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Old 25-05-2016, 08:49   #1
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Is There A Rule About

Hello fellow head users.
I am refitting my head and have a new manual head and bought a new 9 gallon tank. I want to gravity feed the sea cock for this tank and all fittings will be 1 1/2 inches including the sea cock. I am Running pvc pipe up to the tank and over to the sea cock I hang on a bulk head or the hull and will use double clamped sanitation hose at the sea cock,head, and tank to take any vibration or flexing.. Does anyone know of a good rule of thumb about how high the tank should be over the sea cock to gravity feed out the hull? Any insight is appreciated.
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Old 25-05-2016, 09:20   #2
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Re: is there a rule about

It doesn't take much fall at all, if its only a few feet, then only a few inches is needed, don't forget heel though.
Normal plumbing fall is very little like only 1 inch per 10 ft or something, of course a house doesn't pitch and roll about either. I think if you account for boat movement, your fall will be way plenty.

It's called slope of course I guess fall is a local term

TABLE 704.1 SLOPE OF HORIZONTAL DRAINAGE PIPE

SIZE
(inches) MINIMUM SLOPE
(inch per foot)
21/2 or less 1/4
3 to 6 1/8
8 or larger 1/16
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Old 25-05-2016, 09:34   #3
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Re: is there a rule about

Thank You Pilot
I was thinking only a few inches but that would put the tank on the hull above the sea cock. so pitch woul not matter as much. iIt would always be above the sea cock there or on the bulk head.
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Old 25-05-2016, 09:40   #4
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Re: is there a rule about

It's not so much how high it is above the seacock, but how high it is above sea level. My tank is above the seacock, but most of it is below sea level. Guess what would happen if I tried to drain by gravity.

The answer is not a whole lot. If the tank was empty it would fill with water. At best it might drain a third.

I I would guess your tank is not above sea level, so it would not drain well either.
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Old 25-05-2016, 09:44   #5
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Re: is there a rule about

Does anyone know of a good rule of thumb about how high the tank should be over the sea cock to gravity feed out the hull? Any insight is appreciated.

The entire tank has to be above the water line or it can't drain completely...contents will only "fall" down the discharge line to the water level in it. It'll eventually drain as the tank contents mix with the water in the line, but I think you'd want to empty it a lot more quickly.
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Old 25-05-2016, 10:35   #6
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Re: is there a rule about

"Head" pressure (no pun intended) is .433 psi/foot, so a tank 2 ft high has twice the discharge pressure of one at 1 ft. high, both will drain, one's just faster. The gravity systems I'm most familiar with are on the newer French boats where they use a small (6 gallon) green tank mounted about chest high.
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Old 25-05-2016, 10:49   #7
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Re: is there a rule about

thanks SailorChic, Calif Ted and always Peg Hall. I will mount it on the bulk head then. That will put it just above chest high and able to be checked easily.
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Old 25-05-2016, 10:51   #8
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Re: is there a rule about

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Originally Posted by w1651 View Post
thanks SailorChic, Calif Ted and always Peg Hall. I will mount it on the bulk head then. That will put it just above chest high and able to be checked easily.
FWIW, that's the way my friend's Hunter e33 is built. He says it works, no macerator pump.
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Old 25-05-2016, 11:48   #9
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Re: is there a rule about

totally agree with ensuring the header tank is above waterline but if you empty at speed' you might get a sort of venturi effect that might empty the tank even if it is a little below wl.
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Old 25-05-2016, 12:06   #10
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Re: is there a rule about

With the tank bottom no more than maybe 1 ft max above sea level mine gravity fed out quite well. rigid pipe ... leave plenty of hose to avoid issues from flexing and vibration. If not a long run avoid all those clamps, and resistance to flow etc by just using a hose.
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Old 25-05-2016, 12:56   #11
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Re: is there a rule about

...but if you empty at speed' you might get a sort of venturi effect that might empty the tank even if it is a little below wl.

You're far more likely to pull sea water INTO the tank if the thru-hull is open when the boat is underway. I've seen sink drains below the waterline turn a sink into a fountain.
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Old 25-05-2016, 13:12   #12
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Re: is there a rule about

Ours is quite high, chest level, otherwise it won't drain when the boat is healed over.
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Old 25-05-2016, 13:41   #13
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Re: is there a rule about

If you place a shutoff valve in the vent line you can use the head to force the contents of the holding tank overboard. Just close the tank vent, open the discharge seacock, and pump air through the head to force the contents of the tank overboard. This will allow you to place the holding tank below the waterline.
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Old 25-05-2016, 19:09   #14
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Re: is there a rule about

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If you place a shutoff valve in the vent line you can use the head to force the contents of the holding tank overboard. Just close the tank vent, open the discharge seacock, and pump air through the head to force the contents of the tank overboard. This will allow you to place the holding tank below the waterline.

Great idea! I imagine all it would take is a few pounds of pressure to remove a blockage if one occurs. I appreciate all the input guys.
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Old 25-05-2016, 23:35   #15
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Re: is there a rule about

I would be extremely careful about using the head pump or any other pump to clear a holding tank blockage. Tanks are not designed to take any significant pressure and can rupture at fairly low pressure. The consequences of a shower of s**t on the inside of a boat are too awful to risk.
I met a Swedish cat owner where just such an event occurred. Their boat was fitted with the carpet type sidelining and that had to be totally removed. The boat still smelled after some time.
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