Holding Tank Vent Hose & Pumpout Hose Replacement

[Hugh17]

I've replaced my holding tank and all hoses except the vent hose and pump out hose. Both these hoses connect to either a stanchion or deck fitting. I assume the hoses are clamped to these fittings johnson st under the deck. Are these clamps/fittings accessible from inside the boat? How difficult are they to replace?

[Stu Jackson]

James,

The hoses are easy to replace.  They run up inside the teak trim inside the sliding black doors.  Open the doors just forward of the galley and nav station, and remove the few screws holding the trim.  There is wood trim both above and below the shelf.  I took those pieces of wood off when we first got the boat to eliminate the leaks that occur there.  The leaks come from two issues:  those stanchions on either side are usually the ones that folks grab when they come aboard; the hose connections get loose underneath the deck.  I never replaced the screws, just jammed the wood pieces back in.

For the holding tank vent, many have replaced it with a larger hose and a mushroom thru hull below the hull to deck joint per Peggie Hall's suggestion, and have capped the hole either above or below (or replaced) the port stanchion.

[Hugh17]

Stu
Thanks much. I'm glad to hear that access to the hose connections is available. Lois like this is my next project.

[rmbrown]

I've read a lot on here about replacing the vent line for the holding tank and I understand the reasons...
1) Get air in there so the aerobic bacteria win out over the anaerobic (stinky) ones.
2) End up with a line that can be blown out with a hose back into tank instead of pumping out, filling via the pump out until water comes out vent, pumping out, etc.
3) Getting the vent somewhere else than over my electronics since that stanchion will eventually leak (again).

But I want to ask you guys a question that my engineering mind can't quite sort out.  I'll admit, my days of performing calculations are distant... just not what I do anymore... but I feel that I'm forgetting something.

A common theme is to have the vent line as horizontal as reasonable, no dips, etc.  I'm thinking that this is to keep it short, but also to prevent water traps.  What I've observe in my system is that there are times that the wooden cover between the cushion and the holding tank is physically lifted by the tank vent because the tank is pressurized.  Invariably, I remove the vent hose (with much care), a little air comes out, I spray a hose up the vent, and water comes out.  I take this to mean that, while there was some obstruction, it didn't take much to blow it out.

I pump out the boat practically after every weekend away from the dock.  It's never close to full, so I don't think that I'm getting anything from the tank into the vent.  I've removed and cleaned out the fitting to the tank, the fitting to the bottom of the stanchion, and the line in between.

I've also observed that my line has a slight dip in it just before reaching the tank.  This means that air isn't flowing freely, bad from a stinky standpoint, but also from a pressure standpoint.  No pressure in the tank means that I have an unwanted water trap.  (Let's just assume it's clean water from the rinse out for the sake of discussion.)  Pressure in the tank would push that slug of water up the hose and it would have to reach the top and come out the vent before the pressure could get back to 0.  If that was a column of mercury, I could see it resulting in enough pressure in the tank to observe expansion, but even 12" of water would only require about 0.5 psi to hold it there.  What am I missing or forgetting?  Can the water trap effect really make any difference from a tank pressure perspective (forgetting the joy of the anaerobic bacteria).

[Noah]

You mention the tank is "never close to full." How do you know that for sure? Any possible vent issues aside, in my experience, a bulging tank is an overfilled one. Get yourself a tank gauge to be sure.

[rmbrown]

I don't doubt that a tank gauge would be a great add, and it's on the list... but what I mean is that my typical use is to go out for 1 or 2 nights, then dump the tank and put it back in the slip.  And, as gross as it sounds, I've pulled the vent fitting out of the tank and poked around with a flashlight, verifying that the waste was >6" from the top of the tank.  I suppose it's possible that one one weekend, significantly more waste was generated.  I keep close tabs on my guests but not THAT close!

Plus on the 2-3 occasions where the tank has bulged, concerned that the vent might be plugged (and I suppose it was), I pulled that same fitting out, after removing the vent hose slowly, to relieve the pressure.  Again, not close to the top.

Having typed that, it's clear that the vent must have been plugged, or the tank couldn't have pressurized, but I still feel like I'm overlooking or over-simplifying.

[KWKloeber]

MIke

It's simple.  You have had a blocked vent at some location. Either a dip in the line and waste got into the hose and blocked it, or the stanchion (if that's your vent) is blocked. If your vent is free there's no way the top will bulge.

-k

[rmbrown]

I totally agree.  The mental struggle is this... How much pressure is required to make the tank bulge?  And is it possible to contain that much pressure with just water in a dip? (I don't think so.)  If not, then it must be a solid obstruction, but I'm confused as to how it could contain that much pressure, yet blow out so easily with a hose.  Either way, it's not getting air, so fixing it is a no brainer... But I'm curious!

[KWKloeber]

Mike just emperically it doesn't take much. When I fill my stb water tank ("identical" size/shape as my port waste tank) I can bulge the top and the vent is open and the stanchion peeing overboard. The amt of bulge depends on the balance between how much flow I have the nozzle set at (low vs full blast) acting against the back pressure above the tank (height of water column, pressure from the orifice restriction and stanchion restrictions.)

What you have in the head is essentially a very efficient, high volume bicycle pump.

How badly is the question burning up the synapses? If you really? want to know just stick a pressure gauge with hose clamp on the vent and pump a few strokes on a nearly full tank! 🤓

[rmbrown]

Well, the engineer in me would love that pressure gauge.  It would be a great way to know, in advance, when the vent is blocked.

The sailor, in me, thinks that dropping in a gently sloped 1" vent, that pretty much could never get blocked the way I use it, and going sailing is a better option.

[Fred Koehlmann]

Hi Mike,

You mentioned "A common theme is to have the vent line as horizontal as reasonable, no dips, etc. ". I'm going to assume that you meant "vertical"? Right? Definitely no dips in the line. Keep in mind that the line may not be plugged or stopped by water, but also very likely by waste. Us sailboats like to heel and its amazing how the stuff likes to swish about. If it goes up the vent line and later pressure build up forces it further up, it may not come back down via gravity. This was one reason why we implemented a full 1" hose and sent it to a mushroom outlet/inlet by the forward closet. When we pump out we send fresh water down the vent line (important that you don't have line dips here). This flushes the vent line and since we had the vent at the opposite end from the pump out outlet, we also flushed the length of the tank.

Also if you are still using the stanchion vent, check for bugs and spider nests at that tiny outlet. These can also easily block the vent line.

[rmbrown]

I did say horizontal, and still think that's what I read, although I'm too lazy to go confirm it.  Give our tank location though, it will pretty clearly have a large vertical component! 

With the constraints being
1) High enough to avoid the glug-glug when I heel over
2) Far enough from the widest point of the boat that I don't scrape the thru-hull off when I dock vigorously (or increase the odds of dunking it at a given heel)
3) As close as possible to the tank to actually allow, maybe, some fresh air to enter the tank, not just pressure relief

the location of the thru-hull of behind the built in chest of drawers in the forward cabin makes a lot of sense.  I'm just not sure I can get that far forward though... I ran a 4" duct through that space for my heater a few months ago... not sure if there's room for another 1.5" hole or not!

[rmbrown]

Well, to add further to my confusion... I was at the boat today and was happy to notice the lack of boat funk when I opened her up.  (I replaced all sanitation hose a couple of weeks ago, so I was happy to see the improvement.)

While I was working on my electrical panel over the chart table, I kept getting a whiff of "head smell".  Knowing I was near the vent, I sniffed around but found nothing so I went to check the head itself.  I found a solo cup or two's worth of dirty water in the head.  I flushed, filled the head with fresh water from the sink, and pumped it into the tank and, when I was done, it gurgled at me.  Repeatedly.

I assumed the holding tank must be pressurized again, so I uncovered it, moved the cushions, and found the top to be bowed up enough to have raised the wooden cover.  I covered the vent with a towel, and undid the vent hose clamp.  I worked the hose off and relieved the pressure but got no liquid out, only foul air.  I used my foot to mash/pump the top of the tank and the vent elbow breathed fine... again all air, no liquid came out.  I used a piece of clean hose and a double barb to attach to the vent hose so I could blow on it and judge the extent of the blockage.  There was a bit of water in the line at a low point and I had to blow that out, but no blockage.

Now fella's I'm an engineer and while I might have forgotten a lot, I'm certain that if that vent wasn't blocked, the pressure couldn't build, or at least could only build to the pressure needed to blow that water out of the vent (a problem for another day).  I'm equally certain that the pressure it took to blow that water out was nothing compared to the pressure required to expand the tank and force funky air past my joker valve and into the bowl.

Thoughts, ideas, crazy theories... all welcome at this point.

As an aside, while the vent hose was disconnected we had quite a rainstorm and the rainwater started coming down the hose.  I don't know if the vent is so low to the deck that it backfills when a puddle forms by the scuppers or not, but there's another reason for me to replace that vent line with one that comes out through the hull.  No need to fill that tank with rainwater! 

[KWKloeber]

Mike,

Well, at least you seem to have solved the stinkyist part of the problem.
You have some strange things going on.  Maybe it's the carbonaro effect happening with with the toilet, or the tank, or both.

You might need one of these instead of the one you have:
https://youtu.be/01u8fB2FKsE?t=53

-k

[rmbrown]

Well that would certainly be easier to get from the car to the slip, but I think I'd have to put it together on the dock... not enough room for those boxes on the boat.