Anchor Alarm Surprises

Step 1

The anchor is dropped. The red mark is where the anchor is located. The green mark is where the GPS is located. The GPS is 5 units back from the bow. The anchor alarm would show that you are 0 feet from the anchor which is correct. Let's say you set the anchor alarm for 10 units since you intend to put out 5 units of rode. This would make your alarm double your rode length which you'd think is overkill.

Step 1



Step 2

You let out 5 units of rode and set the anchor. Although the GPS is 10 units from the anchor, the anchor alarm will show that you are 5 units from the anchor because you are only 5 units from the original position that was recorded when the anchor alarm was enabled (the blue mark). No alarm will go off and everything is working well.

Step 2



Step 3

Here's where it gets interesting. You swing around and pull back the same 5 units from the anchor. This shouldn't trigger the alarm because you are only 5 units from the anchor. But the measurement is between the position where the GPS was when the anchor was dropped (blue dot) and the position where the GPS is now (green dot). If you count the squares, you'll find that the anchor alarm will show that you are 15 units away from the anchor. Your alarm goes off – you wake up in terror because you know that you way overset the alarm length to 10 with only 5 let out. And yet, you haven't dragged at all. The distance reported, 15, is the 5 units you really are back from the anchor plus 2 times the distance that the GPS is from the bow (2 x 5 = 10). The error is 2 times the distance between the GPS and bow.

Step 3
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