The forward speed of a swell, or wave group. In deep water, it is equal to 1. 5 times the swell period between successive waves in the wave group. The waves within the wave group move twice as fast as the overall wave group at 3 times the swell period. If a swell or wave group has a swell period of 20 seconds, the individual waves will be moving at 60 knots, while the group as a whole will be moving forward at 30 knots. As each wave moves forward within the wave group and reaches the front of the group, it will fall back to the rear and repeat the cycle.