Based on the trials to date, it looks like Chris may manage speeds up to 1.5 kts through the water, maybe a little bit more if we get the hoped for tailwind.

At that speed his passage will be massively influenced by the tide-induced currents. In the Irish Sea the currents predominantly head North on the flood tide (tide coming in) and South on the ebb tide. For the peak (spring) tides, these currents can be as much as 4 kts, especially where the water is ‘squeezed’ as it flows over the shallower areas near the Irish side.

Chris will experience a full tide cycle – slack water to peak Northerly currents back to slack water then to peak Southerly flow – roughly once every 12 hours. Throughout these cycles he will attempt to maintain a constant heading (broadly West), by following a fixed compass bearing. This maximises his progress Westwards, but it does mean that his actual track over the seabed will follow a sine wave type pattern as the currents drift him alternately North and South. In the support boat we will monitor his compass following performance, and offer guidance where necessary.

One key part of the plan is to launch into a Southerly current, to help maximise our separation from the main ferry routes.

Predictions have been made for the amplitude (‘height’) of the sine wave, based on the sea-currents data contained in the relevant Admiralty Tidal Stream Atlas. For such a long crossing (both time and distance) this is a major number crunching exercise, so the data was extracted into a spreadsheet, and the process automated as far as our skills allowed. (If only tides would follow a decimal pattern!).

Our latest predictions say that Chris can expect total North to South movements of at least 8 miles for neap tides, and 12 miles for spring tides.

In terms of predicting where Chris will be at any fixed time, the main additional factors are the actual speed he achieves and the wind effects. For example, an average speed difference of only 0.2 kts could see Chris missing a target aim point on the Irish side by the full 8 to 12 miles. And given the physical efforts involved, one luxury we really don’t have is adding more distance to the challenge by making gross course changes.

So we need a means of live prediction (and minor correction) as the passage develops. In the best traditions of keeping it simple, the plan is to plot Chris’s progress on a large scale chart, and to continually extrapolate this real track data to generate the future predictions. These extrapolations will be used to help finesse the target heading for Chris to steer; we expect to start introducing small changes from the halfway-point onwards.

We do have a target harbour in mind – more on that later – but it’s definitely been chosen because there are safe landing areas (beaches) a long way on either side!

Neil R – support boat skipper.