We advise our customers to order net cutters with their stern tube sealing rings. Unfortunately, they do not always prevent lines or nets entering the AFT seal housing, but at least they make it more difficult.
A regular oil lubricated stern tube seal system will have a rope guard covering and protecting the seal housing against foreign objects damaging the seal housing and even the actual seals inside the system. On the propeller side of the rope guard, net cutters are welded or bolted on. These net cutters should prevent nets or lines entering the space between the propeller and the rope guard. Especially small nylon fishing lines can get easily entangled around the propeller shaft and eventually work its way into the seal housing. When these lines get under the lip of a seal, it will damage the seal and cause leakage.