The first number is usually the size, the second is usually pitch - sounds like a set of 11 1/2 props, and the second is a 15 degree pitch. Make sure the key didn't pinch as for the reason your prop didn't go all the way on....that doesn't sound right. It could be the keyway in the new prop is not as deep and the old key (or a new one) is stopping the prop from seating as the taper is pinching the key before the shaft fully seats. Try putting the prop on without the key and see if it seats to the shoulder of the shaft - that will tell you if you might have a key issue. A lot of times you have to fit the key with a grinding stone - a bitch but worth the hassle. A lot easier than diving to find your new prop or shearing a key and watching an unscheduled disassembly while you are driving it.
If you know all this please disregard. If not there is a great thread on installing a new prop and fitting the key/shaft here somewhere but I could not find the link.