While aloft for the final inspection, I noticed that the lower starboard spreader was in serious trouble!
First, the spreader was mounted upside down. The trailing edge of the spreader was facing forward and the forward edge of the spreader was trailing. The distinction in directions is only evident due to the air foil shape of the spreader. Round and square spreaders create a lot of drag and wind resistance, air foils create significantly less drag. An air foil creates less drag in a specific direction, and slightly more drag if it is set backwards (though still much less than a round or square air foil). Being how this is not a racing yacht, a slight increase in drag is not going to be detrimental to the sailing performance of a cruising yacht.
The reason that the direction of the spreader is so important is due to the drain hole for water in the spreader. The spreader has a drain hole near the mast on the bottom side. If the spreader is mounted with the trailing edge leading, the drain hole will now be on the top side of the spreader and instead of a drain, it would act as a water fill hole. Water inside a spreader will cause more weight aloft but more catastrophically, it will hold water that can turn into ice during the winter. The expansion of the freezing water inside will place great and unnecessary stresses on the aluminum air foil spreader and lead to its eventual and apparent failure.
The other factor that came into play with this particular spreader is the location of the spinnaker halyard and flag halyards. Both of these halyards (which look like they haven't been moved in 30 years) were rubbing on the front of the spreader. While the lines themselves were not significantly chafed, they did grind away at the soft aluminum of the spreader. The lines are full of dirt which acts like sand paper rubbing on the thin trailing edge of the spreader. If you have spare halyards installed on your mast, be sure that they are not contacting anything on their run. Over the years, either the halyard or the rig will become chafed and lead to costly repairs in the future.
The combination of the thinner metal of the trailing edge of the spreader being ground away by the halyards and the ice expansion inside the spreader may have led to the opening of the spreader.
Upon this discovery, the shrouds were relaxed and the spreader unbolted. It was then taken to a metal shop to have new aluminum welded onto the trailing edge and then ground into shape. The spreader was then painted and reinstalled the next day! With the repaired spreader in place, it is time to position the spreaders and seize the tips.