The power of a winch can be multiplied with sheaves block & tackle style. I'd be more concerned with the strength of the winch frame/bearings/drum shaft. For 18" logs I'd anchor the load sheaves to something other than the winch frame so it would only see a fraction of the load. A 3:1 or 5:1 Z-system used in rope rescue would work well with your winch especially in the capstan mode.
Rope Rescue for Firefighting - Google Books MikeD74T