Help with Prop Specs - Quicksilver QS5118 - >>???

I just purchased an '00 Key West 1720 'Sportsman' Center Console boat with an '00 90 HP Merc 2-stroke. The prop on the boat is a Quicksilver Stainless Steel with the markings 'QSS' and the number 'QS5118 18P'. Pretty clearly an 18 pitch prop, but I'm unsure of the diameter. I would have thought I could just look up the part number and Merc would list the specs on the prop.....but they don't. In fact, they show nothing with this part number at all. I fish the Chesapeake Bay.... and the occasional debris hazard is pretty common.....anything from an old crab trap that broke loose to a submerged tree or log floating out to the Ocean. So....I thought a spare prop would be a good idea......and would like to move from the stainless 3-blade to an aluminum 4-blade. I did this on my 15' Key West and was very satisfied with the results. But, knowing where to start is pretty important. So, any help with the unknown factor here would be greatly appreciated. Many thanks!! Tom D.