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Old 09-19-2017, 09:44 AM   #1
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New holding tank size?

Hi All,

OK, we've established I have to get a real holding tank on the boat and get rid of the 1977 era Type 1 head. Going to do a custom installation, what size holding tank would you recommend? Our last boat (sailboat) had a 15 gallon tank and it seemed to be enough, but we never stayed on the hook like we plan to do now. Thanks for the input.

Dave
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Old 09-19-2017, 09:50 AM   #2
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A boat's holding tank can never be too large (within reason). Marinas usually charge a flat fee for a pumpout. It could be 15 gallons, it could be 100 gallons, same fee. Personally, I believe 15 gallons is too small. Ours is 14 gallons and it can be a problem at times.

I suspect when you get into the project, the available space will determine what maximum size is practical. That would be the size I would recommend.

If you're looking for a number, I would say 30 - 40 gallons.
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Old 09-19-2017, 10:14 AM   #3
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Average human daily output is about 1/2 to 2/3 gallon including solids.
Average flush volume is about 1/2 gallon ( electric or light manual flushing).
Figure 10 flushes per day per person = 5 gallons of flush water/day/person.
Aggressive manual pumping will increase that volume.
So, a good guesstimate is each person aboard will fill up your holding tank to the tune of 5-1/2 gallons plus per day.
Now you can figure how many people and how many days in a non-discharge location you need to plan for and compute minimum tank size.
In this situation "bigger is definitely better"!!!
Then unfortunately you have to compromise to what you can fit on your boat!!!
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Old 09-19-2017, 10:23 AM   #4
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Chrisjs' estimate bear out my boat's tank usage pretty closely, within reason.

WesK has the right approach - find out where you want the tank -- how much area you can afford to fill up, then......fill ALL of it up with tank.
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Old 09-19-2017, 10:40 AM   #5
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Originally Posted by boathealer View Post
Chrisjs' estimate bear out my boat's tank usage pretty closely, within reason.

WesK has the right approach - find out where you want the tank -- how much area you can afford to fill up, then......fill ALL of it up with tank.
Thanks for the input. I just do happen to have a lot of available space down below in the engineering spaces. I was contemplating 30 gallons or so.
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Old 09-19-2017, 10:51 AM   #6
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You can always stretch it out by not flushing each time you pee!! Better to leave it in the bowl rather than short flush to leave it in the holding tank hose run.
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Old 09-19-2017, 11:55 AM   #7
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"I just do happen to have a lot of available space down below in the engineering spaces. I was contemplating 30 gallons or so."

IF you can locate the black tank directly under the toilet , (or close) you can install a RV style toilet.

They are of generous size and quality china , but use perhaps 1/5 1/10 the water to flush.

A 30-40G tank that works like a 200-400 gal tank is great for anchoring out.

But it must have the proper , on top of the tank location .
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Old 09-19-2017, 11:57 AM   #8
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Don't know where you plan to cruise, but I went to a federal park with a zero discharge policy. No dumping of grey water. Fortunately there is a Y valve on the grey water line that allows me to dump it in the holding tank. Don't know if this will become more common between the tree huggers and the EPA.

No such thing as too big a holding tank. The tank weighs almost nothing. Having a big one doesn't obligate you to fill it full. Mine is 80 gallons and I would put a bigger one in if it was easily possible.

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Old 09-19-2017, 12:25 PM   #9
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I agree that you should put in the biggest tank you can. Also put in 2 vents one to each side of the boat so you get air flowing through the tank. Make the vents as large diameter as you can, 1.5" is best but sometimes hard to run that diameter hoses.
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Old 09-19-2017, 01:44 PM   #10
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Quote:
Originally Posted by Dave_E View Post
Hi All,

OK, we've established I have to get a real holding tank on the boat and get rid of the 1977 era Type 1 head. Going to do a custom installation, what size holding tank would you recommend? Our last boat (sailboat) had a 15 gallon tank and it seemed to be enough, but we never stayed on the hook like we plan to do now. Thanks for the input.

Dave
We replaced our tank when we bought our boat. We went with the same size the manufacture had put in. It is a 25 gallon tank. With two of us on the boat it will just make it a week. With visitors on board with us (5 to 6 people), it barely makes it a weekend (if that!). Having tried to get a visitors to use the head properly, I can tell you that 1 out of 5 can fathom the issues a marine head/tank system has and so far only a couple have understood how filling a tank is an issue on a weekend anchoring out. We have a brand new 42 gallon tank all ready to go in the boat this winter. It is the largest tank I can get through the floor opening into the bilge area the tank is set up for (without cutting a larger opening in the floor of the galley).

I would go with a tank larger than 40 gallons if you can get it into the area and in the space you are wanting it at. 50+ gallons would be even better if you will have any guests on board overnight at all. You will not be disappointed with a larger tank.
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Old 09-19-2017, 02:27 PM   #11
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Remember that the waste weighs in at 7 + lbs. per gallon. 50 gal = 350 lbs. Place accordingly to support its substantial weight when full and maybe hitting a sea that might pound that weight on its support. Also, have really good straps so no sideways motion can happen.
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Old 09-19-2017, 02:40 PM   #12
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Quote:
Originally Posted by Dave_E View Post
OK, we've established I have to get a real holding tank on the boat and get rid of the 1977 era Type 1 head. Going to do a custom installation, what size holding tank would you recommend? Our last boat (sailboat) had a 15 gallon tank and it seemed to be enough, but we never stayed on the hook like we plan to do now. Thanks for the input.
Quote:
Originally Posted by Chrisjs View Post
Average human daily output is about 1/2 to 2/3 gallon including solids.
Average flush volume is about 1/2 gallon ( electric or light manual flushing).
Figure 10 flushes per day per person = 5 gallons of flush water/day/person.
Aggressive manual pumping will increase that volume.
So, a good guesstimate is each person aboard will fill up your holding tank to the tune of 5-1/2 gallons plus per day.
Now you can figure how many people and how many days in a non-discharge location you need to plan for and compute minimum tank size.
In this situation "bigger is definitely better"!!!
Then unfortunately you have to compromise to what you can fit on your boat!!!
Quote:
Originally Posted by Dave_E View Post
Thanks for the input. I just do happen to have a lot of available space down below in the engineering spaces. I was contemplating 30 gallons or so.

Unless where you boat -- ever, always -- a No Discharge Zone (didn't see an answer to that in the other thread), and if money isn't tight, I'd consider a Raritan Hold 'N' Treat system, with as big a holding tank as you can fit.

In any case, I can tell you our 40 gallon tank (so labeled) actually holds more like 26 gallons -- partly because of the placement of the innies and the outies, partly because of the thickness of the sidealls, perhaps due to some creative labeling on the part of the manufacturer.

And I can tell you that the two of us can fill that rascal up in about 4-5 days. That's with generous freshwater flushing, to manage scale build-up in the toilet discharge line. Still, even with minimal flushing, 6-7 days would have us looking for a pump out.

If your pump out station all charge only $5/shot, it would take longer to amortize the cost of the treatment system... but our normal place wasn't State subsidized, so they charge a portion of what they in turn are charged to pump their on-shore holding tank out... which means we usually pay $15/visit. Plus tips. At that rate, a Hold 'N' Treat system could pay for itself in a year or two... depending on your "rate of deposit" (so to speak).

Back to size, whichever way you go... as big as you can fit. Which means getting it in the door, into the boat, into the space, reachable for plumbing access, etc.

-Chris
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Old 09-19-2017, 03:14 PM   #13
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We can easily go 3 weeks in a 35 gallon holding tank.
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Old 09-19-2017, 04:33 PM   #14
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+1 for 2 large vents...as straight and short as you can....P & Stbd.
I'd also recommend slight slope to corner where pump out ftg is located vs level. Better chance of getting the most out of the tank
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Old 09-19-2017, 05:12 PM   #15
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I agree with Chris js, I have a 100gall and with 4 on board we manage every 5-6 days. Inpractice we pump between islands and plan accordingly, one of the memebers mentioned air flow, very important to keep pumping odors down, Big pipes each side lots of oxy for the good bacteria. Plus the weight factor my 100gall=750lbs. in my 54ft I notice the bow down a inch or two when full. Often though it masy psy to move the hlding tank to the center of the engine room to stiffen the boat a tad when full etc.
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