# Calculation of Center of gravity and Center of buoyancy

Hi there!

I am trying to make materials for some students, to show them what can be done with engineering. So we have bought some BlueROV2 for them to play around with and are then going to teach them about center of buoyancy and center of gravity. The whole idea is then for them to calculate the balance of the ROV, so the big question is if anybody have done som calculations they want to share, or have a list with weight in air and i water of all the relevant components?

Best regards, Robert

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Robert do you want to use solidworks or any other software for this calculation? I have a solidworks manual if you need it please share me your e-mail and Ill send it right away.

Cheers,
Luis

Hi, Robert!
Have you advanced in this direction? It would be nice if you share your knowledge.
Many thank!

BTW, does anybody knows exact location of COB and COG for stock BlueROV2?
It would be also useful to estimate connected mass and other hydrodynamics related parameters as well.

Wow Lucky LUCKY students. !!
You could go down the road and give each part in the STEP file the correct material and density. And let the students calculate by hand or Excel and insert the COG / COB of every part in relation to ROV origo.

Hi Slovak and Skarholm

Yes i have advanced in this but ended up taking a â€śsimpelâ€ť approach to it. So the first thing to understand was that students that we had coming in was around 18-19 years of age so it had to be somewhat simplified to be a good learning experience over the course of the 4 hours we have with the students.

The quite quickly meant not going in the Solidworks direction. Also it meat neglecting most of the plastic parts because of the density of those being so close to the density of water. Also all of the symmetric parts where left out. So the design of the tasks ended up looking like this:

1. The robots weighs 11.751 g and misplaces 12,24 liters of water. Calculate how many weights that are needed to be placed on the robot? (Hint, is there a difference on saltwater and freshwater?)

Introduction to the robot (Further lecturing)

1. Calculate the center of masse for the weight of the robot in water, both in the X and the Y plane.
• Where should the weights be placed?

The questions where accompanied by a drawing of the robot with scales for the pupils to be able to relate to static calculation of center of mass:

The drawing of the side views of the robot, with scales to ensure the directions of placement when calculation the center of mass.

After they had used their calculations to place the weights and balance off the robot. Thet where going out into the harbor and use the robots to search for bottles floating in the water, with a weight holding them down. When a bottle had been caught, then they maneuver to the surface and return to the docks so that the clue can be taken in. As the last assignment, all of the clues that wherein the bottle could be put together to form a sentence.

Its important to note that in our setup we use a newton gripper and the 18Ah battery. Furthermore some of the numbers are estimated as the amount of water it displaces, the weight and distance of â€śSpheres in tubeâ€ť and â€śBottom plate tube incl. Fittingsâ€ť

And now i am kind of lost in my own answer to whether this makes sense. So if anything of this does not make sense feel free to ask.

Looks like more than enough for 4 hrs. One question I would add though is where in the Z plane would you place the weights, and why.
If all buoyant parts are placed on top and all weights sits low, the ROV will have its best best stability with regards to pitch and roll caused by tether drag. But if you want to spend less power from the battery when pitching or rolling a heavy configuration BR2, then maybe one would prefer the COG being closer to COB.
And a follow up question. If you suspend a ROV in air and balance it to hang level. Will this still apply when the ROV gets submerged? Explain why or why not the ROV will be at the same degree of levelness under water.

They are given the information that the weights need to be placed on the bottom plate, where they are suppose to be fitted.
Correct with the best stability, which is also why i want to place the weights as low as possible. But in the case where you want to pitch and roll it is also worth to note that if COG and COB is too close the force needed to pitch and roll is so little that the ROV might be uncontrollable, because the slightest force would make it spin.(i havenâ€™t had anything to do with pitch and roll myself so this is based on what i have heard). So in this case it would be interesting to find the optimal placement, but i think this will depend a lot on the software and hardware in the loop regarding having the lowest response time.
If you balance it in air you only balance regarding the COG and are not balancing with COB in mind, and if you look over the components that are in the list for the calculations, then its the weight in water that is used for the calculations so that is with the COB in mind. If you balance it out in air it might misplace the COG so that it is not below the COB which would give you pitch to some extend.

With regards to COG/COB I have tried to join the two as best I could. The software did handle it very well. But when the tether pulls the ROV either nose up or down depending of where you attach the tether, then more power is needed from the thrusters because the lack of self up-righting moment.
Regarding balancing the ROV in air, the COG /COB is exactly my point. Even in the â€śOffshore professionalâ€ť world I come across mistakes due to lack of understanding of these subjects.

please send it to razasaqib110@gmail.com thanks

Can you share with me ?
gorkem.engin@searover.com.tr

Hi Mr.Luis,
I would like to have the manual that you mentionned, here is my email sassouak477@gmail.com.