Shortribs have lots of connective tissues and can be quite tough. As such they really do require a longer cooking time. First off, don't boil them, they need to be simmered, if you are going that route. Boiling will make meat tough all over again, and cause it to dry out. A slow simmer, like when making a stew is the way to go. For my ribs, short, babyback, St. Louis, etc. (if I am not doing traditional BBQ) I usually bake/simmer them until almost completely tender then finish them on the grill. To do this, I rub down the ribs with the spice mixture I am going to use, then I wrap them in a double layer of foil along with a few shots of beer, or other liquid. I then bake these until tender, in a 350 oven. I would think that about 1 hour or maybe a little longer would be good for the shortribs. Then unwrap them, let them cool slightly and finish them on the grill.