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Old 23-12-2009, 06:19   #1
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Gravity Feed Question

I need to update the holding tank systm on my Hunter. It currently has a V-shaped holding tank below the water line. Sewage can be discharged via two top ports that have tubes that go to the bottom of the tank. One is pump out, the other has a hand pump to a long vented loop to the seacock. It's not working. It's not an actual waste pump, but rather a water pump and it would be hard to put a real waste pump in the current space. I really don't like the lift system anyways.

I have room to install a 6 or 9 gallon tank horizontally that would be just above the water line. I've thought of doing that and having it gravity empty. I've had a gravity feed system before and also used it on many charter boats. I like it: open the thru-hull and drains. However, those boats had vertical tanks with an inline, larger than normal discharge. On my boat, this system would have to have a horizontal tank and and an aproximately 6-foot almost horizontal run to the sea cock with standard 1 1/2" plumbing. My concern is that with such little pressure, sewage may not discharge and a lot of gunk will settle along the horizontal bottom of the tank. (The old holding tank had about 1/2 of sludge everywhere). While it would have little pressure, it also occurs to me that a tank barely above the waterline, might benefit from having raw water flush back up when sailing, cleaning things a bit if one leaves the valve or sea cock open. (4 week - 2 month cruises moving most days) The through hull is inconveinetly located under a bunk, so I'd probalby use a y-valve or ball valve just below the tank to empty it.

Any experience with similar systems would be appreciated. (tank just above water line with 1 1/2 gravity discharge)

(Optoin 2 is to keep the old tank, install a Y-valve to overboard or tank options. I mostly sail the Bahamas where discharge is usually okay - but I do prefer to hold and dump when not in bays, etc. I really dislike having to hunt around for pump out optoins. Space would make it hard to plumb for overboard, tank pump out and tank overboard. Option 3 is to get one of those Sealand MSD heads that are kind of a cross between portipotis and real heads, that flushes into it's own 5-gallon holding tank below which can then be pumped out. I like the simplicity. Don't like having to hand fill the flush resevoir (2.5 gal.) and wonder if a system with so little water introduced would pump out easily. Any experience with these also apprecited.
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Old 23-12-2009, 07:07   #2
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Drainage, Waste, and Vent (DWV) systems are always installed vertically or with enough downward pitch or slope (many Codes permit 1/8" pitch, but I recommend a minimum of 1/4" per foot of run = 1.5" pitch for 6 Ft. run) to allow gravity to do its job and pull the water/sewage quickly through the pipe.
DWV systems are un-pressurised, and hence must be vented.
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Old 23-12-2009, 07:23   #3
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Thanks for the proper pitch information Gord. Easy to achieve that.
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Old 23-12-2009, 08:37   #4
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Any reason you couldn't use a larger drain? I have owned an RV for many years and am very fond of the simple gravity fed systems they use for waste. I have contemplated using something similar on a Cat. Although it would require a very large thru-hull, i would want to go with at least 2 inch or perhaps 2 1/2 inch. I would also put a fitting in the top of the holding tank so that I could pump high volumes of water into the holding tank to flush it out when draining it.

Just a thought, but it would also allow you to use a RV style toilet that is much simpler and really maintenance free. The biggest drawback I see is the requirement to have the toilet mounted high enough to allow everything to be far enough above the water line.

Thoughts??
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Old 23-12-2009, 08:47   #5
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Thanks Steve - a larger through-hull and plumbing is more expense time and effort than I'm willing to take on at the moment. I'm more and more intrigued with the R.V. type optoins (Actually marine sealand). It wouldn't be above the water line, but installing an antisiphon loop and waste pump would deal with that. I like that this eliminates the head pump issues which I find tend to fail more easily than a simple in line sewage pump, makes one coast guard compliant (fasten pump shut) and if all else fails, you can physically carry the tank out to dump until you get things working again. I could even carry a spare tank in less space than the old holding tank used to take up. My concern is how easily it pumps out given much less water is introduced to the tank.
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Old 31-12-2009, 11:55   #6
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Gravity drain

I replaced a flexible holding tank under the V berth with a gravity drain tank. Although I lost some storage in the hanging locker outboard of the toilet, it was a great move. The bottom of the tank is above the waterline even when the boat is heeled (toilet is fwd of mast). I installed a Lavac toilet that pumps directly into the tank. The seacock for the waste pipe is directly beneath the tank, and I have teed into it a hose to a deck fitting so that the waste can be pumped out if necessary. At sea, to empty the tank I just open the seacocks and whoosh - it's all gone. When offshore I leave the seacock open so that seawater goes up the waste hose and cleans out the tank. No smells, no fuss, no bother.
I had the tank (15gal) made to measure by Raritan.

One of the best upgrades I have made to the boat.
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