Variability in the properties of wakes generated by high-speed ferries

The largest impact on the coast of waves generated by high-speed ferries is usually caused by the highest and longest components of transcritical wakes. Building reliable statistics of these parts of vessel-generated waves is usually nontrivial because of the high variability in the properties of wakes and their infrequent occurrence. Tallinn Bay is one of the few places where high-speed ferries frequently operate at transcritical speeds close to the shoreline. We report the results of measurements performed in 2008 at the entrance of Tallinn Bay. The time series of water surface elevations were collected in 2.5-3 in water depth, similar to 100 m offshore from an almost non-reflecting beach, similar to 2700 m from the sailing line. The data from 418 wakes on 15 days allows the construction of distribution functions of different wake properties (maximum height, wake energy,and energy flux) with an acceptable accuracy. The periods of the highest waves vary insignificantly and are closely related to the cruise speed of the vessels. An appropriate measure of the properties and variability of wakes is the maximum wave height. Wakes from `classic' high-speed ships are very variable. Wakes from large, basically conventional, but strongly powered ferries show quite limited variability, thus, both the average and extreme wake properties of such ships can be more easily adjusted by changing their sailing regime.