This article is from the Dolphin
FAQ, by Jaap van der Toorn
The dolphin's fast cruising speed (a travelling speed they can maintain for quite a while) is about 3-3.5 m/s (6-7 knots, 11-12.5 km/hr). They can reach speeds of up to 4.6 m/s (9.3 knots, 16.5 km/hr) while travelling in this fashion. When they move faster, they will start jumping clear of the water (porpoising). They are actually saving energy by jumping. When chased by a speedboat, dolphins have been clocked at speeds of 7.3 m/s (14.6 knots, 26.3 km/hr), which they maintained for about 1500 meters, leaping constantly.
Energetic studies have shown, that the most efficient travelling speed for dolphins is between 1.67 and 2.27 m/s (3.3-4.5 knots, 6.0-8.2 km/hr).
There have been reports of dolphins travelling at much higher speeds, but these refer to dolphins being pushed along by the bow wave of a speeding boat. They were getting a free ride (their speed relative to the surrounding water was low). A recent study using based on the vertical speed during jumps showed maximum speeds for bottlenose dolphins of 8.2-11.2 m/s (16-22 knots, 29.5-40.3 km/hr) prior to a high jump. The maximum speed for wild bottlenose dolphins was 5.7 m/s (11 knots, 20.5 km/hr) and for common dolphins 6.7 m/s (13 knots, 24.1 km/hr).
D. Au & D. Weihs (1980) At high speeds dolphins save energy by leaping. Nature 284(5756): 548-550
J.J.Rohr, F.E.Fish and J.W. Gilpatrick, Jr. (2002) Maximum swim speeds of captive and free-rangings delphinids: critical analysis of extraordinary performance Marine Mammal Science 18(1):1-19
T.M.Williams, W.A.Friedl, J.A. Haun & N.K.Chun (1993) Balancing power and speed in bottlenose dolphins (Tursiops truncatus) in: I.L. Boyd (ed.): Marine Mammals - Advances in behavioural and population biology, pp. 383-394. Symposia of the Zoological Society of London No. 66. Clarendon Press, Oxford