the chain plates for the shrouds are aft of the forward beam...aren't they strong enough to hold the mast up by themselves?
They probably would do the job if the spin halyard sheave was at the same point as the shroud tang. However, most cats have the halyard somewhat to way above this point.
That causes the top part of the mast to go forward, inverting the mast, bending it the wrong way. When that happens, it may fail at the weakest point in it's column.
The mainsail through the mainsheet keeps the mast bend positive and the bend in the lower portion is restrained by the mainsail cloth.
