Stephen Salter’s Legacy: How to Build a Wave Tank That Doesn’t Lie to You

When Stephen Salter from University of Edinburgh set out to harness energy from ocean waves in the 1970s, he didn’t expect that one of his biggest challenges would come not from the sea itself, but from the tank built to simulate it.

He was developing devices to capture wave power, long snake-like machines stretching across the water surface. To test them, he built scaled-down models and placed them in a wave tank. But something went wrong: every time a wave hit the model and bounced back, it returned towards the wave-maker, distorting the entire experiment. In fact, when he replaced the model with a fully reflective wall, the waves amplified rapidly. The tank was interfering with its own waves.

Salter believed that a wave tank should behave like a scientific instrument, as precise as an electronic test bench. He wanted full control over wave size, frequency and direction, and he wanted to be able to repeat rare events whenever needed. He quickly realised that most existing tanks were not up to the task.

The main issue at the time was that traditional wavemakers used displacement as their control signal. They moved a flap or piston with a certain motion and hoped that the resulting wave would match. But it was never that simple. The wave’s final shape depended on many factors, including the hinge depth, reflections from the model, interactions between neighbouring wave-makers and even the aftermath of previously generated waves.

To solve this, Salter in 1981 proposed something different. He used force and velocity sensors to measure how much the water was pushing and how fast the system was responding. This allowed the wavemaker to adapt in real time and even absorb unwanted waves. Instead of forcing the water into a particular waveform, the system provided the correct amount of energy and let the water form the shape it naturally preferred.

His design used asymmetric wavemakers, which generated waves only in one direction and avoided creating disturbances behind the flap. These used less energy, were simpler to build and more efficient. The system was robust too. With stainless steel wires transmitting motion and force sensors immune to water chemistry, it required little maintenance. Even after months in the tank, the components kept working without needing to be recalibrated or cleaned.

Once he had control over force and energy, Salter turned to another challenge: directionality. In real seas, waves come from many angles at once. Inspired by optics and radar systems, he installed 80 wave-makers along one wall of the tank, each with independent control of phase and frequency. By carefully programming them, he could create sea states with complex angular spreads, including simulations of storms, swells and intersecting wave systems.

But perhaps the most fascinating feature of the system was its ability to generate freak waves on demand. Instead of waiting for a rare combination of natural conditions, the system could synchronise wave fronts to meet at the same point and create a massive, destructive wave. Salter called this the “malice aforethought” approach. It allowed engineers to test structures under extreme conditions without wasting time waiting for nature to cooperate.

There was another unresolved mystery that Salter wanted to tackle: wave crest length. While oceanographers had gathered excellent data on wave height, period and steepness, almost no one had measured how long a wave crest actually was. And this mattered. Salter’s energy devices were designed as long floating structures, and the forces from bending under a long crest could be massive.

To estimate crest length, he proposed a clever method using wave gauges aligned side by side. If the output of two gauges was highly correlated, it suggested they were under the same wave crest. If not, they were likely out of phase. By calculating correlation coefficients between sensors, his team could estimate the effective width of wave crests across the tank. It was not a perfect solution, but in 1981 it offered a practical starting point for something the field had ignored.

All of this came from someone who, in his own words, had no prior experience with waves and no preconceived ideas. Sometimes, that’s the best qualification of all. Salter’s work transformed wave tanks from simple tools into sophisticated laboratories capable of simulating the sea with astonishing precision. Today, thanks to that foundation, we can design safer ships, better offshore platforms and more efficient wave energy devices, not in the middle of a stormy ocean, but from the controlled calm of a laboratory.