The Ultimate Anchor WatchBy Gaz Haring
Published: October/November 2013
With today's electronics, you can tell if your anchor starts dragging without leaving your bunk. But make sure you understand your system's limitations .
It's a pitch-black, nasty night with rain blowing sideways, and your concern about your anchor dragging sends you topside again to shoot bearings through lightning on distant landmarks. You escape below only to have your pencil tear a hole in the soaking-wet chart while triangulating the boat's position. Thankfully the days of wooden boats and iron men are behind us. Today, with just a few keystrokes, you can have an electronic box stand the middle watch, allowing a few hours of shut-eye. The challenge lies in understanding the limitations of the different anchor watch systems and choosing which to use.
Almost all anchor watch systems rely on the global positioning system (GPS), a revolutionary aid to navigation that is nonetheless prone to errors that must be compensated for when used to monitor the boat's position. Anchor watch systems work by creating a security zone around the boat, which is the electronic circle the boat is allowed to wander around in without tripping an alarm. To prevent false alarms, that circle must be large enough so the normal movement of the boat, and the inaccuracies in setting up the circle, don't cause the anchor alarm to go off. Remember, the GPS tracks the antenna, not the anchor, and the electronic center for all alarm calculations will be at the set point, the position the antenna was in when the system was activated. To prevent false alarms, the security zone must then take into account the following factors:
- Offset error. The difference between the anchor's resting place and the GPS antenna at the instant you enabled the alarm. Note that when the boat swings 180 degrees, offset error doubles (Figure 1), so you must include twice the offset in your calculation of the security zone.
- Signal errors. The errors inherent in the GPS system itself in generating a position. All GPS systems use the same satellites so it doesn't matter if it is a plotter, handheld, or smartphone; the signal errors will be the same on every device. The signal error varies depending upon the type of receiver you have and whether you're using vertical measurements, but is about 16 feet on average, according to government sources.
- Human error. There are a number of ways the operator can introduce errors, but most common is mistiming the activation of the set point, as happens when the button is pushed to activate the set point and the anchor then drags some distance from that point before holding.
- Scope. The security zone needs to take into account the boat's ability to swing on the anchor to the full length of the anchor rode in any direction.
Consider a boat on 75 feet of scope with the GPS antenna located 20 feet back from the bow, creating a total offset error of 40 feet when the boat swings 180 degrees. With the boat pulling to the limit of its scope, the GPS antenna could be a maximum of 115 feet from the set point. To avoid false alarms, add 16 feet for signal errors and 20 more for human error. The security zone radius is a total of 151 feet (Figure 2). We could set a tighter zone but that invites false alarms.
As a general rule of thumb, if your security zone radius isn't somewhere around double your scope (meaning a diameter four times your scope), it's probably not big enough. With practice, you'll get a good idea of your average human error and may even be able to reduce or eliminate it with good anchoring technique. Most signal errors are beyond your control, but proper equipment installation will eliminate some.
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The Two-Zone Defense
Here's a technique that can be used to greatly enhance GPS anchor-watch security and reliability (Figure 3). Start by setting an anchor alarm on one device by placing a security zone around the entire diameter of your swing. Once the boat has fallen back and is resting to prevailing wind and current, use a second device to enable a set point with a minimal security zone surrounding the boat — a radius of about twice the boat length. This zone will not result in false alarms if wind and current do not change so the boat remains relatively stationary. If the boat drags, both alarms will sound, alerting you instantly. If the wind direction changes, the second device will go off but not the first, so you can start a real anchor watch. Once the boat stabilizes, enable a new set point on the second device and head back below, confident in the redundancy of the two anchor watch units.