Interesting article from ActiveCaptain this morning.
Personally I never set an anchor alarm, and after reading this probably even less likely to!
>>> Anchor Alarms >>>
One of the most controversial newsletters we ever released happened
over 3 years ago - it was about the mathematics of anchor alarms. It was
so popular that we're running it again because there are so many new
ActiveCaptain members who missed it.
It should be simple. Pick the spot to anchor; come to a stop; drop the
anchor and set the anchor alarm. Then pull back until the anchor sets.
Now if you pull away further from the anchor set point than the distance
you specified, alarms should go off, right?
Well, not exactly. The mathematics are surprisingly a lot more complex.
We know. It seems easy and obvious. We've been involved in many debates
until the pencil and paper come out and then, "oh yeah" is heard.
Here's the missing magical point. You've got to notice that the point
where the anchor position is set in the alarm is the position of the GPS
and not the position of the bow/anchor. That one small point ends up
bringing a whole bunch of trigonometry into the calculation. When the
boat swings 180 degrees, the error created by that offset equals twice
the distance from the bow to the GPS.
Let's take an example for a typical 42' sailboat with a GPS on the stern
rail. This is the worst case problem but is very typical and
demonstrates what happens very well.
So we're anchoring in 10' of water with a bow that's 5' off the water's
surface. A good scope for a night without much weather expected would be
5:1. This means 75' of rode will be let out and pulled back to set hard
(we call that power setting). The anchor alarm is set at 125', way more
than the 75 put out. And since we power set the anchor, we couldn't
possibly move 50', right?
At 3 am, because these things always happen at 3 am, the anchor alarm
goes off. You're 127' back. You remember that you way over added to the
75' and start planning what you're going to do in the total black of
night with the moderate wind that's now blowing.
What really happened is that the tide changed at 1 am. During the next 2
hours you slowly swung around moving back. Not realizing this new math
for anchor alarms you didn't realize that the GPS displacement caused 84'
of position error in the anchor alarm. Your alarm went off after moving
back only 52'. In reality, your anchor alarm should watch you move back
another 32' without your anchor moving 1 inch on the sea floor. The
anchor alarm should have probably been set at about 75 + 84 + 10 + 10 =
179 feet. The two 10's are for GPS accuracy error and slop since the
anchor doesn't set immediately. Can you imagine setting an anchor alarm
at almost 200' with only 75' of rode out? And yet, that's the right
We haven't found an anchor alarm that compensates for this GPS
positional error. It's one of the reasons we wrote DragQueen (available
for free in the Apple app store and Google Play). Since the anchor alarm
is on a phone, the GPS position is the phone itself. When deploying the
anchor, we stand with the iPhone at the bow to eliminate one half the
GPS position error. There's still another position error based on where
the GPS is located while we sleep at night (25' back in our stateroom).
Remember too that this positional error happens at all angles. Swing
about 90 degrees to the side and the error is about 1 times the GPS
displacement distance. Even that can be significant.
Given a heading/fluxgate sensor and a few configuration settings, 100%
of this GPS positional error could be eliminated. How come not a single
marine electronics manufacturer has done it?
If you're still saying, "wait a second - there's not a 2x error in the
position" - check out this graphic proof of what happens. We'll wait to
hear the "oh yeah":
One recent note: Since running this original newsletter item, we found
that Vesper now has this swing calculation built into their anchor alarm
on their products.