3 basic types of props out there: No hub for racing applications, rubber pressed in hub, and replaceable hub types for general boating. The rubber hub is friction fitted into the prop casting by a prop shop or OEM (Original Equipment Mfgr.). The replaceable hub has a plastic and brass insert that usually is square in shape, or may be a hexagon (six sided shape) that slips into the prop casting allowing for the consumer to change hubs and the dealer to use one prop on several different engines.
You can't tell much by the rubber pressed in by looking from the outside, but you can on the insert type. You can go to the prop section at the top of this page and find hubs for sale and get an idea as to what they look like. A prop shop will usually have to fix your rubber hub prop or if it's worn and slipping as Faztbullet said, or just get another....iboats sells them...look at the top of this page under props for that too. They also sell inserts if your insert is stripped if you have a replaceable hub type.
The thing you referred to initially is the thrust washer against which the prop delivers it's thrust to propel the boat. On current Mercs it's a tapered washer to fit a tapered shaft and when you put it on you must align the tapered ID with the taper of the prop shaft. Some shafts over the years weren't tapered and used a flat washer for the purpose. They are obvious as the prop shaft isn't tapered right after it emerges from the prop shaft seal in the bearing carrier housing surrounding the prop shaft. If you change types of props, like from rubber hub to replaceable type the thrust washer usually doesn't swap between tops and you need to get a new washer that fits with the new type prop.
HTH,
Mark
