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Old 02-11-2014, 11:23   #1
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Join Date: Oct 2010
Location: Blue Mountain Ont
Boat: Beneteau Oceanis 400 40 ft "Camp David
Posts: 485
Slope Holding Tank

I have a 15 gallon holding tank on my Beneteau, The pump out discharge fitting is located at the lower side of the end wall and thus, it is just above the lower 90 degree corner.

When the boat is pumped out the waste does not completely empty as the 1.5" outlet starts to suck air prior to all of the waste emptying. It is about an inch up the end wall. It must leave about 2 or 3 " inches of waste, that does not seem much however it is about 1 to 2 gallons.

I am planning to build a sloped platform for the tank to sit on so that the waste will run down to the outlet and cover it prior to sucking the last of the waste then air ( no air ).

My question: Is there a suggested slope for the tank, Too much angle might leave solids behind and too little, the waste will not run down hill.

Given the length of the sloped side of the tank is 25" I was thinking about a two inch slope, this does not seem very much.

Any suggestions??

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Old 02-11-2014, 11:53   #2
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Join Date: May 2011
Location: Miami Florida
Boat: Ellis Flybridge 28
Posts: 3,628
Re: Slope Holding Tank

Sewer pipe is sloped between 1/8 and 1/4 inches per foot. Don't know if that applies to what you're doing. Two inches sounds good to me.
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Old 02-11-2014, 12:30   #3
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Join Date: Mar 2014
Location: New Westminster BC
Boat: Windward 914
Posts: 13
Re: Slope Holding Tank

I had a similar issue, but my tank is only 6 gallons, so very small. I put a 1 1/2" slope over the 20" span and it made a difference. I also have my pickup squished lower into the tank, so it picks up at about 3/4" from the bottom. I would think that the boat sloshing around in wave action keeps any solids from not settling near the pickup. I make a point of rinsing the tank every time I empty. Of course it is hard to say for sure what is going on in there. I don't think I really want to know!
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holding tank

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