Tips Sail

New Holding Tank Vent Outlet

On my forward holding tank, the vent line comes out the side of the tank, and because of the small size (<1/4inch) of  the screw in fitting, occassionally becomes blocked. In clearing that blockage last year, I noticed the thread for the elbow, that screws into the tank was stripped. At the time, I secured it back into the tank with my favorite 3M 4200.
This year was the time, to replace that fitting. It was very difficult to get at, being close to the side of the hull, and I didn't want to take the whole tank out to replace it. So I came up with this idea.


I used a plastic fuel tank through hull vent that comes in two pieces. I mounted this on a 1/2 inch plywood disc, coated with 3-4 coats of epoxy. The vent has a tube within a tube and when mounted with the hose tail pointing down , gives an extra protection from water ingress (like a trap). The curved "outer" vent entry has a stainless mesh which I removed. These features, when applied to the holding tank application, should prevent fluid escaping out the vent line.

Diassembled vent and mounting disk.

The disk was located on the top of the tank and the process for attaching with epoxy glue was the same as in this blog here.  Essentially I drilled a hole big enough for the vent intake; sanded the tank and mounting disk; and applied a epoxy mixed with a silica "glue" as a paste between both parts and allowed it to set up. The vent intake was a firm fit in the mounting plate and allowed for the removal (unscrews) of the bigger vent "trap" with hose tail; and also allowed for orientation by turning the vent intake to accommodate.

Finish product

The photo to the right shows the finished product, albeit that I don't have the right type of hose. I have 5/8inch sanitation hose coming from the USA. It's not a size readily available here.
The old vent hole was closed off using a plug they use on hydraulic items. I had one come off a new transmission cooler that was the right size. And of course to keep it in there, I used my favorite product, 3M 4200. One day the tank may be removed and that will be the time to remove the old brass fitting epoxied into the tank.

You might notice the inlet to the holding tank is also on the side, (under the old vent line). I don't under stand why they were built that way. Both the vent and the inlet should be on the highest point of the tank. Next year, I might move the inlet to the top of the tank. And that will give me a couple of more flushes. Doesn't a flush beat a full house?