T&T: anchor watch

[Hal Anderson]

Can't wait!

Hal
 

-----Original Message-----
From: Jeffrey Siegel [mailto:jeff at activecaptain.com] 
Sent: Monday, April 27, 2009 9:32 AM
To: Jake2124 at aol.com
Cc: trawlers-and-trawlering at lists.samurai.com
Subject: Re: T&T: anchor watch

> We have not been happy with using an anchor alarm. Setting it far   
> enough
> so it doesn't go off all time gives me little comfort.

This is one of my most favorite topics.  I've been working on various anchor
watch utilities for 6 years now.  The math is more involved than you'd
imagine.  The application needs to be used while approaching and deploying
the anchor in order to get very accurate results.  This adds to some
user-interface issues.

The basic problem is that no one has their GPS mounted at their bow.   
It's usually back some 15+ feet.  Sailboats often mount their GPS on the
stern rail putting the GPS as much as 45 feet away from the bow.

With every anchor alarm available today, the error caused by the GPS offset
is 2 times the distance between the bow and the GPS.  On my boat, the GPS is
20 feet back on the arch which means my anchor alarm is already off by 40
feet before any other type of error is introduced.

In addition, no anchor alarm today takes the depth of the water into
consideration and most handheld GPS solutions (phones, laptops, etc) don't
provide heading data.  Course over ground (COG) is different and almost
meaningless when moving very slowly around an anchor.  Getting a proper
orientation is critical to knowing where the anchor is actually located.
The existing anchor alarms are all just dumb circular alarms.  They force
you to create much larger circular areas than needed - or the alarm goes off
too often.  Everyone who uses an anchor alarm has experienced this.

My test software has gotten the full error down to about 9 feet.  If the
boat moves more than 9 feet, it means the anchor is moving and it lets you
know immediately.  I'm really hoping that when I'm back on my boat for an
extended period, I can create some versions of this for people to try.


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

www.activecaptain.com
The Interactive Cruising Guidebook









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