On Sun, 04 Jan 2009 10:35:46 -0500, John H
wrote:

OK, my explanation probably sucked. I'm not worried about fore and aft
motion, either while underway or at anchor. It's the side to side motion of
the bimini that puts stress on the mounting bracket, because there is not
enough play in the interface between the bimini frame and the deck bracket.
The side to side motion gets transmitted through the bracket to the
mounting screws.

"Side to side" motion is the most difficult to prevent because there
is typically no lateral bracing. The screws are working loose
because of the high stress on them, and they will crack the deck
eventually as others have pointed out. The answer is to through bolt
the fittings with backing plates and/or oversize fender washers.
That will solve the fitting problem and may reduce swaying some what.
Ultimately the answer is more bracing, with at least three attachment
points on each side. I'd get the recomendation of a good canvas shop
that does a lot of biminis.