Reading your post, Mato, I wonder if the reason why the fenders seemed to make no difference could be that, as you were already inverted, the thrust of the bouyancy fenders was directly upward. If the mast were at an angle to the surface, the bouyancy effect would, I think, be more effective.
If the fenders had been raised before the inversion was attempted, perhaps it would have been much more difficult to actually invert the boat.
Has anyone actually tried to invert a boat with the built in sail buoyancy raised and/or eg. the 9 litre Crewsaver masthead device? Regards, Dave Doran