Anchor Dynamics and Anchor Watches — Not There Yet

Among the Crowd

Among the Crowd

Even when we aren’t traveling, as long as we are afloat, we are moving. Whether it is surging in a slip (as we are now) or wandering around a mooring or anchor, which we prefer, we are moving. When it’s at anchor, we can be moving a lot.

The picture below shows how much. And remember this picture was among as many as 30 other anchored boats. Some of these were charter boats that snuggled in too close, dropped some amount of rode, and went to the beach. The worst of these swung within 8 feet of us a couple dozen times in one morning. Then the wind switched, and they came back absolutely clueless of the danger they had created for us and the trawler anchored on the opposite side.

real anchor watch1

The tracery of black dots is the sum of all our positions while anchored in Treasure Cay’s basin. Note these are the positions of the AIS GPS antenna, not our bow and not our anchor. For this experiment, we set the warning ring very large to capture the entire picture.

real anchor watch2The range and bearing provided is between the current GPS position and the one when we anchored–the lower black arrow. The upper black and blue arrows point to the GPS position we recorded when we dropped the anchor. The larger translucent circle [graphic addition encompassing 1.1 acres] bounds all our maximum (taut rode) distance from the anchor. The inner circle our minimum (slack rode) distance, except…

real anchor watch3When the strong pre and post frontal winds abated, we entered a two day period where the wind circled the compass. As this period began, the center of our swing circle transitioned and we were no longer swinging to our anchor.

real anchor watch4

The circle became much smaller and it spiraled (as in wound around something on the bottom) into a set of smaller taut-slack ellipses. How the swing circle returned to the larger one we can’t be sure as there are no time stamps with these positions.

So what did we learn?


  • 121 ft taut circle (84 ft of chain in a catenary curve and 37 feet to GPS antenna)
  • 97 ft slack circle (97-37=60 ft of chain wandering across the bottom when the wind dropped.)


  • 80 ft taut circle (43 ft of chain in a catenary curve and 37 feet to GPS antenna)
  • 74 ft slack circle (74-37=37 ft of chain wandering across the bottom when the wind dropped.)
  • Creates the appearance we are anchored on ~40 feet of chain.

Net taut chain reduction due to suspected chain wrap 84 ft-43 ft =41 feet. If this wrapped in perfect circle, the circle dia. would have been 13 ft +. If it was wrapped around a square block, the block would have been 10 ft+ square.

TreasureCayThe bottom here has abandoned concrete mooring blocks roughly 4×4 ft. We believe we wrapped two of these and that’s why they we have elliptical swing behavior — these accounted for roughly N-S major axis. People with more experience here have now told us to anchor along the channel edge to avoid these blocks.

And no anchor watch would have told us what was going on! The hand held Drag Alarm based on anchor drop position would not have seen this, the boat moved toward the anchor position, not away. The AIS Swing  Alarm would not have seen this, the swing was reduced not increased nor even shifted relative to the larger swing behavior.

Why is this a big deal? It happened on days when we went ashore because the weather improved. It gave arriving boats a false picture of how our boat would swing when things returned to normal. [This was not the case with the bare boat that nearly hit us. We were at full radius. They were more than clueless but less than competent.]

None of the technologies employed worked.  What is needed is the ability to define a space — a fence — of any shape desired and to do so without having to buy yet another piece of electronics. (see below) We can do this roughly with our chart plotter by saving waypoints with overlapping proximity warning circles, but it is crude, and the chartplotter takes too many amps.

A Fence (smartphone apps need not apply)

One response to “Anchor Dynamics and Anchor Watches — Not There Yet

  1. Pingback: Anchor Watch Aha! | Periodically Peregrine

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