Has anyone out there installed a flapper or some type of cover on the discharge for the bilge? We have what seems to be a universal problem with black soot on our transom for the Yanmar diesal. However we also appear to get a backwash of soot and water into our bilge, expecially when the stern is low in the water,or if waves lap against the stern when engine is idling. Would appreciate anyone's comments?. Thanks