To protect the propeller blades from hitting something like shallow sand/rock and logs. If you hit them you will damage the prop. I hit a log with my 2HP motor and it broke the split pin in the hub.
It may have some value in protecting objects and your legs from getting in the way of the propeller too.

See here - looks like they are for the "Dual Thrust" props which have high thrust in reverse as well as fwd due to the function of the hub design or in the case of a hub with this deflector ring fitted - exhaust gases are deflected away from the blades when in reverse so that the blades bite clean water. Expect phoning Crowley Marine will confirm.

www.crowleymarine.com/parts/450862.cfm

and 4th pdf page of this

static.perfprotech.com/files/store/app_themes/ppt/images/product-specs/Yamaha-Propellers.pdf

R13 is right, its a feature of the high thrust, and to do with the exhaust and reverse.
Also, treat the prop as a consumable item, with a lifespan.

It drives through a rubber moulded in doughnut, which will in the end perish, leaving maybe 10% of drive left based on friction b/n inner and outer hub.
We just managed to slowly motor back to the mooring one time, and now keep a spare onboard, and change it about every 2 years.
great outboard.

you just need to know the model number, on the sticker just near the leg pivot.
Having said that, I think all 9.9 high thrust 4 stroke props are the same.
No, separate donut part sorry, the rubber is formed in the prop when made, like an infill.
You cant really suss out its condition either (well, I couldnt).
But its easy to change.
Then do your impellor too, which involves dropping the leg off (4 bolts, and disconnect the throttle lever) but thats for another day.
Also, take some whipper snipper line, and insert it into the pee hole teltale, it will clear any crusty salt, so you can ensure the impellor is pumping.

And finally, at the bottom of the leg, are 2 small 'drain covers', held in with a self tapper, one on each side. They cover the water inlet water intake. In the past, I have found some of the impellor pump vanes, which was a good sign to change the impellor.

hey Ben, what cat do you have?
Mine is a Schionning Wilderness 9.3m, and the Yamaha pushes that along fine.
If yours is in a pod, or otherwise unaccessible, the other thing you might consider is adding a temperature guage, especially if you cant see the telltale.
I'll find some pics.

same as Phil.
My boat was delivered from QLD to vic, and had long periods of motoring when the wind died out.
I wasnt fussed about engine hours or anything, just to get the boat here.

I did notice when it was arrived, the engine cowling cover was cracked open abit just to allow some extra cooling, the last few hours where in sheltered waters. I think fuel wise, it used about 2l/hour from memory (4 stroke).

Nowdays its only used in and out of the mooring, the boat sails quicker than motors.

motoring 21 hours/day for 27 days is a good effort.