> a kool dinghy anchor would be a house brick with some holes in it and 20 feet of 5/16th nylon rope :)

They do sell really small foldable anchors, all down to [700 grams](http://www.biltema.no/no/Bat/Anker-og-kjetting/Dregg/Dregg-2000017757/), mostly meant for holding fishing lines in place I suppose.  

I tend to leave the dhingy around for several days in a row, and it's not always practically possible to lock it (and sometimes I also want to leave behind things like life wests, in the motorized dhingy I have a petrol tank as well as a loose canister ... or, do I?  Possibly the canister has been stolen, can't remember to have seen it last I used the dhingy) had an idea of some "security by obscurity" - so I had this "security by obscurity"-idea - leave the dhingy by an anchor (using a kellet on the line) some ten metres from land, with a small preferably invisible rope going from the boat, down into the sea and being attached to land.   From a legal perspective, that's a completely different story than leaving the dhingy on private or public land, and from a security perspective it makes it a bit more difficult for someone to just steal the whole thing.

Apropos brick and thin nylon rope - I have this anecdote, from my first sail boat - a 24 feet "Grenada 24" with a broken engine - me and a friend from Portugal was sailing throughout the night, we did shifts so in the end it was my friend sailing and me sleeping.  I woke up in the middle of the day, there was no wind at all and we were lying still.  My friend was fishing.  I noticed something was odd - there was quite a lot of a tidal current on the place where we were fishing - and I got rather stressed from the fact that the current wanted us through a straight with lots of reefs - but, we were not moving we were standing perfectly still.  Guess what?  Some of the hooks on the fishing line had gotten a hold of the sea bottom - we were anchored up by the fishing line! :-)