Swim-and-Breach at Palm Scale: A Rudder-Steered Two-Propeller Underwater Robot Platform with Differential-Thrust Pitch Control

Nature has long mastered the art of moving between worlds. Fish do not merely swim; they leap. To escape a predator, clear a waterfall, or snatch prey from the air, a fish must pitch its nose upward, accelerate with a sudden burst of power, and hold that steep angle until it breaks through the water's surface. This maneuver requires a delicate balance of forces: the ability to generate immense thrust in an instant, the precision to control its angle as it leaves the water, and the stability to keep its path straight against the chaotic turbulence of the surface. For decades, engineers have built underwater robots that can swim with impressive speed or mimic the graceful undulation of a fish, but few have successfully replicated this specific act of leaving the water. Most underwater machines are designed to stay submerged, and those that attempt to jump often lack the control to do so reliably, tumbling out of alignment or failing to clear the surface entirely.

A team of researchers at the University of Colorado Boulder has now built a tiny robot that solves this problem, creating a machine capable of swimming underwater, leaping into the air, and landing back in the water with the same fluidity as a fish. The device is palm-sized, measuring just 65 millimeters in length and weighing only 34 grams, yet it performs a complex sequence of actions that pushes the limits of what small underwater robots can do. By stacking two propellers vertically and adding a tail rudder, the team created a platform that can not only swim at speeds of 13.9 body lengths per second but also pitch its nose up to a vertical position in less than a second. When it launches, it clears an obstacle 1.6 body lengths high and travels 3.7 body lengths horizontally before splashing back down. In a natural stream with flowing water, the robot managed to leap even higher, reaching 2.8 body lengths above the surface, proving that its design works even in the messy, unpredictable conditions of the real world.

The secret to this robot's success lies in how it mimics the physics of a fish without copying its shape exactly. Instead of using a flapping tail, which is difficult to control for sudden bursts of speed, the researchers chose propellers. They mounted two small, high-speed motors one above the other inside a waterproof, 3D-printed body. By spinning the top propeller faster than the bottom one, or vice versa, the robot can tilt its nose up or down with great precision. This differential thrust acts like a lever, allowing the robot to change its angle rapidly. To keep the robot from spinning sideways as it moves, they added a small tail rudder between the two propellers. This rudder acts like the rudder on a boat, steering the robot left or right to keep it on a straight path. The body itself is shaped like a streamlined airfoil, a design borrowed from aviation that allows water to flow smoothly over its surface. Computer simulations showed that this shape reduces drag by five times compared to a simple box of the same size, allowing the robot to move faster and use less energy.

The researchers tested the robot in a laboratory water tank first, where they could measure every movement with high-speed cameras and sensors. They programmed the robot to follow a series of commands: swim straight, pitch its nose up to a specific angle, accelerate toward the surface, and then leap. The robot's onboard computer constantly checks its orientation using a sensor that measures rotation and gravity, adjusting the speed of the two propellers hundreds of times per second to stay on course. In these tests, the robot could hold a steady pitch angle even when the researchers tapped it to simulate a disturbance, correcting its position within a second. When the robot was ready to jump, it would pitch its nose up to 65 degrees, accelerate, and burst through the water. Without the rudder, the robot would often twist sideways as it left the water, losing its direction. With the rudder engaged, it stayed perfectly aligned, shooting straight up and forward.

The team then took the robot out of the lab and into Boulder Creek, a natural stream with moving water and turbulence. Here, the challenge was much greater. The flowing water tried to push the robot off course, and the surface was choppy. Despite these conditions, the robot successfully executed the jump. It swam against the current, pitched up, and launched itself out of the water, reaching a height of 2.8 body lengths. When the rudder was turned off during these outdoor tests, the robot failed to maintain its path, yawing wildly and landing far from its intended target. This confirmed that the rudder was essential for stability, not just in the calm of a tank but in the dynamic environment of a real river. The robot's ability to perform this entire sequence—swim, pitch, leap, and return—demonstrates a new level of agility for small underwater machines.

This work suggests that small robots could soon be used to inspect areas that are currently inaccessible to them. Many underwater systems, such as pipes, dams, and flooded structures, have barriers like weirs or dry gaps between pools that stop a robot from moving forward. A machine that can swim underwater and then leap over these obstacles could navigate through complex environments, inspecting infrastructure or monitoring water quality in places where it was previously impossible to go. The researchers note that their current prototype is still tethered to a power source by a thin cable, but the principles they have demonstrated provide a blueprint for future untethered versions. By refining the battery size and the propeller design, a fully autonomous version could one day hop from pool to pool, exploring the hidden corners of streams and industrial systems with the same ease as the fish that inspired it.