Holding Tank Second Vent

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Rick Johnson

Quote from: Paulus on April 02, 2012, 04:55:30 AM
Hi Ken, I changed my vent line last year from the stanchion to a thru hull. 

So where did you place the thru hull?


Rick Johnson, #1110, 1990, s/v Godspeed, Lake Travis, TX

Indian Falls

Yep,,, not a very long thought out reply.  In Kens pic, it just looks like the hose is near the top of the tank.  Chalk it up to a Senior moment, I haven't seen my tank since November. I stand corrected!

Secondly, I'm trying to respect Peggy's time like I would respect my own... when I get a chance to set up an appointment to call roughly between 8-5 M-F,  I will.  I may be a sailboat owner but I'm far from RETIRED!
Dan & Dar
s/v Resolution, 1990 C34 997
We have enough youth: how about a fountain of "smart"?

Dave Spencer

I added a second vent to my holding tank this weekend.  I gave Peggy a call earlier this year and got some good advice before starting the job.  One thing that is recommended is to keep the vent hose rising at less than 45 degrees.  I'm not sure that this is possible or practical on our C34s.  Boats with the holding tank under the V-Berth could likely do this easily but our tanks under the port settee make this virtually impossible without a very long length of hose.

I picked the location for the new vent just below the rub rail and aft of the bulkhead.  It's easy to get to, is hidden by speakers under normal use and "looks right" on the outside just below the rubrail.  I got the fitting on ebay.  It has a stainless cover and a marelon-like body.  About $8.50 or so.  The hull is about 1 1/2" thick at this point including the ceiling battens.  I bedded the through-hull fitting with Butyl Tape.
- 1st Picture – inside view
- 2nd Picture – Stainless cover on the outside

I used 3/4" Trident XHD sanitation hose for the job.  I could have made 1" hose fit but I figured the fewer and smaller holes I had to drill, the better.

I routed the hose down into the cubby behind the settee back and then through the forward locker into the holding tank locker.  Drilling into the cubby took some patience and some careful probing with small test drill bits to avoid the structural elements (steel?) supporting the rigging loads through the chainplates.
- 3rd Picture

I connected the new vent hose to the old vent fitting on the holding tank.
- 4th Picture

With the speaker in place, and the cabinet closed, the hose is invisible and there is a very minimal reduction in usable space in the cabinet and cubby.
- 5th Picture

I'm at my picture limit for one post.  Adding a new vent hole to the holding tank and renewing the original hose for the vented stanchion in the next post.
Dave Spencer
C34 #1279  "Good Idea"
Mk 1.5, Std Rig, Wing Keel, M35A Engine
Boat - Midland, Ontario (formerly Lion's Head)
People - London, Ontario

Dave Spencer

In addition to adding a second vent, I replaced the freshwater grade vent hose to the vented stanchion with proper sanitation hose. 
- 1st Picture – old and new hose

I also had to add a new fitting to the tank.  Peggy recommended Uniseal for a new vent hole in the tank, but I had a couple of extra through-hull fittings lying around and I had already installed an inspection port in the holding tank so the through-hull was very easy to install.  I drilled a hole near the aft inboard corner of the tank because that is where I had the most clearance between the tank and the settee seat board. The top of the tank is 3/8" thick.
- 2nd Picture – new hole and through hull into the holding tank

The finished job is shown in the 3rd picture.  I'll let you know how it works throughout the summer.  Next unpleasant job – replace the 1 1/2" hose from the head to the tank and from the tank to the pumpout deckplate.
Dave Spencer
C34 #1279  "Good Idea"
Mk 1.5, Std Rig, Wing Keel, M35A Engine
Boat - Midland, Ontario (formerly Lion's Head)
People - London, Ontario


Why 2 vents?  As Peggy has noted that we need a good vent that works.  A 2 vent system is a good idea but has not addressed the problem, the original vent is not working as it should.  The problem as I see it:
1. the old vent has 4  90 degree angles
2. the opening in the stanchion is small and prone to filling with water and dirt
3. the 2nd 90 degree angle after leaving the tank forces the hose to dip down below the top of the holding tank and acts as a trap
To correct the problem, I did the following:
1. took out the 2 corner panels that hide the pump out, vent line( 4 screws in each set)
2. removed the old vent line
3. checked out the space and how I could get a new thru hull next to the pump out hose behind these panels.( about 1'  from the deck) marked this spot.
4. took off the old pump out hose and put my new thru hull fitting in place
5. at this time I also drilled a new 1" hole in the storage bin at a 45 degree angle for the new vent hose.  This was to keep the vent line from dipping down.
6. replaced all the hoses with new, tested the vent system to be sure that it drained back into the tank.  Replaced all parts.  Worked great last season.
PS:  My previous boat was an Irwin 34 with a vent system that travel about 14 ft with numerous low spots.  Constant blockage.
Cool Change 1989 #944


very nicely done Dave, pictures are great, Paulus, I think I understand what you did, but, do you have pictures as well, were you able to increase vent size to one inch as peggy suggests?  Is your new thru hull just below the rub rail?
Bob Gatz, 1988 catalina 34, Hull#818, "Ghostrider" sail lake superior Apostle Islands


Bob,  I stayed with the existing size.  The thru hull is about 8'' to 12" below the rub rail, as high as I could get it before the pump out hose no longer would flex to allow for a thru hull.  I did not want the hose showing in the cabin.  The vent  is only used to let gas out.  The only time that air travels down this vent is during pump out.
Cool Change 1989 #944


I have read several post about members repositioning the holding tank vent to just below the rubrail either just forward or just aft of the port bulkhead and it is my intent to do the same this week.  My only question is has anyone found this location to allow seawater to enter the vent when sailing deep angles on a starboard tack.  It is rare for us to sail for any length of time with the rail under water it does happen on occasion and I don't want to fill my holding tank with lake water.  The addition of a clamshell comver should negate this concern, but I do't like the look of them so I would prefer to not install one.

Thanks in advance
Cory Mainnprize
Hull # 1344
Midland Ontario

Indian Falls

Dave Spencer has two identical vents for all intents and purposes.  He doesn't really say, but it would seem he kept the stantion vent intact.  According to the experts he now has two vents that won't work due to not being at less than 45degrees slope. With any luck you may get a breeze through your tank if air pressure hits the opening on one vent, maybe.  Is that a fair assumption?

Paulus has only eliminated the stantion vent and shortened the run to the opening about a foot. 

I'm thinking that if folks are making this modification putting the thru-hull below the rub rail, that the odor is somehow being, more often that not, directed away from the cockpit... unlike the stantion vent which dumps the smelly gas right at nose level when the head is pumped.  Nobody has really pointed out that the tank odor smells all the time or just when the head is pumped.  In my case, I only get assaulted by tank smell when the head is pumped.  My stantion does not have a slit. It is 7/8ID until it exits the stantion vent hole which is the same diameter as a 1/4 pipe thread.  In fact, I threaded my stantion hole with a 1/4 pipe tap and put a 1/4x3/8 hose barb in the hole, for which I slip on a piece of 3/8 clear tubing to hang over the side when we are onboard.  This puts smelly gas exiting the tank at the waterline.

After talking with Peggie about conquering the aerobic/anaerobic tank environment I was fairly sure no vent mod in my c34 will accomplish what is needed to achieve this.  So the next best thing is to direct the smell away from the deck.  There are tank aerators that run off 12 volts pumping fresh air into the tank all day... but, I don't know about that solution... 

Dan & Dar
s/v Resolution, 1990 C34 997
We have enough youth: how about a fountain of "smart"?


A brief addition.  I also changed the water tank vent.  Filled all holes with epoxy and then moved the opening gate forward so that it was more at mid ship. Note the rectangular foot print on the support stanchion.  This was changed to cover the vent hole.
Cool Change 1989 #944


I had the vent line off on a recent trip... because I was using the elbow access to view inside tank to determine level... and had a thunderstorm.  When the rain really started to fall, and the scuppers stopped being able to keep up, I was shocked to get water flowing back down my vent line.  Has anyone else seen this?
Mike Brown
1993 C34 Tall Rig Wing Keel Mk 1.5
Just Limin'
Universal M-35AC


Interesting thread, but a second vent line for the holding tank, what's the point?
Kevin Quistberg                                                 Top Gun 1987 Mk 1 Hull #273


What is the problem all this is trying solve?


Why would there be backflow their the vent??
Twenty years from now you'll be more disappointed by the things you didn't do, than by the ones you did.
So throw off the bowlines.  Sail away from the safe harbor.  Catch the tradewinds in your sails.
Explore.  Dream.  Discover.   -Mark Twain


Perhaps an answer in search of a problem.
Kevin Quistberg                                                 Top Gun 1987 Mk 1 Hull #273