Vans catalogue mentions under wood prop extensions that a 6 inch interface between the extension and prop has proven to be marginal and they have started using a 7 inch interface. I'm guessing the 6 inch was what many older RVs were built with, have owners that used a 6 inch tended to stick with it, and if so what were the issues that made them "marginal", or have most been changed over time to the 7 inch, or indeed some other type of prop?