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Using a GPS to assist with piloting tasks

Jan 1, 2003

Now that SA is turned off, there are several piloting functions a simple non-chartplotting hand-held GPS can assist with that are worthy of comment.

1. Non-SA GPS is accurate enough to calibrate knotmeters. Set your speed filtering function (if available) to about five seconds and run steady for at least 15 seconds. Do this in calm, still water (no current/wind) or once up-current and once down. Add 1/2 the difference between speeds to the speed recorded going up-current. Do this at 1/3, 2/3, and full speeds to check your knotmeter for linearity.

2. While we still prefer using a sun compass, GPS is now accurate enough to assist with determining compass deviation. In calm/still water, motor on a series of compass courses in 30° (or less) steps. Do this with your track filter (if available) set for about three seconds and your GPS set for local magnetic variation. Hold a steady course for 15 seconds to ensure proper track display. Note the difference between compass course and GPS track (track = course in this case because of 0 current and 0 wind). The difference is deviation.

3. We use the routes function to store danger bearings and keep-out zones. We create waypoints that mark the ends of danger lines of bearing or corners of areas we don't want to get into (or the limits of areas that are safe to pass). We then store them as named routes. When we approach these lines or areas we activate the requisite route. The route course lines are displayed and we can pilot more effectively around or away from them. This is a nice capability where low, featureless coasts offer few opportunities for bearings or where a slightly off DR can generate fines of astronomical proportions (Florida reefs!).

4. We also create virtual tide boards and tide gauges. In an area a safe distance from a tidally restricted feature (bridge or sand bar) with a generally flat or slowly sloping bottom, we set up a new waypoint and record the depth at that waypoint just after having successfully cleared (up or down) the obstruction. This waypoint is our early warning mark for whether it is safe to proceed or not. This has been impressively useful in areas where height of tide is substantially affected by barometric pressure and wind, such as our Chesapeake.

5. Entering a new waypoint (just using the default number) each time you come head to wind when tacking is a good way to assist with plotting and logging a windward track (and it may be legally useful too). We also find it useful in assessing windward sailing performance after the fact.

6. In a similar vein, when sailing to weather, entering a waypoint for the destination and hitting the GO TO button will give you the shortest (usually unachievable) linear distance to the mark. As the beat proceeds, watch the distance to mark (we set ours to display hundredths of a mile). As it reaches a minimum and starts to increase, it's time to tack based on track made good. This is a nice help when beating in a current (where ignorance of set and drift would lead to early tacking). Remember, however, that hydrographic reality (shallow water) may intervene before the GPS indicates it is time to tack.

7. Finally, GPS units all fail eventually, and keeping an independent paper plot is proper practice. To assist with this we have created a set of virtual reference marks (using waypoints) at specific 00 minute lat/long intersections, and we plot range and bearing from each on a regular basis.

None of these constitute a navigational epiphany, but since enough of our sailing friends have responded "Wow!" (especially after watching us cross a bar most consider unpassable to keel boats), we thought we'd share.