Leaking holding tank causes boat to almost sink? - SailNet Community

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  #1  
Old 02-12-2010
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Leaking holding tank causes boat to almost sink?

This thread Buyer Beware mentions that a holding tank leaking caused a boat to fill up with a lot of water because of reverse siphoning.

I can almost visualize a holding tank springing a leak that as the water leaked out caused a vacuum that could pull water in from outside, but not quite.

Where exactly would it pull in the seawater?
The head is between the tank and the raw water inlet.
The massarator outlet would have a shut off through-hull on it, plus a pump.

The tank would have a vent that would not allow a vacuum to build up in any event.

Sucking the wast out with a pump out station would cause more vacuum than any leak in any event.

So exactly what combination of events could have happened to cause the problem reported.
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Old 02-12-2010
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In my system, which I designed so blame no one, the tank itself is below the water line. The hose from the thruhull to the pump (and then tank) is also below the water line. It is that 1" hose that would need a anti-syphon. As it is the seawater can leak by the pump, slowly but it does and then fills the tank.

In my case the hose from the head is a bit higher and seems to be just above the waterline but I would expect the seawater to fill that hose as well.

So with the valve left open the only thing preventing the boat from sinking is the hoses and fittings connected to the tank, the tank itself and the valve at the toilet which is not designed for that so I would expect it to leak or fail first. A failure of any of those items that result in water coming in at a rate higher than the pumps pump it out gives Davy another ship.

If the 1" hose from the mac pump was to rise to an anti-syphon device all that could be avoided.

BTW when I bought the boat the thruhull was left open. The tank was full. It was an old aluminum tank and I found a spot so weak I could break thru the tank with my thumb. That is all that kept the boat afloat (if the pump had failed).
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Old 02-13-2010
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I think in a properly installed system if the below the waterline through hull was left open the holding tank would fill with water but as long as the hoses were still attached to their connections that is all that would happen. There should be 2 anti-siphons installed for a conventional head. The first should be between the pump and the back of the bowl. The second should be in the outlet hose between the bowl and the holding tank. If this is done the water that enters the holding tank from the open through hull cannot siphon past the holding tank into the head itself. An anti-siphon is not required between the holding tank and through hull and in most under V-berth installations is impractical as well.
On the other hand if there is not an anti-siphon between the bowl and the holding tank the boat could flood if the through hull was left open. The solution is to install one.
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Old 02-13-2010
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Or keep the valve closed. As you suggest there is not a one size fits all solution which is why there are so many differences.

So many differences that I'm not sure what is conventional. It seems conventional has only one hose to the thruhull, from the mac pump? Thats what I did in part because it seemed to be the most common.

The older style, which seems to be so old that most of them have been changed over the years had the head connected to the thruhull, some had tanks some did not.

And then there are other differences, depending on boat. At one point ours had a Y valve going to the thruhull which I think had a single anti-syphon because that is all that style would need.

And there are newer ones that have no thruhull at all. I think that is because they are required in some areas.

I like to keep the ocean out of my boat as much as possible so would not be comfortable counting on a tank and it's hoses. Though I might if I had designed it for that.

Speaking of keeping the ocean out the thruhull for the sink drains seem to be a poor idea. It looks like the only purpose is to avoid having to clean a sump.
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Old 02-13-2010
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With a conventional pump head I know of no safe install that doesn't have 2 anti-siphons installed. One from pump to bowl and one on the outlet. Of course if the through hulls are ALWAYS closed when the head is not in use there is no problem, but how often does that happen. I have no problem with a through hull as long as it is properly installed, has 2 good hose clamps on quality hose and has an anti-siphon installed where required.
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Old 02-13-2010
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This is a Wooden Boat Graphic

http://i321.photobucket.com/albums/n...dm203/head.jpg
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Old 02-13-2010
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That is almost right, but the intake anti-siphon should be in the line from the pump to the bowl, not in the hose from the inlet to the pump. The hose from pump to bowl isn't shown in that diagram.
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Old 02-13-2010
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The safest setup for a holding tank system is also the easiest to use. Step 1: get a Lavac toilet. Step 2: install a holding tank above the waterline. The intake hose for the toilet is looped above the waterline with an anti siphon valve. The outlet from the toilet goes directly into the holding tank (no outlet to the water) from the top of the tank. There is a typical breather from the tank. The tank then has an outlet at the bottom which is connected to a y valve. One side goes to a drainage seacock, the other to the pumpout fitting on deck. With this system there is no way for the water to backflow unless the tank is already under water (in which case the toilet is the least of your worries! The lavac toilets have no joker valve. Instead they rely on a vacuum created by a henderson bilge pump. NO clogs....ever!!!! It goes like this. Poop....then close the lid with a seal on it. Pump the pump 5-6 times and the water will be pulled into the bowl...crap goes out and into the holding tank as the bowl is re-filled. Simple.....nothing breaks and the crap is macerated by the big pump. Again, there are no small valves to be clogged and there is no direct path for water back-flow. Now the real advantage to this system is this; Say you have been in and out of harbors...... cruising for several weeks.. your tank is full and there are no pumpouts. All you do is turn the y-valve ( 3 miles out) and the tank will gravity drain (and wash itself out). Again, it is not possible for the water to backflow into the tank or the toilet! I installed this system on my SC 31 and it worked flawlessly!!! To bad i was forced to sell the boat . Now my Oday 25 c/b has a portable toilet....yeah......no more thru-hulls at all! At least i'm still on the water...in April that is.... and i'm not taking this boat offshore!
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I agree the Lavac is the best solution. This is what I have on my boat (my previous posts were for Jabsco type heads). I will also be installing a gravity drain holding tank across from the head in the back of the large hanging locker. But I disagree with the Y valve below the tank. In my to be built tank there will be a drain at the tank bottom directly to a through hull with seacock. The deck pumpout will come from the top of the tank and will be directly above the bottom drain. There are several advantages to this. One, there is no Y valve which requires 3 additional connections that are possible leak issues. Two, if by slim chance the drain clogs just open the deck pumpout fitting and as it is above the drain a rod pushed down will clear the clog. I have the inlet hose looped to just under the deck with the Lavac supplied vent at the high point. The outlet line goes from the pump to a anti-siphon fitting also just under the deck then down, across and up to the top of the gravity tank. There will be a vent on the holding tank of course.
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Old 02-13-2010
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Quote:
Originally Posted by mitiempo View Post
That is almost right, but the intake anti-siphon should be in the line from the pump to the bowl, not in the hose from the inlet to the pump. The hose from pump to bowl isn't shown in that diagram.
In fact Jabsco specifically says it is wrong to put the loop in the inlet side.
Apparently it will not even work as the pump will just suck air.

http://www.myboatsgear.com/newslette...ted%20loop.pdf
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