T&T: Anchoring and coll regs

[Jeffrey Siegel]

>  A feature I wish chartplotters had was an anchor alarm 
> position setting feature, whereby I could say the anchor was 
> "x" feet in front of the boat.

There are other problems with existing anchor alarms too.  I have never seen
one that was done correctly.  That's why I've been writing my own for the
last 2 years.

Most alarms don't allow you to set the position of where you originally
dropped the anchor.  But even worse, there isn't an alarm that takes into
consideration that your GPS isn't on the bow - it's further back.  On my
boat, my GPS is 30 feet back from the bow.  If you don't take heading data
into consideration, the position error is the GPS error (9-30 feet) plus two
times the distance between the GPS and your bow.  This means that a 45'
sailboat with a GPS on the stern is starting out with a 90 foot error before
they've even considered swing.  Anchoring in 10 feet of water and letting
out 7:1 chain will mean that the error of the GPS is larger than the error
of swinging.  This is why we all lengthen the anchor alarm distance when it
goes off prematurely - it always does when stressed with wind (just the time
when you really need to know if you're dragging).  It usually happens when
you swing to the opposite side of where you were when you set the anchor as
the GPS location error really starts to pile up.

Even if you have heading data, you must know the position of the GPS
relative to the bow or else you have a 1x distance error between the bow and
GPS.  This is all quite easy to show on paper.

Having heading data - not COG but real heading - isn't impossible to
integrate into a PC or cell phone.  I do that with a Bluetooth multiplexer
to my Raymarine autopilot.  Combining that with position and software that
understands the actual position of the GPS relative to the bow allows you to
eliminate all error except the GPS error.  I've found that to be pretty good
- to within a foot when averaging and differential is used.

Putting this all together gives you a pretty good idea about whether your
anchor has moved more than 6" or so.  I've tested it with engines engaged in
reverse to get a maximum backwards position.  Then idle the engines and let
out 1 more foot of chain.  Put the engines in reverse again and the alarm
goes off.

The thing I can't figure out yet is how to know if your neighbor's anchor is
dragging.  This is often a much larger problem...

==================================
Jeffrey Siegel
M/V aCappella
DeFever 53PH
W1ACA/WDB4350
Castine, Maine

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