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Old 25-03-2019, 12:38   #1
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Holding tank?

I have 3 separate water tanks on my boat. Iím wondering if it would be possible or if there were a reason not to convert 1 of them into a black water holding tank? I was told I have to have one and would like not to have to take up any more of my not so much space. TIA.
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Old 25-03-2019, 13:11   #2
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Re: Holding tank?

Welcome aboard! In theory what you propose is possible, but I reckon the issue will be whether the plumbing has room to fit, and do you have the access needed to install the needed plumbing. What kind of boat do you have? Personally I’d probably really try to avoid that change, and I am wondering where was the waste water going before? There are heads that have a tank attached below the bowl, that if it fits, might be a solution too.
Check “Dometic model 711-m28 toilet system”
I don’t have much room in my boat either, and I have considered that one. Or you can try a dessicating type head too like “C-head.”
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Old 25-03-2019, 13:29   #3
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Re: Holding tank?

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I have 3 separate water tanks on my boat. Iím wondering if it would be possible or if there were a reason not to convert 1 of them into a black water holding tank?
It may be possible unless the water tanks are metal...metal is not recommended for sewage holding because urine is so corrosive that it'll turn any metal tank--even 316 steel--into a colander in as little as 5 years.

Location relative to the toilet is another factor 'cuz the last thing you want is long and/or convoluted convoluted plumbing runs.

I can prob'ly give you a definitive answer if you can give us a bit more information.

--Peggie
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Old 25-03-2019, 14:47   #4
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Re: Holding tank?

Peggy is right.
I purchased our trimaran when it was 5 years old.

The seams of the ss tank were already leaking in several places.
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Old 25-03-2019, 15:49   #5
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Re: Holding tank?

The boat I have is a 1976 Pearson 35. Where my side tank is located is just below the port side couch. There’s plenty of room to get to the tank. I also have a tank on starboard same location and v berth. As far as I know the system in it now is a direct dumping type where it just goes outside. I recently purchased it and the previous owner said there was no holding tank.


All my tanks are molded in fiberglass. The tank I’m thinking of is about 3ft from the head on the same side of the boat.

I have tons of work to do and am no where near the boat atm. I bought it for $300 and found $200 while cleaning it up. It sank at the slip due to no functional bilge pump in hurricane Florence. I’m just trying to figure a few things out while I’m away so I have a game plan when I’m able to return the boat and begin the refitting since I now have it cleaned.
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Old 25-03-2019, 16:40   #6
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Re: Holding tank?

Perhaps a porta potti is in your future for now just to have something functional and legal while you work on the boat? They work fine and they are not too expensive.
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Old 25-03-2019, 17:34   #7
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Re: Holding tank?

The marina I’m in has a restroom. The boat is in the water.
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Old 25-03-2019, 19:32   #8
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Re: Holding tank?

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Perhaps a porta potti is in your future for now just to have something functional and legal while you work on the boat? They work fine and they are not too expensive.
Don may be onto something, especially if the existing toilet is more than a few years old: A self-contained system...an "MSD" portapotty

The "MSD" designation in the model name/number means it has fittings for a pumpout line and vent line, and is designed to be permanently installed (actually just sturdier brackets than portables, so you could still take it off the boat if you absolutely have to), which means that although it's still called a PORTApotty, you don't have to carry anything off the boat to empty it. If you want to be able to dump it at sea, a y-valve and overboard discharge pump (macerator pump or manual diaphragm pump if you don't want electric) in the pumpout line to the thru-hull used by the existing toilet is all you'd need.

A 5-6 gallon model holds 50-60 flushes...you'd need at least a 30 gal tank to hold that many from a manual marine toilet. No plumbing needed except a vent line and pumpout hose--so no new holes in the boat...and -0- maintenance needed except for rinsing out the tank--which you can do with a bucket while it's being pumped out. Total cost including the pumpout hose and vent line is about $200--a fraction of what you'd spend for toilet, tank and all the related plumbing needed. And the best part is, you have all the advantages of a toilet and holding tank without giving up a single square foot of storage space.

Check out the Thetford 550P MSD (if you can still find one, Thetford discontinued it) and the Dometic/SeaLand 975MSD Sanipottie. SeaLand SaniPottie 975 MSD (shown on retailer's site because it's impossible to find anything on the Dometic site any more).

It may not be as elegant as a "real" marine toilet, but it solves your problem for very little expense.


--Peggie
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Old 25-03-2019, 19:55   #9
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Re: Holding tank?

Yeah that's why I mentioned the Dometic model 711-m28 previously, I believe it has a full size or nearly full size bowl. The 975 MSD bowl is a bit smaller than the Thetford model and smaller than full size. The Thetford MSD is still available at Defender.
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Old 26-03-2019, 04:39   #10
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Re: Holding tank?

Greetings and welcome aboard the CF, Tia.
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Old 26-03-2019, 05:08   #11
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Re: Holding tank?

3 water tanks on a Pearson? Any chance that the one closest to the head is, in fact a holding tank and the previous owner just didn't know his boat very well?
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Old 26-03-2019, 06:34   #12
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Re: Holding tank?

Quote:
Originally Posted by Don C L View Post
Yeah that's why I mentioned the Dometic model 711-m28 previously, I believe it has a full size or nearly full size bowl. The 975 MSD bowl is a bit smaller than the Thetford model and smaller than full size. The Thetford MSD is still available at Defender.
Here's the link to the 550P at Defender Thetford 550P MSD on Defender

The 711-M28 SeaLand Traveler at Defender could be considered an "msd portapotty," but it's a highly glorified one that draws flush water from the fresh water supply with a china RV gravity toilet atop a 9 gal tank that has way too big a footprint to fit in most sailboat heads including his. But it's one of my favorite systems for boats with heads big enough for the tank to fit. Unfortunately Dometic doesn't offer it with a choice of tank sizes.

--Peggie
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