Vertical thruster position and buoyancy balance

[rokenbuzz]

My overall ROV design is a PVC rectangular box.
The front has a waterproof box which houses electronics.
Behind that is a vertical thruster as well as thruster on the left and right.
Like this:

frame_concept.PNG (20.41 KiB) Viewed 6395 times

The air in the electronics box makes the front very buoyant. I estimate it will take about 11 pounds of ballast to make the ROV neutrally buoyant.
I'll add the weight to the bottom.

Notice how the vertical thruster is not centered from front to back.
My brain sees this as a problem at first glance. I don't want the back end to move faster vertically than the front.
Will adding weight more to the front to give the ROV a horizontal attitude solve this?
Can anyone confirm this? Should I be worried this design?

Thanks for helping me!

[rokenbuzz]

I'm finding some great information from the Cornerstone Robotics website.
Specifically the Buoyancy, Stability, and Ballast lessons dealing with center of gravity and center of balance.

[rokenbuzz]

I did some testing in my pool and found this to be a significant problem. With so much weight in the front to counteract the front's buoyancy, the sub pivots on the front when using the vertical thruster. So technically the sub goes up and down, but it doesn't stay parallel to the ground/surface. So the sub's view changes and that's no good. And the pivoting means the vertical thruster is no longer vertical, so that's no good either.
So I'll probably redesign things.

[Wolf91]

Why dont you move the horizontal thrusters next to the box, and make the vertical double in series, and put one on each side?

[rokenbuzz]

Thanks! That's actually where I'm headed. I'm changing it to have two vertical thrusters, one on each side. I have an unused motor controller output available already, so I think I'll use that.
I wish I had this insight earlier. Seems rather obvious now.
Thanks again.

[Ianth3impler]

Do you have any pictures of your frame?