So it finally didn't rain here for a weekend so I got to go finish my fiberglassing and while I was there I checked out my mast. The base of mine is only held on with 2 rivets (I guess there are a few different types). But what happened what, the rivets didn't actually shear at all, the holes on the mast were just so close to the edge that they tore all the way out! Now I'll have to drill new rivet holes in different places, I think I will put these a little further from the edge.