Prior to our extended Chesapeake cruise last year, I installed an anchor Washdown largely based on the experiences and recommendations of Allan Field, Sea Shadow, #808. His project can be seen at http://tinyurl.com/mqxxjh.
At the time, I removed the hose to the marine head and connected the thru-hull to the suction side of the washdown pump, since we avoid using saltwater to flush. Since that time, I have plumbed the washdown into the sink outlet using a “Y” connector. This also allows the ability to easily flush the washdown hose by introducing fresh water through the sink. Winterization is also easily accomplished by using the sink to introduce antifreeze to the system. BTW, I would have liked to double clamp on the “Y” however the length of the fitting did not allow for sufficient area for a second clamp. Fortunately, it was an extremely tight fit for the hose and I doubt it would slip off even without a clamp!
Karl Mielenhausen, Silver Lining, #690