This crack is now gone.
The top rail of this fiberglass tub was causing leaking into the unit below.The strength of the top rail was greatly diminished . The first thing we did was to remove and clean where the silicone repair was done.
We then bored out the crack to remove any loose fiberglass and to open up the crack wide enough to to accept the underside patch.
The patch is then adhered to the underside of the crack. The adhesive creates a reaction which melts into the patch and to the underside of the top rail fusing the two surfaces together.
A polyester glazing putty is then applied to the top of the rail to make it smooth and level.