I have a 2008 Nordic 42. The exhaust silencer, which is manufactured by Sundown, has at it's base what appears to be a drain. It is covered with a large washer/plug and is attached by a large brass bolt through the centre. The washer portion has become heavily corroded and needs replacing. Any help on confirming this is nothing more than a drain and the correct procedure to be followed to replace would be very much appreciated. Can this be done in the water or does the boat have to be on the hard?