Since reading “The Celtic Ring” 10 years ago I have always wondered what it would be like to sail through a really strong overfall1. Now I know. Before embarking on the rather short leg from Lymington to St Peter I did an exemplary thorough route planning, taking into account the tidal state both in the start and ending port, currents along the way, and the forecasted winds. The marina in St Peter Port has a sill in its entrance, so access is only possible in a 4 hour window around high tide.
We evaluated 3 possible crossing times, having different aspects of “Too much adverse current in the Solent”, “Too much waiting at the arrival”, etc. The chosen time (crossing at night, starting 4 hours after high tide Plymouth and arriving in the morning around high tide Jersey) did have side currents for the first half of the channel crossing.
Everything went exactly as planned, except that I was looking at average levels for the tidal currents. Turns out tonight must have been an extreme. My tidal atlas has the note “17,35” for the earlier part of the crossing. This means that currents should be 1.7 knots at neap and 3.5 knots at spring. We got almost 4.5 knots, with almost 4 hours above 4 knots.
Having 4.5 knots hitting you in the side as you are beating upwards in 7 knots is a very strange experience. It turns what would have been a comfortable closed reach into an impossibility, throwing the course over ground more than 40° off from your heading. The usual super-stability of the Journeyman doesn’t work any more. Steering is more like a guesswork, with the boat kind of jumping between different wind angles without any logic.
And then, as we passed west of Alderney (in the middle of the night, sorry, no pictures :-( ) all of a sudden it looked like we were about to drive onto a rocky shore. Large breakers in front. Heart beating I quickly checked the navigation and depth. Everything was perfectly OK. Except we were about to pass through a field of 3 meter breaking overfall waves created by the currents. We do have the overall current in our backs, but in the overfalls the turbulence is really really strong. Boat speed drops from 8 to 4 knots because of the sheer barriers of water we have to plough through. It’s a bit hard to keep the boat on course, but we are really more going with the flow than actually driving somewhere. And the flow is in the right direction so it takes us through without any problems. Fun experience!
 From the dictionary: Overfall: A turbulent stretch of water caused by marine currents over an underwater ridge. As far as I know there is no word for this in Swedish since the phenomenon does not exist there.