587Re: Tacking with a privateer, self-steering, and various other technicalities
- Sep 11, 2012--- In privateer20@..., "Oliver" <oliver_shaw@...> wrote:
>Another Tacking Angle track loaded, and a reminder that we are all of us welcome to add our own tracks.
> Aart has also posted a number of other photos of technical interest, including one of a sheet to tiller self steering system, to which he has also added a commentary.
> These are well worth a look.
> Even after a lifetime of sailing, and a quarter of a century teaching sailing and seamanship, I now find myself still learning from some of this material. I commend them to the rest of the membership.
One problem however with adding our own tracks is that I suspect that comparatively few of us sail in waters that are free of tidal streams, and the latter complicate the situation. Aart is probably in a very small minority in sailing a Privateer on inland (non-tidal) waters.
In order to do an accurate correction to the raw GPS data you need accurate data on the tidal stream at that location and time, and while approximate data is comparatively easily deduced - either from the combination of tide lozenges or tidal stream atlas with the relevant tide tables or alternatively from prediction software - this is at best only a prediction, and is usually a combination of a prediction and an interpolation. The actual streams can vary from the predictions, under the influence of a number of well known factors, and interpolation from one point on the chart to another is at best a matter of informed guesswork.
Obtaining data on the actual tidal stream at a given location and time by direct measurement is often not a realistic option.
Hence when producing Tacking Angle tracks in tidal waters it is very difficult to properly correct for the tidal stream, and without that correction the track is potentially very misleading.
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