Years ago there was a ring balancer sold for auto wheels.. It was a circular tube partly filled with a liquid and ball bearings..the weight of the balls was supposed to go to the unbalanced side..Don't know if it worked, haven't seen them for years..We had a small, about 4 inches around, steel ring filled with mercury, that was bolted to the crank on top of the prop, to do the same thing..It didn't seem to make any difference....