In principle, the sterntube aft seals can function without a lubrication system. However, the aft sealing functions better and has a longer life if it is fitted, like the forward sealing, with a lubrication system. Although dated, permanent oil lubrication without circulation is still used. Oil loss can occur if there is no counter pressure from the outboard water. The system with the circulation is safer. The aft sealing is connected by means of two oil lubrication lines to a settling tank. The oil is circulated by a small oil pump. Any seawater that has leaked through the damaged or worn water seals into oil system can be separated out in the settling tank. Similarly, any oil that has escaped from the oil seals can be drained to the waste oil tanks. Low and high level switches on the settling tank allow clear observation and monitoring of the functioning of the aft seals.