How to Build a Buoyancy Engine for Seaperch Underwater Gliders

Pressure in fluids (liquids and gases) differs from solids - fluids exert pressure in all directions and pressure increases with depth. Density is mass per unit volume (Density = Mass/Volume), with SI unit kg/m³. Buoyancy is the upward force exerted by a fluid on an immersed object. An object floats if its density is less than the fluid's density, and sinks if its density is greater. However, an object with higher density than water can still float if it displaces enough water to create sufficient buoyant force. This explains why ships (made of steel, denser than water) float - their shape allows them to displace a large volume of water.

When two immiscible fluids are in contact, the pressure at the interface is the same from both sides. For example, if oil floats on water, the pressure at the oil-water interface is P_interface = P₀ + ρ_oil × g × h_oil. This must equal the pressure from the water side at the same level. The height difference between fluid surfaces can be found using this principle, taking into account the different densities of the fluids.

When an object floats at the interface of two immiscible fluids with different densities, the buoyant force is the sum of the buoyant forces from both fluids. For a cylinder of length L and cross-sectional area A/5 floating at the interface between fluids with densities D and 2D, with L/4 submerged in the denser fluid and 3L/4 in the lighter fluid, the object's density is found by equating total buoyant force to weight: D × (A/5) × (3L/4) × g + 2D × (A/5) × (L/4) × g = ρ_object × (A/5) × L × g, giving ρ_object = 5D/4.

This concluding segment explores the physics governing ROV buoyancy and environmental adaptation. The presenter explains that ROVs achieve neutral buoyancy through carefully sized air-filled buoyancy tubes that counteract the total weight of the vehicle. Since water is incompressible, density remains constant regardless of depth pressure, though temperature affects buoyancy—colder water at greater depths is more dense, increasing buoyancy as the ROV descends. The segment contrasts freshwater and saltwater environments, noting that saltwater's higher density makes objects float more easily. The presenter discusses how larger ROVs require proportionally more powerful propulsion systems while maintaining similar maneuverability principles. Active ballast tanks could compensate for density changes but add complexity. The video concludes by encouraging viewers to experiment with their own underwater devices and participate in the SeaPerch student competition.

Oil and water are immiscible liquids with different densities. Oil floats on water because it is less dense. This property is demonstrated when adding colored water to oil, where the water forms distinct layers. Understanding immiscibility and density is important for understanding fluid behavior.

Buoyancy force (F_A) equals weight of displaced fluid: F_A = ρ_fluid × V_displaced × g. It acts upward through center of gravity. Objects float if buoyancy > gravity, sink if gravity > buoyancy. This explains why objects behave differently in various fluids based on density comparisons.

Buoyancy force (Fk) equals the weight of displaced fluid: Fk = Vbat × D × g. Objects float when Fk equals their weight (Fk = G). Objects sink when G > Fk. Apparent weight in fluid equals actual weight minus buoyancy force. When objects are submerged, the displaced fluid volume equals the submerged object volume. The buoyancy force depends only on submerged volume and fluid properties, not on object material.

This section provides comprehensive coverage of buoyancy principles. Students learn the buoyant force formula F_b = V_submerged × D_fluid × g, which equals the weight of displaced fluid. The video explains that for equilibrium, buoyant force must equal object weight. Students learn how to determine object density relative to fluid: if object density > fluid density, it sinks; if equal, it remains suspended; if less, it floats. The section covers how adding salt to water increases density, causing floating objects to rise. Students learn that buoyant force depends on submerged volume, not total volume, and how to calculate object volumes from buoyant force measurements.

A small iron nail sinks in water while a large iron ship floats because the ship's large volume contains much air, making its overall density less than water; according to Archimedes' principle, an object floats when it displaces a weight of water equal to its own weight.

Buoyancy is the upward force exerted by a fluid on an immersed object, equal to the weight of the fluid displaced (Archimedes' Principle). Whether an object floats or sinks depends on comparing its density to the fluid's density: if the object's density is less than the fluid's density, it floats; if greater, it sinks. Fish control their buoyancy by adjusting the volume of gas in their swim bladder, while ships float despite being made of dense iron because their hollow structure reduces their overall density below that of water.

This video demonstrates how to waterproof three 12-volt SeaPerch ROV motors using electrical tape or stickers, solder wires to motor terminals, prepare 35mm film canisters with wax for motor housing, and assemble propellers using threaded couplers, T-nuts, and hex lock nuts, then attach propellers to motor shafts using super glue after roughening the surface.

Underwater gliders operate using a buoyancy engine that enables vertical movement without traditional propellers. By extending or retracting a piston in the nose, the glider changes its volume, displacing more or less water to ascend or descend. This principle mirrors human swimming—when lungs fill with air, the body displaces more water and floats higher. The glider uses only about 0.5 watts of power, allowing operation for months on battery power.

SeaPerch is an underwater ROV (Remotely Operated Vehicle) competition where student teams design, build, and operate their own remotely controlled underwater robots using PVC frames, thrusters, and waterproofed motors, competing in challenges like mission courses and obstacle courses while developing engineering skills through hands-on innovation.

To balance the SeaPerch underwater vehicle for both diving and surfacing, trim approximately half of the flotation foam from the floats while the vehicle is in the water, using a knife to cut and slit the foam sections; this allows the vehicle to achieve neutral buoyancy where it can be driven down and will return to the surface, with the vent holes ensuring air escapes from the frame so only the foam provides buoyancy.

An underwater glider achieves controlled vertical movement through a ballast tank system that uses a stepper motor to pump water in and out, allowing the vehicle to sink or float by changing its overall density; achieving neutral buoyancy requires balancing the glider's weight with its volume, which may involve adding ballast weights like lead pellets when the initial design proves too light.