Monday, January 16, 2006

Favorite Gadget # 6 - GPS Alarm

There always comes a time when cruising sailors find themselves in somewhat dicey anchorages.  The cause could be poor holding (rocky or oozy bottoms come to mind), high winds, or nearby dangers (rocks, reefs, coral heads, etc.).


When these situations occur it can be hard to get a good night’s sleep.  In the past (1990-1993 cruise) we tried using the anchor alarm on our depth sounder, which would go off if the water depth changed by a preset amount.  But that only worked if we were anchored on a sloping bottom where the depth changed rapidly.  In an anchorage with a flat bottom but with dangers nearby, it wasn’t much of a help.  The only other solution was to set an “anchor watch” which meant that one of us either got up periodically to check our position or stayed up for a period of time and alternated watches with the other crew person.  Neither of these options was very popular aboard Kavenga, although we did it when we had to.


Now, in the age of sophisticated GPS units (Global Positioning Systems), relief has arrived.  Once we arrive at a new anchorage, we leave our GPS on and after a half hour or so the boat has settled into its initial anchored position.  Using the GPS alarm feature, we can set it to go off at increments of 60 feet.  In other words, at the lowest setting, if the boat moves 60 feet from where we initially anchored, the GPS beeps to inform us of that fact.  Sometimes, conditions are such that the winds are variable and the boat is naturally shifting around.  We typically have out anywhere from 100 to 200 feet of anchor chain.  So our “swinging circle” can be as much as 200 to 400 feet in diameter respectively.  If the boat is moving around a lot, but not dragging its anchor, we might set the alarm for 120 feet or even 180 feet if there are no reefs or rocks astern of us.


In addition to the alarm, the tracking feature of the GPS tells us what’s going on.  As the boat moves, it leaves electronic “bread crumbs” in its path, little blips on the screen.  As the boat naturally swings from side to side and pulls on the anchor, it eventually leaves an arc of bread crumbs, which we have dubbed the “smiley face”.  This arc is in fact a electronic image of part of our swinging circle.  As long as Kavenga stays somewhere on that arc, we know we aren’t dragging anchor, we are just moving around the anchor.  However, if the bread crumb trail starts to resemble the pattern of a falling maple leaf that gets farther and farther from its initial arc, we know something is going on.  This happened to us recently in an anchorage called El Cardenal on Isla Partida, north of La Paz.  We were anchored close to the rocky, high cliff shore.  We moved to the center of the cove and re-anchored with the wind still blowing 25 to 30 knots and had no problems thereafter.  We subsequently reasoned that the bottom closer to the cliffs and rocky shore, probably had less sand over the rocks that had fallen from the cliffs over the years, making for an irregular and less penetrable bottom.  The GPS bread crumb pattern alerted us to what was going on even before the alarm when off.


So, score another one for modern technology.



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At March 25, 2006 at 5:34 PM, Blogger jon said...

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