Replacing the Holding Tank Vent Thru Hull

Over on s/v Astraea, Nate tackles the problem of getting adequate holding tank ventilation.

Last year I replaced the holding tank. I kept the original vent through deck fitting, but installed a 3/4 inch hose from the tank to a 3/8 inch hose and then coupled that to a woefully small through hull. A larger diameter vent hose helps promote oxygen flow in the holding tank encouraging aerobic bacteria to flourish. Aerobic bacteria break down the waste, but don’t put off a foul odor. There’s plenty more information about holding tanks at The West Advisor and from Peggie Hall’s book How to Remove Boat Odors.

What I did today was remove the old through hull fitting and replace it with a 3/4 inch fitting to match the hose size and encourage as much air as possible to get to the tank.

First I removed the hose from the through hull fitting

Here’s the hose with reducing coupling from 3/4 to 3/8 inch

Next I took the nut from the new through hull and traced the inside so I knew how much to enlarge the hole

I taped around the hole before cutting with a jigsaw to preventscratching the gel coat. I used an 18 tooth fine metal blade for cutting the fiberglass hull.

After cutting I roughed up the surface with sandpaper and cleaned up with acetone

Then I applied some 3M 5200 around the inside of the lip and slid the through hull in to place. Next I cleaned up the excess 5200. I also need to scrub off the stain on the side of the boat from the old through hull corroding. I chose 3M 5200 sealant because the fitting is above the waterline but could be submerged on a starboard tack.

The last part was connecting the vent line hose with a hose clamp.

Hopefully this new vent through hull will make the holding tank less stinky. We’ll have to flush and see.