TidePlan is a passage planning tool for sailors crossing the English Channel. It is intended to provide the best "course to steer" from a departure point to a destination at a given date, time and speed.
The program uses a model of the tidal flows occuring in the English Channel. It relates this model to the magnitude and state of the tide at intervals during the planned passage and calculates the tidal vector that is likely to affect the vessel at each interval. It integrates this with the vessel's speed and heading for the same interval and re-calculates the vessel's course accordingly.
It repeats the process above and adjusts the heading at each iteration until a heading is found that takes the vessel to within approximately 100 yards of the destination position.
It uses this last iteration to calculate the waypoints and ETA.. It then displays the ground track on the minichart.
The program is tested repeatedly each year and the results compared with those obtained by careful manual plotting.
Departure and destination points may be entered manually by clicking on the minichart, or by selecting from a list of user defined waypoints. You can also use your current position as the departure point by reading it from your GPS receiver, if this is connected to a serial port on your PC. There is a facility for importing the departure point and/or destination point from the shareware OziExplorer chart program. Departure and destination points can also be imported from Maptech's Offshore Navigator and Chart Navigator Pro navigation software. There is a function to swap the departure and destination points to reverse a route.
Once a route has been calculated there is a facility for optimisation of the start time. This will find the start time, within 6 hours of the initial start time, that produces the fastest passage. The route is then recalculated using this start time.
The list of waypoints produced by the program can be viewed, printed, saved and exported to the OziExplorer program or to your GPS receiver. They can also be exported in formats suitable for Maptech Chart Navigator Pro and Offshore Navigator, Euronav seaPro and SeaClear II plotter programs as well as the G7ToWin and Waypoint+ GPS management programs.
OK, so that's somewhat exaggerated, but I suggest that most of us are likely to cut a few corners doing the chartwork for a trip we're familiar with.
In fact, because of the difference in relative tidal flows across the English Channel, the assumption that one will be swept equal and opposite distances during each ebb and flow is very unreliable. Add to this the improbability of our estimated log speed being such as to make our passage last a whole number of tides and we see that some planning is certainly desirable.
Of course, in these days of sophisticated electronic navigation systems one could say, "So what? We always know where we are so we can just head down the rhumbline and let the tide do what it wants...". Perhaps I exaggerate again. I don't suppose anyone really navigates like this but I'm sure we're all aware that GPS can give us a warm, fuzzy feeling that may not always be justified. It's easy when steering a rhumbline course to end up with wind and tide conspiring to prevent us reaching our destination. It's also inefficient and will mean a later arrival time.
My intention in developing this program is to create a planning tool that will give an accurate "course to steer" (or, as we used to say before the electronic age - a "heading") that will take us to our destination in the most efficient manner. I also wanted to provide a list of waypoints so that GPS could be used as a regular check that the vessel is following the ground track predicted by the program.
The end result is much the same as we would get using the traditional methods with chart, dividers and parallel rule or plotter. There are, however, some subtle differences. The traditional method first assumes a straight line course and then uses the tidal vectors along that line, at intervals decided by the predicted speed of the vessel, to produce a cumulative offset. The offset is then applied to the departure point. The bearing from the offset departure point to a point projected the appropriate distance along the rhumb line is then taken as the "course to steer". There are two minor inaccuracies inherent in this method: the predicted speed of the vessel is not the speed over the ground and the tidal offsets are taken from positions along the rhumbline rather than where the vessel will actually be at the time.
Admittedly errors due to these two factors are likely to be quite small but the method used by TidePlan avoids both these problems.
TidePlan works by initially steering the bearing from departure to destination. It plots the vessel's ground track by continually recalculating the tidal vectors as the vessel's position changes. It checks how far the vessel misses the destination by. It then uses this to generate a new heading aimed to reduce this "miss distance". This process is repeated over many iterations until a heading is found that 'nails' the destination. This heading is then used in a final iteration during which the waypoints are generated and the ground track is plotted on the chart.
The result should be at least as accurate as that obtained by very careful chartwork. It is certainly much faster and is unlikely to cause headache or eyestrain!
I hope you have a great sailing season this year.