When sailing I may anchor in a different bay each evening and take a dinghy in to land to sleep in a tent overnight.
I always worry that when I wake up in the morning the boat may no longer be there. What can I do to choose a suitable sea bed to anchor in, or to ensure the anchor is firmly affixed, even if the tide changes?
Set the anchor by reversing the engine until the anchor chain is tight, and then run the engine at medium speed to make sure the anchor has dug in. You can watch a GPS to make sure the speed is around zero (and the anchor is not dragging) when you're doing this.
However, if the tide and/or wind changes, it can pull out a set anchor. If you use a LOT of chain and it's not too windy and the current is not too swift, the boat will only drag the chain around and won't actually pull on the anchor. Also, if it does pull the anchor up and you have a lot of chain out, it's likely to catch again somewhere nearby (unless it goes over a ledge into deep water).
Some people will set two anchors, one in each direction of the tide. I don't think I could do that without ending up with a tangled mess of chain.
Maybe you could get a wireless anchor alarm that goes off in your tent when the boat moves.
What xpda writes about setting the anchor is right (although hopefully you know this if you are sailing your own boat). As well as what he says, make sure you have the right kind of anchor for the bed, and you have enough rode (rope or chain). For overnight it should be at least 7 times the water depth. The longer the better, within reason. Here are some sailing specific websites.
If you are in any doubt about the security of your anchor, you should set an anchor watch. Basically this means checking periodically that your boat position is not moving. Use a compass bearing from a fixed point. Obviously that means not leaving your boat unattended for the night. If you have electronic navigation system you can set an anchor alarm - one that will sound if you get more than a set distance from your anchor point. There are also some smartphone apps that will do the same job. Some specialist boating websites will give you more information. Here is an example.
You can also set two anchors (though 'one for each direction of the tide' may not be necessary - two anchors might be set for a different reason if the current or wind might change and you don't want the boat to swing).
Another action is to dive on the anchor - swim down and visually check that the anchor is properly set.
Plenty of sailing or boating websites will give you more information about this.
Some set a Mediterranean Mooring, with one anchor out to sea and the other end of the boat tethered securely to the shore. The outward anchor keeps the boat away from the shore. It does prevent the boat from swinging with the weather.
You can ensure the anchor is firmly affixed by increasing strain on the shore line, and then ease the rope to let it ride easily. The strength of the shore line could guarantee that the boat will be there in the morning.
