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Old 15-05-2010, 17:46   #1
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Gravity-Out Holding Tank Thru-Hull Size

I am going to install a new holding tank for the head and wish to make it a pump-in, gravity-out system to keep things simple. It will have the option of deck pump-out but normal use will be gravity out from the holding tank to the thru hull which will be directly below the tank, about 2 feet away.

My current thru hull is 1 1/2 inch with the usual restrictions with the tail pieces to 1 1/2 hose. Should I change it to a 2 inch thru hull and hose or is the 1/1/2 inch thru hull sufficient? If I am going to change it, I would rather do it now than later but a 2 inch valve looks huge. Thanks.

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Old 15-05-2010, 17:53   #2
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My boat has a gravity fed system and the through hull is larger. However, I've read several posts here from people who use 1 1/2. When one thinks about it, many systems gravity feed out. The pump pushes waste up one side of the anti-siphon loop, but the discharge side works only on gravity.

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Old 15-05-2010, 18:41   #3
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Gems have 1.5" hose.
Seems to work fine for me.
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Old 15-05-2010, 18:50   #4
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I had 1 1/2" gravity on my last boat. Emptied very fast - less than 10 seconds - I guess that's why a broken 1 1/2" thru-hull can sink your boat so fast!

Be sure to have a good size air vent to avoid suction. And, as you may already be planning, big air vents (1" or even 1 1/2") increase oxygen flow to the tank which greatly reduces the stink (much of which is caused by anaerobic activity).

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Old 18-05-2010, 20:27   #5
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A macerator pump is a good investment in the output line of the holding tank for several reasons. One you can use an anti-siphon loop to prevent sinking the boat should you get back flow or too much heel on the boat. And also it "chews up" the solid matter and paper in the tank so that you don't get "floaters" hanging around your vessel in an anchorage.
- - The MSD has a pump system the sends the waste to the tank so a positive macerator pump on the output side quickens the pump out time and also disperses the waste material.
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gravity-out, holding tank, thru hull

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