Catherine Brahic reports on some interesting new research in an online article at New Scientist:
“”There are certain limits on swimming speed that are imposed irrespective of power,” explains Iosilevskii. One of these is the frequency at which the swimmers can beat their tails to propel themselves forward.
The other is the formation of microscopic bubbles around the tail, a phenomenon known as “cavitation”. According to Iosilevskii and Weihs, for animals such as dolphins that have nerve endings in their tails, cavitation can be the most important limiting factor.
The bubbles form as a result of the pressure difference created by the movement of the fins. This process is what produces the ribbons of tiny bubbles that stream behind a ship’s propeller (see image).
When the bubbles collapse, they produce a shockwave, which eats away the metal in propellers. To dolphins, it is painful. According to the researchers’ calculations, within the top few metres of the water column, this happens when the dolphins reach 10 to 15 metres per second (36 to 54 kilometres per hour).”
Reports of dolphins overtaking boats can be apparently be explained by dolphin’s “cheating” when they surf the bow waves.