** Need to know if my whirlpool tub with hairline crack is repairable and how. **
It may be able to be repaired using the right technique. The (usually) ABS plastic can often be welded using a plastic repair kit which should be available at a local commercial plastics supplier.
There is also available a product called "SlapStik" (or something similar) which is applied like a tape to the outside of a plastic outer washer tub to cover cuts in the middle of a flat area. Check a local appliance parts supplier to see if they know of or can provide such a repair item.
Is the cost of a replacement tub so high that is not an option? Usually Whirlpool replacement parts are priced quite reasonably.