cracked bathtub

I have a crack on the bottom of a bathtub in the middle of the tub about 6 inches long. I attempted to repair it with a fiberglass repair kit, but it reopened. It appears that the tub may have been installed incorrectly and has a gap between the bottom of the tub and the wood support. The previous owner didn't weight much and that may be why it's now broken. Any cheap hints other than having the old tub pulled out and another one installed? I sometimes find these projects challenging and have considered filling the area below the tub with a fiberglass blow-in foam to give it support underneath and then repairing the crack again. I'm in no rish because it's a flip than I'm living in and I have 2 other baths in the house.