Proper Head configuring on an O'day 25 or similar

[Ran-Dee18470]

I have an O'day 25 from 1976. It had a thru the hull head disposal system. It was half way fitted for a holding tank. The tank is under the Vee berth. It has a standard venting 5/8" outlet on the top that enables the vaccum necessary for pump action using the head lever. this outlet had no gear on it at all. In a big heel you'd risk waste coming thru it. I added a check-value that is for fuel systems that lets air pass into the tank and only releases a minimal amount of air from the tank. I am told this really should be vented to a thru the hull fitting high above the water line. I am reluctant to put another hole thru the hull and don't even know the hull thickness that I need to know to get the right thru hull fitting size. Another idea I see on west marine is the Sanigard HOlding tank filter (part # 447144). It looks like I could install that under the Vee berth right on either side of my check valve and not do a thru hull fitting. Has anyone had experience with to give advice?

 

[TT23844]

Holding tank vent

Ran-Dee I'm a bit confused by your statement that " It has.....outlet on top that enables the vacumn necessary for pump action using the head lever."The purpose of a vent line on the holding tank is to allow the free movement of air in & out of the tank. The vent hose should run from the tank up as high as possible to an appropriate fitting thru the topside of the hull, usually located just below the rub rail. As the head pumps waste into the holding tank,it displaces air which exits via the vent. When the tank is emptied air must be allowed to enter the tank as the waste is removed. Additionally, waste in the tank will generate gasses, which also must be able to vent overboard.The function of the vent is to prevent either a build-up of pressure in the tank, or the creation of a partial vacumn as the tank is emptied.I can't see the logic of the check valves you mentioned.