Other PicoMicroYacht

Sunday, 29 July 2012

Navigating the PicoMicroYacht for techies

I had a very experienced support boat (Will and crew: http://www.fullthrottleboatcharters.com/), who could have done all the navigation, but I preferred to do it for myself. This was partly because I am used to navigating at sea, and also because I found the navigational aspect part of the challenge. The arrangement was that I followed my own route and Will monitored it very carefully to check how we were doing and focussed on us tactically avoiding the other ships. I had full confidence in Will and we worked well as a team. As a responsible support boat specialist, he would have taken over the navigation at any moment if he thought we were in danger and was discrete about allowing me to dictate the course.

The key to the crossing is dealing with the strong tidal flows and it is best to go at neaps. I went a bit before neaps to catch good weather. The main issue with crossing the English Channel is the east-west flow of the tide, or in the case of the Dover Strait, more northeast - southwest  (for short hand east or west below). If you are going slowly you have to let the tides push you one way and then the other, and not resist them or your eventual distance is increased very substantially. The art is to establish one direction to travel in using a compass and be able to stick to this, whilst the tidal streams cancel each other out. There are slightly more sophisticated approaches, where if you start at the right time from the right place, the tide will help you in total. In my case the tide ended up not hindering me very much.

I used a paper map, with traditional techniques involving a parallel rule and a brass divider. First I drew a line on the chart from Folkestone to two nautical miles east of the Cap Gris Nez. This latter position was on the assumption that the tide when I arrived would sweep me west past the Cap, so some leeway was needed. I then measured this distance (approximately 20 nautical miles) and assumed an average boat speed to take me there in seven hours. This was 3 knots, adding in a little for breaks. I then used the Channel East Tidal Flow Atlas to determine approximately the east or west tidal speed for every hour of the voyage, keeping in mind the approximate shift in position east or west on the atlas. These tidal speeds were translated into hourly distances and subtracted from each other to produce a net tidal drift, which turned out to be 6 nautical miles west. I then plotted out a new vector for the total crossing (115 degrees) allowing for this drift and made some extra adjustment for the drift caused by a force 3 to four wind coming from the north east. Then for each hour of the voyage I plotted onto my paper chart a vector using the direction and distance to be travelled, adding onto it each time the tidal vector for that hour. This produced a total course, a series of joined up waypoints plotted on the chart.

The result was a semicircular course, the tide sweeping me up Channel to the North Sea to start with and back down at the finish. The advantage of this course was also that I was crossing the shipping lanes almost at right angles, as required by law. In the chart above is shown the shipping lanes delineated by purple marking (in between two thin purple lines with a thicker line in between). The lane closest for England is for ships travelling up the Channel and the one closest to France is for down Channel travel. The area outside the delineation is for local ships. Further down the channel (bottom of chart) the lanes are separated from each other by a large amount (more purple). The chart shows how the shipping lanes narrow in the Dover Straight. This is the busiest international seaway in the world, with approximately 400 ships passing through every day.

I then read off the waypoint coordinates from the paper chart and fed these into my GPS, to have the electronic record. The GPS is a cheap but very effective black and white screen Lowrance H20, which has mapping for the UK and English Channel. I set up a route by stringing the waypoints together and then used a screen on the GPS that told how much I was deviating to the left or right of the route and how far it is to the next waypoint. It also told me the time left to reach each waypoint assuming the current speed.

The in boat navigational strategy was to use the 115 degree compass course, with some adjustment for wind drift, but also use GPS waypoints to check whether I was on track. Provided I kept approximately to my estimated 3 knot boat speed and I had calculated the tide approximately correctly, by going from waypoint to waypoint, I would maintain this course. Conversely, provided I stuck to the 115 degree course adjusted for some wind drift, I would go from waypoint to waypoint.

In practice this strategy worked out very well and I was able to keep this up until the last two waypoints, when the northeast wind had more effect and the wave height increased, slowing the boat and sweeping me down the Channel too much. I started to fear being swept past Cap Gris Nez. At this point the two mile leeway saved me and by rowing flat out for the last three miles I arrived almost exactly at Cap Gris Nez. The final course is shown in dashed lines, which I had to do without being able to see the French coast until one mile out. One operational error was to be fixated on the screen that provided the distance to the next waypoint, rather than looking at the map, making me think I was going to overshoot. When I flipped to map mode, this reminded me that I didn't need to achieve the last waypoint and had the additional leeway that I could use. I could see on the map I was heading directly for Cap Gris Nez. As someone interested in operator error, I realised this was a classic mistake of narrowing your focus too much when fatigued, in this case the process of rowing occupying me greatly.Will didn't know what was going in my head and he just thought I was rowing hard in the direction of Cap Gris Nez.

On reflection, I should have allowed more leeway, but because things went fairly smoothly, the two miles turned out to be enough. My course was almost exactly the fastest one to Cap Gris Nez, given the starting time and position, even though my judgement was slightly wrong.

To navigate I have a compass and the GPS mounted just behind the rowing stretcher, easily seen. The compass, the Silva 70 UN, is designed for small boats, but is sufficiently accurate and visible that it does for serious navigation. It was beautifully smooth and clear as PicoMicroYacht bounced around in the sea. I was looking at it facing the back of the ship, so had to recalculate my bearing by adding 180 degrees.
I also carry a spare GPS. The support boat had their GPS and other navigational equipment. There is an issue as to how much you should rely on GPS without paper charts. I don’t carry paper charts on PicoMicroYacht, which would not be very practical, but if I am doing sea rowing I normally keep well inshore and only go out in good weather, having researched the coastline thoroughly beforehand.

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