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Old 08-17-2020, 09:10 PM   #1
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Flushing Holding Tank at Sea

We have a 430 aft cabin trawler with two ~44 gallon tanks and the ability to discharge the holding tanks out side of the USCG limit at sea.

We have never even tried to use this feature but we're wondering if this option exists for others and what your experience has been.

I'm concerned that the waste line from the holding tank is below the waterline and that if the pump fails it would flood the holding tank.

Looking for best practices here...

Thanks, - Jim L
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Old 08-17-2020, 09:17 PM   #2
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Jim, we do it regularly and our through hull is just below the waterline.

Go have a look at your set up. You should have a macerator pump that pumps the sewage overboard via a hose that is in a n (inverted U bend) shape taking the sewage up above the waterline and then down to the through hull.

If your pump fails the n bend will prevent any water intrusion.

Additionally you have a ball valve at the through hull that you can close.

BTW, while within the three mile zone that valve must be closed and secured by a lock or by taking the handle off. It will be checked during a CG inspection.
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Old 08-17-2020, 09:21 PM   #3
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Assuming you have a washdown pump.... First pump out or dump the tank. Then...

1.open the deck pumpout cap and use the washdown pump to put about 6-8" of water into the tank (putting water into a tank via the deck fitting sends it into the tank at the bottom to stir up sludge and hold it in suspension so it can be flushed out).

2.Open the discharge thru-hull.

3. Turn on the macerator pump while you keep the water running into the tank.

4.When you see only clean water near the discharge thru-hull (it'll prob'ly be cloudy when you start), turn off the water and macerator pump and close the thru-hull.

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Old 08-17-2020, 09:28 PM   #4
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Assuming you have a washdown pump.... First pump out or dump the tank. Then...

1.open the deck pumpout cap and use the washdown pump to put about 6-8" of water into the tank (putting water into a tank via the deck fitting sends it into the tank at the bottom to stir up sludge and hold it in suspension so it can be flushed out).

2.Open the discharge thru-hull.

3. Turn on the macerator pump while you keep the water running into the tank.

4.When you see only clean water near the discharge thru-hull (it'll prob'ly be cloudy when you start), turn off the water and macerator pump and close the thru-hull.

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You may want to read his post again.
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Old 08-17-2020, 09:56 PM   #5
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He should be able to repeat the operation for the other one. There should be a vented loop--or at least a loop--in each tank discharge line. What did I miss?



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Old 08-17-2020, 10:46 PM   #6
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It’s done all the time and I’ve never heard of anyone having a problem. It would be very hard for sea water to flow backwards through the pump.
Even if the holding tank filled with water, it isn’t a disaster.
Just follow Peggie’s instructions.
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Old 08-17-2020, 11:46 PM   #7
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Even if the holding tank filled with water, it isnt a disaster.
That is exactly my wife's definition of a disaster.

(I follow the same routine at a pump out station. Pump out, refill, pump out again. Don't think the operator is too happy with the extra volume but solids settle.)
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Old 08-18-2020, 01:38 AM   #8
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Hi Jim. Our 430 has two 26 gallon tanks with a diaphragm pump on top for overboard discharge. When underway, open the seacock for one of the tanks, turn the pump on at the electrical panel, and watch the amp gauge. When it is pumping fluid it will move slowly. When the tank is empty the gauge will jump quickly. Turn off the breaker and close the seacock. Repeat with the second tank. Never run the pump with the seacock closed and never leave the seacock open when not being used. If you have a different setup then disregard all of the above.
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Old 08-18-2020, 06:03 AM   #9
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Thanks everyone. I know the forward tank has the loop above the waterline, I'll check the aft tank. Thanks for the information.

Jim
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Old 08-18-2020, 06:20 AM   #10
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He should be able to repeat the operation for the other one. There should be a vented loop--or at least a loop--in each tank discharge line. What did I miss?

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This was his question.

Quote:
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I'm concerned that the waste line from the holding tank is below the waterline and that if the pump fails it would flood the holding tank.

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Old 08-18-2020, 08:55 AM   #11
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99% of holding tanks are partially below the waterline. Worst case scenario, water will only fill the tank to the boat's waterline even if there's no loop in the line. It would have to seep through the macerator pump, which is possible, but would take a long time and could only happen if the thru-hull is left open when not actually in use to empty the tank.



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Old 08-18-2020, 03:00 PM   #12
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One thing I liked about my vacu-flush system was the robust pump. I installed a Y in front of it and used it all the time to empty the holding tank.

It was a very reliable system. Are VFs still popular these days or are they not so?
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Old 08-18-2020, 03:08 PM   #13
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One thing I liked about my vacu-flush system was the robust pump. I installed a Y in front of it and used it all the time to empty the holding tank.

It was a very reliable system. Are VFs still popular these days or are they not so?
Did you not have a separate macerator pump between the holding tank and the through hull for overboard discharge?
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Old 08-18-2020, 04:10 PM   #14
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No I solely relied on the VF pump. My understanding is the the soil gets macerated during its travels from the head to the holding tank
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Old 08-18-2020, 04:32 PM   #15
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No I solely relied on the VF pump. My understanding is the the soil gets macerated during its travels from the head to the holding tank
Not quite. There is no macerator in a Vacu Flush system. The sewage does get broken down somewhat by the force of the vacumn as it forces it through the system into the holding tank.

If pumped out through the deck then it never does go through a macerator. If going overboard via a through hull it should be moved via a macerator sitting between the holding tank and through hull.

So it is important in a VF system to use Marine/RV paper if putting into the system at all.
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Old 08-18-2020, 06:01 PM   #16
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Thanks everyone. I know the forward tank has the loop above the waterline, I'll check the aft tank. Thanks for the information.

Jim
If you have a diaphragm pump for overboard discharge then you don't need a loop above the waterline. There is a duckbill valve that will keep water from going back up the hose.
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Old 08-23-2020, 06:51 PM   #17
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On our MS 400 our anti siphon loop is only for when you're going directly from head to overboard. The macerator pump sucks out directly from holding tank to the through hull. Pumping your tank out probably does not go through anti siphon loop.
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Old 08-23-2020, 07:52 PM   #18
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Not quite. There is no macerator in a Vacu Flush system. The sewage does get broken down somewhat by the force of the vacumn as it forces it through the system into the holding tank.

True. There's no macerator in manual toilets either...but it doesn't matter because solid waste is 75% water anyway, plus it gets broken up going through manual toilets and VFs, so it dissolves very quickly in the tank...and so does quick-dissolve TP. All of which makes maceration, whether in the toilet or the overboard discharge pump totally, unnecessary except in the those rare occasions when the tank might be dumped with an hour or less after solids and TP land in the tank.


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Old 08-23-2020, 08:42 PM   #19
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Take a good look at the pump location. Some are tied to the tank at the tank bottom. They could back flow.

Some like mine, a Sealand diaphragm unit, are mounted atop the tank which may make the pump above the water line. With this the seawater will not backfill the tank even if the pump goofs up.

One way to find out for SURE whether the pump and/or tank is above or below the sea level is connect a clear vinyl hose to the nearest seacock. Install a small valve on the other end of the hose and close it. Drag the vinyl hose over to the tank and raise it above the tank and pump as far as you can. Open the ball valve. The seawater will rise to the same level as the sea water outside. Now you know whether or not you need worry.
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Old 08-23-2020, 08:44 PM   #20
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I have to pump out at sea fairly often because there are no pump out stations where we go. I treat the tank with Noflex to help neutralize it. I usually close the thru Hull valve, but sometimes leave it open if I will be using it again with the next couple days when we are on the move. As previously noted, there is the loop, and these pumps are positive displacement which act like a check valve. So I don’t think not closing the thru Hull is an issue. Long term, yes, I do make sure the valve is closed.
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