I don't want to create a definition "what works" or in this case, "sufficiently reliable", what I would like to know and think would be helpful to know is what kind of quantifiable track record do these gear drives (airboat only, and geared only, please...) have when used in aircraft. Reliability is a degreed assessment, so to what degree are these devices reliable. So the key identifiers for this thread are "airboat", "gear drive", and "used in an aircraft". Are there documented experiences (logs) of usage, type of usage, flight hours, any failures, durability (time between rebuilds), etc. I only know of two airboat gear drives used in aircraft: an RV6 and the Radial Rocket derivative. Surely there are others - how did they work out in the long term?