When it comes to catching prey or escaping predators, top speed can be vital. The study of maximal speed swimming in fish, dolphins and whales is fraught with difficulties: there are no equivalents to racing greyhounds and horses, which regularly and predictably achieve near-maximal speeds. It is also often unclear whether free-swimming animals are cheating, using bow waves from the observation vessels. It is therefore brave for a study to approach the physical limits of swimming speed, and exciting when its conclusions are moderately robust and relatively straightforward.
Gil Iosilevskii and Danny Weihs re-visit fish and cetacean swimming, using a simple approach that models a tail's movement as a waggling wing. Using best-guess empirical observations, and the hydrodynamic theory of lift, drag and stall, Iosilevskii and Weihs explore the limits to swimming speeds for animals such as dolphins, tunas and mackerel sharks. They also consider cavitation, which presents a limit to speed rarely considered in the biological literature. When a fluid experiences a pressure below the vapour pressure, it cavitates, forming a pocket of gas within the water. This phenomenon is undesirable: if the pocket collapses, there is sufficient energy to cause damage; if the region of gas is maintained beyond the fin,performance crashes.
The authors consider three potential limiting factors to speed. The first is simply down to the power requirements, which change according to scale: the drag-producing surfaces scale approximately with the length of the swimmer squared, but the muscle volume – and hence the muscle power –scales with length cubed. Thus, larger fish are less speed-limited by power because of their greater muscle bulk. Precise predictions are more difficult because of the uncertainty of the value of mass-specific power: the authors used values ranging from 10 to 160 W kg–1, noting that this value will be species and conditions dependent.
The second and third potential limiting factors to swim speed are both based on stall. Stall for swimming fish occurs when the flow over the low-pressure surface separates and is associated with an increase in drag(bad) and decrease in thrust (very bad). Typical stall occurs when the fluid arrives at the aerofoil at too high an angle, causing a sudden reduction in thrust. For a given maximum tail beat frequency, there is a maximum body drag that can be overcome and a maximum swimming speed before the required angle of attack becomes too great and stall is inevitable. The precise speed limit relies on another difficult-to-measure physiological parameter: tail beat frequency. If a swimmer could increase this frequency thenhigher speeds should be achievable before this form of stall occurs.
Unlike the previous two limiting factors, the third, cavitation, cannot be avoided by higher powers or frequencies. Top swimming speed before cavitation occurs when the tail is waggling neither too quickly nor too slowly. By making reasonable assumptions about a fish's hydrodynamic properties, the authors conclude that cavitation provides a real speed limit to larger swimmers near to the water surface. Further below the surface, cavitation is less of a problem as it takes lower pressures to produce the vapour pockets. Smaller fish, meanwhile, are more likely to be speed-constrained by power.
While marine propeller design has been influenced by issues of cavitation for many years, it is exciting to think that fish and cetaceans may also have been developing under the selective pressures of not only power and efficiency but also cavitation avoidance. With this insight, further relationships between behaviour, fin design, physiology and hydrodynamics are ripe for discovery.
