relative to the blade radius) for each section.

The whole thing is done for performance optimization, for a flow that runs through the propeller, along a given path, into a speeds range.

If that flow path is reversed, the whole design optimization becomes broken. By the way, a propeller is designed for an inflow rate range. Many times, when working on reversal, the propeller is not fed with fluid enough for a correct working.

A propeller with flat blades, would almost work the same in both sences (neglecting the inflow question), but would be optimal for none of both conditions.

If a high performance is wanted, a choice must be made, becouse .... an optimized propeller for turning clockwise, will result in a bad one when turning the counter sence.

Your numbers look very good. We usually divide AHEAD by 5 when making initial assumptions about ships manouvering capacities related to Propeller thrust astern.

Best regards

-Ion-


#8

I am also curious as to how issues of buoyancy have been resolved. Given that there is an extreme difference at varying depth levels. Is the top prop the only mechanism available to produce lift or change elevation? Is there no "atmospheric air chamber" or is this in development?


#9

Hi Dean:

For undewater vehicles, there are only two systems for lifting power control, Dynamic and Static.

Dynamic systems provide power by vertical thrusting devices, or by control planes, as airplanes do.

The second one, is the simpler, but requires the vehicle to be moving at a minimum manouvering speed, that for a ROV is not very useful.

The first one, has been the choice for the OpenROV project, due to its building simplicity, easy control and low price, together with its good performance.

The fixed and releaseable weight, is a good option, albeit its a One Way only procedure.

Static systems use a change in the averaged vehicle density for buoyancy/weight balance control. That's the system actually used by Submarines.

Those systems, are the best for underwater operations, but are complex to be built. A good manteinence is required, and are expensive.

For big depths, variable displacement systems are not the most suitable. High counter pressure devices must be fitted in the vehicle in order to control ballasts, at the same time, very strong circuits (piping/valves ....) and tanks are required.

Wouldn't it be an intermediate solution carrying a lifting line, attached to the surface "unit" with the ROV, hooking the weight, releasing the line from the ROV, and heaving that line up from the surface ?

Making the ROV able to lift heavy loads, means a radical design change, but making it able to carry a lifting line, does not look so complex.

Regards.