The purpose of having a spare aboard is of course to be able to continue without (long) interruption. Looking at the cost of a new propeller, we opted for a fixed blade propeller which is less expensive, but will certainly do as "an emergency prop" aboard. If nothing ever happens, this is a good insurance. and piece of mind. Considering that the delivery time of the prop we bought was 13 weeks, having one aboard is a good thing.
Were to store it was the question. We've got a couple of ideas from fellow Outbound owners. Sail locker or forward part of bilge. We tried the sail locker first, but it did not fit under the floor board. Maybe since we have a later engine and the fact that the bow thruster takes some of the space. We managed to get it into the forward part of the bilge and it fits well in there. We did cut up some plastic tubing to protect the bilge (and the propeller). Then strapped it in place to avoid motion and chafe. Happy to have it aboard, out of the way, and low and center in the boat.