Gee the conversation re fences, winglets etc is getting interesting. Winglets and fences stop lift flowing from the wing root to the tip and thus forming vortices that generate drag. The "fences" you talk about Old fart (or at least I assume so) are actually vortex generators that re invigorate the airflow in the conventional sense and improve the response/effectiveness of the flight controls, actually the ailerons. Fences are very crude winglets. Now re hoerner tips, on STOL moded a/c the sensitivity of the ailerons is greatly reduced at slow speed and this was an attempt to improve the performance (flight controls) of single engined a/c. Unfortunately the loss of top end speed reduced their popularity.