Not in the Delta, but last month I was chartering a 36' trawler in BVI. The weather was just about perfect. The last time we chartered we were annoyed by a tropical wave, which necessitated a lot of tacking in the flat-bottomed Heritage trawler. Anyway, on our final day, we motored back into Road TOwn Harbor to the fuel dock on the opposite side of the harbor from our marina. While it was sunny and light winds all the way in, just about the time we reached the dock, a sudden squall hit us with winds that I estimated at 30 knots. I had to go past the dock and turn back into the wind, only to discover that most of the cleats on the dock were useless. It took us quite some time to safely tie up the boat, and then the horizontal rain hit. The dock worker waited for the rain to pass, which it did as suddenly as it arrived about 10 minutes later.
We fueled the boat and departed. I radio'd the marina for a dock assignment, and then squall part B hit. This time, the winds were 30 knots sustained with 50 to 60 knot gusts. It was kicking up some good waves, coming directly into our beam. Across the normally tranquil harbor I had some troubles making headway, especially as my mate was trying to move the dinghy to the starboard side of the boat. As I was wondering how in the world I was going to dock under these conditions, the marina called me and changed my assignment to the first slip, so at least I would not have to dodge another boat. As it turns out, the squall died down just when I got to the marina.
I later learned that a 40' Moorings monohull sank outside the harbor in the Drake channel. Three guys from the UK were on it - they had maximum sail flying, they were heeling over by about 40 degrees, with hatches open. When the squall hit them, they got caught by a microburst, which broached the boat. With the open hatches, it sank in one minute. It is now 160 feet down with the sails still up. There were a lot of boats that sustained minor damages throughout the Tortola area during these squalls. Everybody said the same thing - they literally came out of the blue.