Relying on GPS Anchor Alarms can be risky

It was a warm sunny day (27° Celsius) with a light North East breeze blowing at around 7 knots. It was predicted that the breeze would shift around in the evening to the south.

Our boat was a 6.5 meter day cruiser with a double bed, head and a gas bar-b-que which we had fitted to the back of the boat. It was our intention to anchor in the afternoon, and stay overnight. We intended to do some fishing in the afternoon and then relax with a few drinks and dinner and then retire at about 9.00pm

The spot where we had chosen to anchor was a deep estuary approximately 500 metres wide, with about 20 Metres of water under us. As is the norm with anchoring, we let out 60m metres of anchor line, which gave us an effective swing radius of around 57 metres.

For anchoring purposes we had a 25 kg sand anchor with 3 metres of chain and a 20mm rope. To monitor our position we had on board a mid priced GPS Satellite navigation system that included an anchor alarm feature. Once the anchor was deployed we set the anchor alarm on the GPS at the minimum distance of 30 metres.

Basically it works like this. In setting the Anchor Alarm on the GPS if the boat moves more than 30 metres in any direction, then an alarm will sound. This creates an effective coverage area around the boat of 60 metres in diameter.

Now, as it would happen within an hour of anchoring, the wind seemed to have shifted slightly with the result that the boat has now drifted further to the South. While we weren’t worried by this, it sent the Anchor Alarm into a frenzy.

So of course this necessitated that we reset the anchor alarm, but this time I decided to increase the setting to 40 metres and went back to our fishing.

It was on about my third beer and Kim had finally caught her dinner, when the alarm on the GPS went off again. It seemed that the wind was now starting to come from the east and we had yet again, drifted out of the GPS’ coverage area.

I went to the bow and a quick tug on the anchor line and I was comfortable that the anchor was secure.

So this time I set the alarm for 60 metres. I then settled back and then started the Bar B Que. We still had about 2 hours of light left and it was now a fairly still and balmy evening.

We finished our dinner and sat and talked for a couple of more hours and we didn’t decide to go to bed until about 10.30. Before turning in I decided to check our position, and lucky I did.

The breeze had now shifted to almost due south, moving us closer to the rocks. Sure enough the GPS was showing our new position, but because of the previous false alarms, I had enlarged the Anchor Alarm distance to 60 metres and it was yet to sound an alarm. Of course at this stage I decided to check the anchor and sure enough it needed to be re set.

So, I then re set the anchor alarm to 30 metres again and settled down to a good night’s sleep.

The Solution?

The only solution to not setting the anchor alarm every couple of hours was to set the anchor alarm on the GPS to a radius that was twice the diameter of my swing radius. which effectively covered the area so large that it rendered the alarm ineffective.

This problem can be compounded when other boats arrive and decide to moor within my effective GPS diameter. My boat can move, not sound any alarms and then bump neighbouring boats.

Of course this doesn’t take into account that once an anchor starts to move, by then it is too late and a crew must react very fast.

In our experience GPS anchor monitoring is a great idea but not accurate enough for any mooring situations.

The only way to be accurate is to have some sort of device attached to the anchor point.

 

 

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