# Help with buoyancy and gravity

Hi,
I did my first ever fly with my ROV yesterday in a calm shallow lake. Everything work fine except the stability of the rov. After numerous foam/weigth addition-movement, I realize I should understand the final goal before trying different things.
I assume that we aim for this:
Goal: slightly positive buoyancy and homogenous weigth repartition.

In my case, the ROV is buoyant positive until I add the heavy batterie pack in the rear compartment. Then the ROV become buoyant negative and because the batterie is at the back of the system, I end up with the rear sinking first and the front looking to the sky at about 30 degree angle.

I added foam to the rear end and lift the rear of the ROV and achieve a slight positive buoyancy. I also add some weigth at the front to make it more horizontal when floating. It work but was not stable. Specially when I tried to dive vertically. I guess this is because the setup is unbalanced?

So my question is : what are the step to reach a stable setup? Is there a guide somewhere about this? What do you guys means by sligthly positive? How many second to come back to surface when pull 10-20cm below the surface?

Instead of my unbalance attemp, I now think I will try to increase the weigth of the front end of the rov to counterbalance the batterie pack weigth and try to make the ROV sink horizontally and then add some foam at each corner to reach buoyancy. What do you suggest?

flying the ROV was so fun! Can’t wait to go back there.

Thank you

Hi @Charles,

Great to hear you had a lot of fun with your first ROV dive!

There’s a quite extensive post about both active and passive compensation of mass/buoyancy tuning here, which will hopefully be helpful to you:

“Slightly positive buoyancy” just means that the object will float back up if submerged in a liquid of the specified density. The speed of floating up depends a lot on the amount of buoyancy/density of liquid, along with the shape and size of the object.

For the BlueROV2 the technical details tab has a section at the top for physical properties, which specifies that a standard BlueROV2 has net buoyancy (without ballast) of 1.4kg, which means basically that when it’s submerged it has the equivalent of 1.4kg worth of gravitational force pointing back up to the surface. The weight without ballast is 9-10kg, so if you assume 10kg the acceleration upwards should be \frac{1.4kg \times 9.81m/s^2}{10kg} \approx 1.37m/s^2. If you assume no drag and a zero starting velocity then the time taken to go up 20cm should be about t = \sqrt{\frac{2\times 0.2m}{1.37m/s^2}} \approx 0.54s, but you should expect it to be longer than that since drag will slow it down. Note that that’s based on the water properties that the buoyancy was measured in, so is only a rough estimate, and is also only valid for the no ballast case. You can do similar calculations for different configurations or ROV designs if you want to.

Hi Eliot,

thank you for this well documented reply. I still have to learn a lot!

About the “sligthly positive buoyancy” then 0.54s / 20 cm is rather fast to me. This mean that as soon has you stop maintaining the ROV 20cm below the surface it will pop out almost immediatly. I was aiming for about 2-3 sec and this is difficult to achieve. In any case 0.54s is a very good starting point and reference to reach a stable setup. Thank you very much.

Charles

For reference, the ‘with ballast’ configuration instead has a weight of 10-11kg and a net buoyancy of 0.2kg, which becomes a=\frac{0.2kg\times9.81m/s^2}{11kg} \approx 0.18m/s^2, with a release from 20cm down taking t=\sqrt{\frac{2\times0.2m}{0.18m/s^2}} \approx 1.5s to reach the surface (assuming no drag) - a bit of extra mass makes a significant difference.

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