Wake on Ullswater

Whilst walking above Ullswater, I spotted this unusual wake from a boat. It's not an even v-shape but has a scalloped edge, certainly on the right hand side as seen above. I wondered if perhaps the boat has accelerated and thus whether the speed of the boat has an effect on the width of the wake. That must surely be the case as I think that's what results in the sonic boom of aeroplanes. The wind was blowing from left to right across the upper end of the lake. The lake gets shallower as it approaches the right hand side. I doubt it's a refraction effect because my map doesn't show it getting shallow very suddenly.