You will most likely find that the winches are bolted into a metal plate that has been embedded into the GRP construction of your boat. The metal plates most likely have been drilled and tapped to take the thread of the bolt such that there are no nuts on the under side of the plate. On mine, these metal plates are aluminium and the bolts used have been stainless steel which have become seized into the aluminium to the point where the previous owner, when wanting to remove a rope clutch, broke the securing bolts off. The ends of the broken bolts were below the surface of the GRP, and the broken off ends were irregularly shaped such that it has not been possible to drill out what is left of the old bolts without the drill skating off and doing more damage. Ideally the threads of such bolts would have been coated with "Duralec" to prevent them from seizing, but there is no evidence to suggest that Bavaria would have done that during construction, and so unless the original owner took the precaution of removing all new bolts and coating them as described, then like as not you are stuck with what you have already got. Maybe the bolts are not impossible to remove, just exceedingly difficult. Good luck.
