Buoyancy and ballast for custom ROV and AUV research platforms.

On a research vehicle built from a frame, pressure housings, thrusters and sensor payloads that keep changing, buoyancy is a running total. Track it from the first structural decision. Leave it to the end and it turns up as a vehicle that floats nose-down, sinks under a heavier payload, or has weights bolted on the night before a trial. This guide covers the net buoyancy calculation, choosing syntactic foam by depth rating, and trimming ballast on lab-built and small-series ROVs and AUVs.

Calculating net buoyancy.

Net buoyancy is the upward force from the water your vehicle displaces minus its weight in air. Teams usually get the weight wrong. It includes the frame and the pressure housings, and also every thruster, cable run, fastener and small bracket added during the build. An accessory mounting bracket or a cable clamp weighs a few grams, but a research vehicle can carry dozens of them, and together they are a real line in the weight budget.

Build the weight table alongside the mechanical design. List the frame, the enclosures with their trays and boards, the thrusters, connectors and cabling, and the sensor payload, each with a measured or published mass. Work out displaced volume the same way, part by part, from the outside dimensions of everything that sits in the water. Aim for close to neutral with a small positive margin: a few hundred grams to a kilogram or two, depending on the size of the vehicle. Then a power or thruster fault leaves the vehicle rising slowly to the surface, and the thrusters don't spend their effort fighting excess lift the rest of the time.

Leave room in that budget. Research payloads change between deployments: a camera swapped for a sonar head, a sampling module added, a bigger battery for a longer mission. If the vehicle is trimmed to the gram for one payload, every change means redoing the ballast sums and often ordering more foam. Size the budget for the next one or two payloads as well as the one on the bench today.

Choosing syntactic foam by depth rating.

When the weight table shows a shortfall against your buoyancy target, you make it up with syntactic foam. Sealed air volumes don't survive depth. Blu-Float solid buoyancy foam is epoxy filled with hollow glass microspheres, moulded under high pressure. It comes in grades rated to 500 m, 1,200 m, 1,500 m, 2,000 m, 3,000 m and 6,000 m. Density runs from about 0.36 to 0.57 g/cm³ and minimum compressive strength from 12 MPa at the shallowest grade to 70 MPa at the deepest, and every grade absorbs 1% water or less. Standard blocks are 500 × 500 × 100 mm and weigh about 8.95 kg in the 500 m grade and about 14.55 kg in the 6,000 m grade.

That spread in density is the trade-off. The lighter, shallow grades give more lift per kilogram of foam. The denser, deep grades give up some of that lift so they can take the pressure without soaking up water and losing buoyancy over repeated dives. Choose the grade for the vehicle's real maximum depth plus a sensible margin. Over-specifying to the deepest grade means more foam for the same lift and a heavier vehicle.

Trimming ballast in practice.

The weight-and-displacement sums get you close to the target trim, but rarely exactly there. Real parts weigh a little more or less than their datasheets, cables end up longer or shorter than planned, and a vehicle that balances on paper can sit nose-heavy or list once it is in the water. Fine-tuning after assembly is normal and doesn't mean the sums were wrong. It is easiest with ballast you can add and remove: small weights at known positions, or an electronic tray set inside an enclosure that you can move or reload to shift trim and internal weight together.

Check trim again every time the payload or the enclosures change. A heavier sensor, an extra enclosure or a different battery all move the balance. Treat ballast as a setting that belongs to each configuration, and re-check it after each significant change, the way a lab re-checks calibration after changing hardware.

How enclosure and thruster choices change the budget.

Your enclosure and thrusters move the numbers directly. A subsea acrylic pressure vessel tube is much lighter than a subsea aluminium pressure vessel tube of the same diameter and displaced volume. That helps buoyancy, but acrylic has a lower depth rating and has to be clamped rather than bolted; our subsea enclosure selection guide covers those trade-offs. A subsea watertight enclosure box is heavier again for the same internal volume, which matters when compute and payload electronics move from a tube into a box. Whichever you use, it mounts to the frame through an aluminium enclosure mounting plate, and that plate's weight belongs in the same table.

Thrusters work the same way. A Blu-Thrust Z60 weighs about 245 g and gives up to 4 kgf forward at 24 V. A Blu-Thrust Z80-170 weighs about 500 g for up to 5.3 kgf. Adding thrusters, or moving to the bigger model, adds weight and displaced volume at every thruster position, plus the mounts and cabling that come with each one. Our underwater thruster guide covers thrust, voltage and mounting. Decide the thruster count and the buoyancy budget together, before either is fixed.

Sources.

• Density, depth rating, compressive strength and water absorption grades for Blu-Float syntactic foam, from the manufacturer's published specifications.
• Archimedes' principle, with seawater at 1025 kg/m³.

Published 11 August 2026. Last revised 25 September 2026. Corrections to sales@vebixautomation.com.

Talk to us about buoyancy and ballast.

Getting the weight and displacement right before foam is cut or an enclosure is machined saves a round of rework. We sell the Blu-Sub foam, enclosures and thrusters in this guide, and we can work through depth rating, foam grade and trim for your frame and payload. Ask for a quote.