Nick, Dave, et all:
I'll state that the Box Anchor is the SHIZNIT.
They can be found at:
I have the large on my 37' AT, and I have held at least 5 boats (for a period until others get anchors out) in current waters of the Potomac River and a few other places. I can't tell you how many times I see my friends Effing around with their Danforth/Fortress anchors, tugging and pulling, tugging and pulling, tugging and pulling trying to get them set.
With the box anchor, you throw it overboard, and that's it. The harder the boat(s) pull against it, the tighter it gets. If the currents flop, and you turn 180*, the anchor flips over and resets. I'll chuck mine overboard, and give it a tug just to make sure its taught, and then I'll let the current/wind pull the boat out the rest of the rhode.
The ONLY time my box anchor let me down was in a creek on the Potomac River when there was at least 4 feet thick of Hydrilla covering the entire bottom of the creek. You could not walk through or on this stuff, and the box anchor would not sink through it. Guess what, the danforths and fortresses were not working either. It was like trying to set an anchor into a sponge.
I don't trust a single anchor when 'sleeping on the hook', but that's because I had an anchor line snap on me during an afternoon of anchoring in front of a restaurant/hang out. If I plan on sleeping, I'll bring the Danforth, and set it after I set the Box Anchor. That's more for my piece of mine. The Box has never let go.