Wave frequency focusing has been used in two-dimensional (2D) laboratory wave tanks to simulate very large waves at sea, by producing large energy concentration at one point of space and time. Here, three-dimensional (3D) frequency/directonal energy focusing is simulated in a fully nonlinear wave model (Numerical Wave Tank; NWT), and shown to produce very large waves. This method alone, however, cannot explain why and how large waves occur in nature. Self-focusing, i.e., the slow growth of 3D disturbances in an initially regular wave train, is shown to also play a major role in the formation of "freak waves". Self-focusing is studied in a more efficient space-periodic nonlinear model, in which long term wave propagation can be simulated. The combination of directional/frequency focusing and self-focusing, and resulting characteristics of large waves produced, could be studied within the same NWT.
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