What happens when a big boat drags anchor and slams into you?

What causes anchor dragging and how can you prevent it in places like Roche Harbor?   

Anchor dragging is often caused by poor holding ground, insufficient scope, or shifting wind and current. In busy areas like Roche Harbor, boats may drag when anchors fail to set properly or conditions change overnight. Preventing it requires proper scope, a well-set anchor, and checking swing room to avoid nearby boats.

The quick answer is, "Nothing good." This is a short synopsis of what happened to us one 4th of July in the San Juan Islands at Roche Harbor 

     In about thirty minutes the Roche Harbor staff will be lowering the flags and firing the cannon. 

Several times in past years, circumstances or poor planning have caused me to miss the ceremony. The firing of the cannon is what I really want to see. The belching smoke, the jagged dagger of flame, the explosive echo’s ricocheting across Roche Harbor. I anticipate a fantastic kick off for my best fireworks ever. Finally, the stars have aligned for me, and on the fourth of July at that. This is already a great trip. The kids will stay on the boat while Linda and I paddle ashore.  The lowering of the flags and cannon firing will be at sundown.

      I am in the cockpit tending to the dinghy, anxiously waiting to leave. When I look over at the boat anchored near us, I sense that it looks different. It is about thirty-five or forty feet long and easily over fifteen feet tall at the upper bridge.  The wind has been steadily increasing for the past hour and I suspect this apartment size power boat is catching the wind and straining at the anchor lines which would make him appear to move a little closer to us. I ask Linda, “Do you think that boat is getting closer?” As I’m watching, I become sure it is half the distance it was when I first noticed. Now I know for sure, we have a problem, the boat is only ten feet away. There is nothing I can do. The other boats anchor has broken out and it is dragging into us. The last ten feet closes rapidly, and suddenly he is against us. I hold him off long enough to grab a fender and place it between us saving us both from damage. I am yelling and banging on his hull with my fist trying to get their attention, but Linda tells me she saw all of them leave earlier.