1985 Johnson 4HP prop spins upon load

[fullerroad]

hello I have a 1985 Johnson 4HP model J4BRHcob which runs well but at 1/2 throttle the prop spins freely up to 1/2 throttle it works ok but I can't go pass 1/2 throttle as the prop spins and the engine revs up. is it a bushing that needs to be replaced or the spring inside the lower unit is worn or weak?

thank you

 

[Texasmark]

Classic definition of a spun hub in the prop.

 

[QBhoy]

More likely a shear pin broken up, on an engine like that. At a guess.