Hi @toosat,
It seems like the mass and buoyancy distributions of your vehicle are sufficiently unbalanced that the BlueROV2-Heavy frame configuration thrust contributions are quite incorrect, which means the control algorithms can’t function properly.
I’d recommend you read through this post to get a deeper understanding, and from there you can determine whether it’s best to adjust the vehicle design or create a frame configuration that better matches your actual vehicle’s distribution (or some combination of the two) 