What Are Amphibious Robots?

Amphibious robots are mobile robots designed to operate both on land and in water, and to cross the boundary between the two without human intervention or a change of hardware. They draw on both marine vehicle engineering and terrestrial mobile robotics, and they exist because the transition zone itself, whether a surf line, a river bank, a flooded street, or a marsh, is exactly where a purely aquatic or purely ground vehicle fails. Designing for two media means accepting compromises in each: a hull shaped for low drag makes a poor chassis for rough ground, and legs or wheels sized for soft sediment add drag and mass underwater.

The engineering problem divides into three coupled parts: a propulsion architecture that works in both media, a structure sealed against water ingress and pressure, and a control system that recognizes which medium the robot is in and switches gaits accordingly. A review of locomotion strategies for amphibious robots groups existing platforms by how they resolve the first of these, which remains the design decision that determines most of the rest.

Locomotion and Propulsion Architectures

Three approaches dominate. Dual-mode designs carry separate mechanisms, typically wheels or tracks for ground travel and propellers or water jets for swimming, which gives good performance in each medium at the cost of dead weight in the other. Transformable designs reuse one mechanism in two configurations, as in wheel-paddle hybrids whose rims fold out into paddles, or leg-fin modules that rotate from a walking pose into an oscillating fin. Undulatory designs use a single serial chain of joints that produces anguilliform swimming in water and lateral or sidewinding crawling on land, an approach demonstrated in an amphibious snake-like robot tested on ground and in water. Bio-inspiration is common across all three, drawing on salamanders, turtles, crabs, and frogs, since those animals solved the same transition problem.

Sealing, Materials, and Power

Waterproofing drives much of the mechanical design. Rotary joints need dynamic seals, usually O-rings or lip seals, that add friction to every actuator; static enclosures need pressure ratings matched to the intended depth, and buoyancy has to be trimmed so the vehicle neither sinks nor floats too high to submerge. Corrosion resistance matters in salt water, pushing designs toward anodized aluminum, polymers, and sealed brushless motors. Energy is the persistent constraint: swimming and crawling have different power profiles, batteries cannot be swapped in the field, and thermal management is easier underwater than on hot sand. Soft robotics offers one route around sealed joints entirely, using compliant bodies actuated by pressure, dielectric elastomers, or magnetic fields, at the cost of lower force output.

Control and Autonomy

An amphibious controller must detect the medium and select a gait, either by direct sensing of water contact and hydrostatic pressure or by inferring the transition from motor current and inertial measurements. Central pattern generators, coupled nonlinear oscillators that produce rhythmic joint trajectories, are widely used because a single network can be retuned continuously between swimming and walking rather than switched discretely. Navigation is harder than in either medium alone, since satellite positioning is unavailable underwater and acoustic or inertial methods degrade in shallow, turbid, wave-driven water. Multimodal bio-inspired platforms that combine fish-like swimming with wheeled crawling show the sensing and gait-switching logic in a working system.

Applications

Amphibious robots have applications in a range of fields, including:

• Coastal and riverine environmental monitoring, including sediment and water quality sampling
• Search and rescue in flood zones and disaster sites where terrain and water alternate
• Inspection of harbor structures, dams, pipelines, and offshore platform legs
• Mine countermeasures, reconnaissance, and other defense tasks in the surf zone
• Marine biology field work in tidal flats, wetlands, and shallow reefs
• Amphibious exploration concepts for planetary bodies with liquid surface features