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Old 03-02-2016, 22:35   #1
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Holding tank discharge via thru-deck opening

I am thinking of removing the thru-hull discharge of my holding tank and pumping out my holding tank only thru-deck (either by vacuum at pump-out stations or by means of a Whale pump). In the latter case, I would need to attach a short hose in the deck opening to divert the waste over the starboard side.

On my Hunter 31 the waste from the toilet goes directly to the holding tank. The holding tank discharge hose goes to a Y-valve, one end goes to the thru-deck pumpout port, the other goes through a Whale pump, a siphon, and then to the thru-hull valve. My thru-hull valve is broken in the closed position (unlikely to repair it soon) and the space for hoses from the tank under the V-berth to the toilet is very limited. It was a pain to remove the three hoses and it would be a bigger pain to push three new hoses and do the siphon.

I am thinking of connecting the two output hoses from the Y-valve with a T-connector (one goes through the Whale pump, the other straight to to T-connector) then having a single hose going to the thru-deck opening. When I want to empty the holding tank manually (not often), I would divert the flow through the Whale pump and then insert a short hose/fitting in the thru-deck opening (from the deck) that will lead the waste over the starboard side to the water. It will not be a pretty sight but will do. In this way, I can eliminate the thru-deck valve and siphon completely and have a much simpler system.

I have two concerns: 1) can the vacuum pump-out damage my Whale pump and 2) will the Whale pump have enough power to push the liquid up to the deck opening (about 2.5 feet vertical distance, 5 feet horizontal distance)? What do you think?
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Old 04-02-2016, 08:19   #2
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Re: Holding tank discharge via thru-deck opening

1) can the vacuum pump-out damage my Whale pump?

Yes, it could. What you want to do would also make the pump a blockage in the pumpout line that would interfere, if not prevent entirely, a pumpout from pulling anything out of the tank.

2) will the Whale pump have enough power to push the liquid up to the deck opening (about 2.5 feet vertical distance, 5 feet horizontal distance)?

Check the owners manual for your model for those specs. Even if it can, the pump will still be a blockage in the pumpout line.

There is a third consideration: Even if you come up with a way to rinse it out, you're gonna have about 5' or more of drippy hose that's still likely to be smelly to store aboard. Depending on the wind and other factors, you're also likely to have a stained hull after dumping.

So all things considered, this is NOT a great idea. Bite the bullet and repair--or replace if necessary--the seacock.
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Old 04-02-2016, 09:16   #3
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Re: Holding tank discharge via thru-deck opening

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Originally Posted by Pizzazz View Post
... I would need to attach a short hose in the deck opening to divert the waste over the starboard side...
Don't be "that guy."
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Old 04-02-2016, 09:39   #4
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Re: Holding tank discharge via thru-deck opening

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Don't be "that guy."

No, be Los Angeles and dump 230 million gallons a day! Treated Sewage to Be Dumped Out Just One Mile From Los Angeles Beaches For a Bit - The Environment - Curbed LA
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Old 04-02-2016, 09:53   #5
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Re: Holding tank discharge via thru-deck opening

Fix your seacock
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Old 04-02-2016, 09:56   #6
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Re: Holding tank discharge via thru-deck opening

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Corruption in city hall and in the water.
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Old 04-02-2016, 10:43   #7
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Re: Holding tank discharge via thru-deck opening

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Originally Posted by peghall View Post
So all things considered, this is NOT a great idea. Bite the bullet and repair--or replace if necessary--the seacock.
I think you're being too kind. This a BAD idea!
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Old 05-02-2016, 08:26   #8
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Re: Holding tank discharge via thru-deck opening

If you have a ball valve screwed to a thru-hull, it's easy to change the valve while the boat is in the water. You can plug the thru-hull from outside the boat or use a Seabung to do it without going over the side.
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Old 05-02-2016, 08:54   #9
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Re: Holding tank discharge via thru-deck opening

We carried a spare manual bilge pump connected to a banjo fitting that could be used on deck to empty the holding tank offshore. We would only use this when leaving (outside) a country that didn't allow direct discharge (ie leaving US for Bahamas). Once you leave the US with a half full holding tank, it's difficult to find a pumpout in the next country... You'll need a way to empty the tank.

I bought the banjo fittings from Mcmaster-carr to connect to the standard on deck fitting. There was then about two feet of 1.5" hose, the manual bilge pump and then another few feet of 1.5" to run overboard. After use, all would be disinfected and put in a Rubbermaid container in the bilge storage. We only had to use it in a few areas, but saved travelling for months with a holding tank of old waste.

The typical option is to have a 3-way valve mounted below deck, but I didn't want the permanent hose run, as we didn't have the room for it in the holding tank/head area.

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edit: reread the OPs post. Fix the seacock!
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Old 05-02-2016, 09:24   #10
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Re: Holding tank discharge via thru-deck opening

Oh yuck. Deep dodo all over the deck, in the cockpit, and even perhaps down below. A hull mount opening way cleaner and easier to wipe down when the inevitable happens, and it will. Saw a yacht getting sucked out when the system "burped" due to an air lock. Covered that boat with a lot of flying, well you know what.
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Old 05-02-2016, 09:57   #11
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Re: Holding tank discharge via thru-deck opening

Thinking down the road, making a change this significant will likely make it more difficult to sell the boat when the time comes.

We replaced a total of 37' of sanitation hose on our Norseman 400. Not that bad of a job really and very much worth the effort. The Trident brand was a bit stiff to work with but slid into place nicely and completely contains any odor.
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Old 05-02-2016, 10:09   #12
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Re: Holding tank discharge via thru-deck opening

Ever see the movie "Christmas Vacation", Cousin Eddy with a hose at the curb...

very appealing
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Old 05-02-2016, 12:50   #13
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Re: Holding tank discharge via thru-deck opening

Thru-hull fitting much easier and cleaner to use.
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Old 05-02-2016, 13:01   #14
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Re: Holding tank discharge via thru-deck opening

Unless you repair as you should, why change anything? Leave the thru hull shut, same as your mods right?
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Old 05-02-2016, 13:05   #15
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Re: Holding tank discharge via thru-deck opening

If this was such a good idea, more boats would have it.

They don't make 'em the way they do for the convenience of the builders alone, let alone the owners.

Mostly, they do it because it's been proven to WORK.

Fix the seacock. I had the same leaking seacock, I replaced it. I actually had the yard do it, one of the few times I EVER let anyone else work on my boat. It was worth the $200. WELL worth it.

The other two times: standing rigging replacement and a macerator pump.

Seeing the pattern here?
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