Anchor Alarm: Computing the Swing Circle

[koliver]

I note that the OP is in Florida, anchors in 10' of water, has little tidal range and a boat that is longer than the fully deployed rode, even at 5 to 1 scope. Those parameters are foreign to west coast conditions, where having a rode of less than a boat length will never happen. Here, water depth is usually more than a boat length.

 

[Portage_Bay]

I note that the OP is in Florida, anchors in 10' of water, has little tidal range and a boat that is longer than the fully deployed rode, even at 5 to 1 scope. Those parameters are foreign to west coast conditions, where having a rode of less than a boat length will never happen. Here, water depth is usually more than a boat length.

Yes you are correct. I did note in my first post on this thread, #19, that I boat in a different area and conditions are different. I offered my technique with the hope it may be useful to OP and others.

 

[Portage_Bay]

Your graphic indicates that the GPS antenna is directly over the anchor roller and that the anchor sets immediately with zero dragging to bury. There are anchor alarm apps that allow you to move that point but I’ve yet to see a boat with a GPS antenna on the pointy end of the bow pulpit.

Correct. I do not allow for the distance from anchor roller to GPS antenna. Your profile does not say where you boat. In the PNW where we mostly anchor in deep water the short distance from roller to GPS antenna, in my case approx 9 ft, is of little concern. Tidal range would be of more concern. However I have found this technique to be nearly fool proof.

 

[psneeld]

Some are arguing minutiae when exact anchoring point or length of scope or depth where the anchor actually sets or actual GPS error, etc, etc..... are hardly exact in real life.

If you guys are cutting things THAT close, glad I am out of big boating for now at least.

The principles some are using are sound as long as you aren't counting errors in feet.

This isn't strictly a math problem, it's a captain's intuition problem involving many variables. For both coasts....well..... anywhere you anchor. Can't forget our world wide cruising friends...

 

[markpj23]

I think this subject has been picked over numerous times and with numerous articles. Here’s but one I vaguely recall. https://www.boatus.com/expert-advice/expert-advice-archive/2014/may/the-ultimate-anchor-watch Figure 2 gives a good visual of what’s going on and describes what some here have said.

Thanks for this. Great illustration of the error I made: swing circle not large enough for the scope and size of boat.
Looking forward to more restful nights on the hook..

 

[catalinajack]

Your graphic indicates that the GPS antenna is directly over the anchor roller and that the anchor sets immediately with zero dragging to bury. There are anchor alarm apps that allow you to move that point but I’ve yet to see a boat with a GPS antenna on the pointy end of the bow pulpit.

Quite correct about the GPS location but, as I mentioned earlier, the Watchmate app that is companion to the Vesper 8000 AIS transponder takes into account the distance from the Vesper dedicated GPS and the bow. Its in the setup of the Vesper.