Back to my heap of a '72 Johnsorude.....
Older props had an anti vibration rubber bush 'tween the shaft splines and the prop casting proper, which i believe is injected in at the time of manufacture (i.e. not a replacable part). I suspect your engine might be newer than this technology, but although there is (should be) a shear pin, if the previous owner had repalced same with a sawn off 6" nail, then continually buried it in sand / mud , the knobbly bits on the rubber that stop the whole lot spinning on its own axis shear off, and so you end up relying on the surface friction between the rubber & metallic parts.
Way tou can tell is that round the hub at the joints, you'll see the paint flaked off or badly crazes, with hints of extruded rubber round the perimiter. It's basically the friction can't hold the torque, and it slips, generates heat, and extrudes the rubber. You probably won't notice it below about half throttle, and it would be a continuous effect of lots of revs combined with not a lot of action........
Having said that, it's a sib with a 40 - air time is an integral part of the trip!!!
(I'd try a trip on flat water first before you start dismantling things)