Holding Tank Second Vent

Ken Juul · March 28, 2012, 08:01:24 AM

I'm planning on adding a second vent line when I pull this spring. I borrowed Fred's picture because I forgot to take one last weekend. The actual space in the MKI is different, but the space above the tank appears to be about the same, which causes problems. Peggy has said on numerous occasions that more vent is good, so I intend to keep the original stanchion vent in use. I see these as my options.
1. replace the current "L" with a "T" and vent both lines using the existing vent hole.
2. Move the current vent line to the end of the tank (A). It will be easiest to add the vent to the back wall because of vertical clearance. But worried about it plugging when the tank gets full.
3. Add the second vent somewhere on the top, perform surgery on either the removable cover or fiberglass to allow clearance.

If option 2 or 3 is chosen, what is the best method for putting the fitting into the tank?

Bobg · March 28, 2012, 09:04:05 AM

very interested in what you come up with, I am also at the point you are. Been thinking about point A for me, I just don't see the room for method 3 if you are going to install the one inch that Peggy recommends.
Curious what Peggy has to say about method 1, the tee instead of the L.
I have already installed a clear see through removable 3" beckson port in the top of my tank so I can get my hand in there to tighten locknuts or what ever. Also I find it invaluable to really, really, clean the tank before winter layup.

Ken Juul · March 28, 2012, 11:55:02 AM

Thinking about it somemore. I think I need to modify choice 2.

2. Add new vent line to near the top of the back tank wall near A. If the vent gets plugged when the tank is full, it will be easy to clean out with a hose because of the easily accessed thru hull.

Stu Jackson · March 28, 2012, 12:37:19 PM

Ken, with #1 would would be restricting the opening to the original diameter, like a bottleneck, so I doubt the new vent line wold be effective.

Peggie Hall · March 28, 2012, 07:06:13 PM

1. replace the current "L" with a "T" and vent both lines using the existing vent hole.

Definitely NOT an option...nor is any other idea that would connect two vent lines a viable option...'cuz the air will just travel across the top of the tee instead of going through the tank.

I like #3: Add the second vent somewhere on the top...you'd need about 3" clearance above the tank.

IMO a Uniseal http://www.aussieglobe.com/uniseal1.htm is the easiest way to put a new fitting into a plastic tank--which is what your tank SHOULD be if it's a Catalina OEM tank...no locknuts or anything else that would require acess to the inside of the tank needed.

We may need to have a phone chat to sort out the best way to do this. 30 minutes on the phone will accomplish more than 50 more posts here. Send me an email if you'd like to take me up on that offer.

Ken Juul · March 29, 2012, 04:37:55 AM

I thought that #1 was a non starter, but wanted confirmation.
Also not real keen on either version of #2, unless I can get the vent on top. But the area around the back of the tank is getting crowded. One more line will complicate replacing the macerator or new cable runs to the electrical panel.

So #3 it is. The Uniseal is a neat product!!! Thanks, that will save a tremendous amount of work. It will be a couple weeks before I'm back to the boat to measure and sort out the best way to accomplish this.

Lance Jones · March 29, 2012, 06:17:27 AM

My question is that bringing both to the same vent would equal only have one vent line anyway as they are exiting the same size, single opening.

Jim Hardesty · March 29, 2012, 08:18:19 AM

My thought is with 1 vent line the tank would only vent. In..Out no sizable exchange of air. With 2 vents, an exchange of air(oxygen). Is this a good theory?
Jim

Peggie Hall · March 29, 2012, 10:57:32 AM

Quote from: Jim Hardesty on March 29, 2012, 08:18:19 AM
My thought is with 1 vent line the tank would only vent. In..Out no sizable exchange of air. With 2 vents, an exchange of air(oxygen). Is this a good theory? Jim

That depends on how long the vent line is, size of the diameter, whether it's straight or has bends in it, and how steep the rise is.

A 1" vent line that's absolutely straight, no more than 5' long, doesn't rise more steeply than 45 degrees, AND has nice open bulkhead thru-hull with no cover or screen in it will exchange air with the gasses in the tank just fine. The longer and/or steeper the line, the harder time it has exchanging air...bends in the line make it impossible. When none or most of the above is possible, a second vent may be needed...sometimes aeration is the only solution.

In a perfect world, all toilets would be right behind the v-berth...the tank would be under the v-berth...and the vent line would run forward off the top of the bow end of the tank to exit the hull about 2' aft of the stem, about a foot below the toe rail. 'Cuz that guarantees that air will be continuously forced into the vent any time the boat is underway or on a mooring or anchor. Unfortunately, boat builders often have a different idea what makes a perfect world, so we have to work with what the give us.

Bobg · March 30, 2012, 09:54:39 AM

Ken, I have 1988 and I see no viable way of using option 3 with a 1" fitting on my tank, there simply isn't any room on top of the tank under the hardboard cover for a fitting and hose, I am curious on what you come up with there and where you vent it to the outside, please keep us informed thanks Bob

Peggie Hall · March 30, 2012, 10:48:22 AM

...I have 1988 and I see no viable way of using option 3 with a 1" fitting on my tank, there simply isn't any room on top of the tank under the hardboard cover for a fitting and hose...,

Where on the tank is the existing vent fitting--on the top, or AT the top of a vertical surface? In either case, is it closer to the hull or closer to the centerline of the boat?

Ken Juul · April 02, 2012, 04:15:46 AM

The tank has a recessed area on the top for the original vent. Looks like easter will get in the way of boat work, so be at least 2 weekends before I can take a look at how to do it.

Indian Falls · April 02, 2012, 04:46:16 AM

The physical location of an added second vent seems very similar to having as a test, a modified pump out cap with a vent hole in it.

I wonder if this would be a fair test to see if it's worth the trouble to punch new holes in tank and hull and then find out it's not really working.

I can't seem to find a way to add another vent either.

I'm trying to have the "phone call with Peggy" about this very topic also, I just can't find the time during the day to call, maybe I won't have to if you get yours in and it solves the problem.

Paulus · April 02, 2012, 04:55:30 AM

Hi Ken, I changed my vent line last year from the stanchion to a thru hull. The problem we had was that the vent in the stanchion seemed to plug easily. Used a small hand pump to clean it out. I took the stanchion off and noted that the small diameter in the base of the stanchion. I filled all the holes and ordered new stanchions so that I could move the gate more to mid ship position. We have always used odorless in our boat and motor home. Never had problems with odors.

Peggie Hall · April 02, 2012, 07:31:50 AM

The physical location of an added second vent seems very similar to having as a test, a modified pump out cap with a vent hole in it.

Won't work...'cuz the pumpout line comes from the bottom of the tank...it can't provide a source of air above the surface. At best it could prevent a tank implosion from the suction of an excessively strong pumpout.

I'm trying to have the "phone call with Peggy" about this very topic also, I just can't find the time during the day to call.

So what's stopping you from letting me know what WILL work for you?