If we were to replace the Buoyancy foam with a empty box which is filled by normal air, will there be a increase in buoyancy? If so are we using buoyancy foam because it will not compress under pressure like air can?
Hi @dew, welcome to the forum
Buoyancy is determined by the weight of the volume of liquid an object is displacing, minus the weight of the object itself. An empty box could have a higher buoyancy than an equivalent block of buoyancy foam, but whether it does depends on the thickness and density of the box walls (and their shape), and the density of the foam being considered.
From a cost perspective, it’s generally more expensive to have an extra enclosure in the place of buoyancy foam, but the required wall thickness depends on the material properties, shape, and required depth capacity. It can also be possible to have extra room in an existing enclosure to increase the buoyancy (there’s an example here), which may reduce the cost increase by not needing extra components (e.g. end caps).
For a modular and configurable vehicle like our BlueROV2 it makes sense to have buoyancy foam and multiple ballast weights, because it’s much easier to tune the buoyancy and balance by cutting foam and moving / removing weights than it would be by changing the size or shape of an enclosure. For a custom-designed vehicle with known and fixed components it makes sense to consider more fixed buoyancy and weight distribution options, that can be part of the vehicle body design.
It depend on the depth you aim. I don’t go much deeper than 30m and at the moment I use plumbing pipes (ABS) to modulate the buoyancy of my ROV and this work fine. You can adjust the length of the pipes and it’s diameter. It work for my application because it’s shallow water (low pressure). Once I find the raw pipe configuraion, I fine tuned the buoyancy and equilibrium with small pieces of foam and extra weight.
Of course if one of my pipe broke then the ROV will have hard time going back up. Foam is a lower risk business from that perspective!