Water Tank Venting

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Joined: 2/29/24
Posts: 1
Water Tank Venting

New to this forum although I owned a 320 for 14 years, so this may be an already worn-out topic. My 36 water tanks will not vent once full. They begin to swell and leak and the fill hose doesn't back up (so without visually viewing the tank when filling you cannot tell it's full until it leaks into the bilge and the pump comes on. Not sure I see a solution to this as the vent fittings all, because of their location, seem to plug with water as the tanks gets full. Never had this problem with the 320. Filled it till the fill tube overflowed on to the deck and stopped filling. Vents were all located so as to vent the tank properly when full. Anyone else have this problem and any solutions. For now I am simply timing the tank fill and shutting it off after so many minutes so I don't have to tear the boat apart to watch the tanks fill.

Jimmy's picture
Joined: 1/12/08
Posts: 75

Not sure if you have a MK I or MK II.  My MK II leaks out the vent when the tank is filled.  Then water leaks out of the tank until the level goes down.  It doesn't brother me too much.


Hull No. 2058

Joined: 8/31/21
Posts: 37

I have not had this problem either.  Are you saying that you don't have vents?  Or are they clogged?  My fill hose flows faster than the vents so I do get backflow out the top when filling, but otherwise everything works as it should.  

KevinLenard's picture
Joined: 1/28/15
Posts: 213

We had a few issues with our tank vents on #1120 (1991).  First of all I was applying WAY too much volume/pressure of water to fill the tanks.  This was not allowing the air/water pressure to vent out before the incoming water pressure overwhelmed the 1/2" vent hose.  Led to split fittings, leak around the 3" access port, other issues.  So relatively slow fill is important. I had to fill the cracks with a hot melt glue gun and extra strength glue sticks. 
Two, the two vents from the center tank and the starboard tank were not only connected via a T-splitter, but they were too long and had been pulled down by gravity to a level below the top of the tanks, aft of the tanks under the center access panel below the steering pedestal under the aft mattresses. 
I had to shorten the vent tubes and use some zip ties and drill holes for the zip ties to keep them tight against the underside of the fiberglass liner (the platform that the mattress rests on) , but I also moved the T-fitting up on the starboard side to a spot at the level of the top of the wet locker 'ceiling' back in the starboard aft lazzerette.  This meant running some new clear 1/2" vent hose from the T-fitting down to the tank vent fittings.  With the eliminations of the loop that was dipping below the level of the top of the tanks the vents started working properly (in addition to slowing down the filling pressure).  Not sure if that's all clear...

Kevin Lenard
'91 C-36 Mk. "1.5" Tall Rig, Fin Keel, Hull #1120, Universal M-35 original (not "A" or "B")
CBYC, Scarborough, Lake Ontario, Canada

Splendid Isolation
Joined: 9/13/20
Posts: 31

After trying to clear my vent hoses as described above and doing way too much boat yoga, I found a much more effective solution: I now leave my infill caps cracked half a turn to prevent a vacuum from forming in the tank. Makes my fresh water pump work a lot less, too.

This doesn't solve your problem of the tanks inflating when you fill them up, but you've already found the solution to that: I usually set a timer for 5 minutes, open the nozzle on my spray hose about a quarter of the way so it's not putting too much pressure on the tanks, and then go below to watch the tank as it fills. It's usually up to the top within 5-7 minutes, depending on the level when I start. 

Good luck, Joe

1994 Mk 1.5
SF Bay

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