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Discussion Starter #1 (Edited)
I have read most of the articles here on this issue, but still need a bit of advice.

Toilet
It looks like a Lavac vacuum toilet uses a lot less water than a macerator evacuated toilet. But do they use enough water or to do they clog?

Configuration
I like the set up where all waste going through the holding tank and then to a Y valve with one line to the deck and the other to a macerator pump and overboard.

Size
For living aboard for two people.
berths * days between emptying * 2= size in litres
2 * 15 * 2 = 60 litres -> /3.8 = 16Gallons

This seems pretty large, what do other people think.
I am not sure I have room for a tank this big.

Material
I have seen the HDPE (bread board) tank stuff, but the limitations of the shapes and fit would make it very difficult to get a large enough tank without effectively losing an entire hanging locker (at least in a functional sense). So I was thinking of making the tank out of fiberglass and glassing it into the inside of the hull, below the toilet. There is already a semi structural closed area of close to the right size. It would be a much bigger job than a HDPE bread-board prefabbed.
My question is what internal lining in an FRP tanks should be used to ensure no leakage?
Any construction techniques or tips to avoid leakage?

Shape
The FRP tank would also be long and flat. i.e. 3 feet long, 2 feet across and would follow the hull from 1 inch to about 10 inches high. So size would be 3*2*10/2=30cubic feet or 91cm*5cm*25cm/2=568cm (57 litres)
(Divide by 2 because bottom of tank follows hull form.)

Position
Better under the toilet or above?
Better close to the toilet?
The output uptake should be at the lowest point in the tank and run directly outside.

Venting
How long can the vent run be, e.g. to the top of the windgen pole?

Tank Level
What indiactors are useful here?

Siphon Valves
These have never worked properly for me since they clog up with salt.
If the toilet is above the waterline, they should not be required. Do you agree?

In Practice
If you do all this do people still get problems with smells?
 

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Fiberglass is not a good material for the tank. You will have a lot of osmosis damage and most probably the tank will leak very soon. HDPE is a better material.

Your size is big. 5 liters/day/person is the minimum.

Makesure you use high quality hoses otherwise they will cause odor. You can cover them with aluminum tape (used for air ducts in buildings) to overcome odor.
 

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celenoglu,
My boat is a fiberglass tank and has been keeping water on the right side for 40 years.

Matt, there are a couple of articles on the internet on builidng a wood/FRP tank and an all FRP tank. Seems pretty simple. What I took away from them was to use a few layers of mat and resin rich on the wet side.



Here are a couple of links. There's more out there.

http://www.epoxyworks.com/18/pdf/tanks.pdf
MARKSTANK
 

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Fiberglass is fine for a holding tank, provided it is properly constructed... after all fiberglass boats have lasted many years...

Long flat tanks have a problem in that they'll be harder to empty completely. If you leave much waste sitting in the bottom for extended periods of time, it can eventually build up and reduce tank capacity. A taller, more vertically oriented tank is usually a better idea IMHO.

If the tank inlet is high on the tank, as it should be, having the tank higher is probably a good idea, since it may allow you to gravity drain the holding tank.

The vent line should be at lest 5/8", preferably 1", and as short and direct as possible to maximize oxygen getting into the tank. Anaerobic bacteria is the stuff that makes the tank stink... Aerobic bacteria are good...

Tank level—can be managed in several ways, but I'd recommend putting an inspection hatch in the top of the tank with a clear deckplate, like the Beckson I have installed, since that makes it easy to visually inspect how full the tank is as well as gives you an option if you need to clean out the tank.

I like the Lavac, as it is almost bulletproof and generally uses less water than traditional designs.

Siphon valves— if the head is above the waterline on all angles of heel, you may be able to eliminate it...

One major source of head odors is not using fresh water to flush the head when the system is going to be left alone for an extended period of time. Salt water micro-organisms left in the bowl and hoses will die and leave an awful stench.
 

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Discussion Starter #5
Size

Your size is big. 5 liters/day/person is the minimum.
So for 2 people, 15 days between pump outs would make it
5*2*15=150 litres? -> divide by 3.8 to get 40 Gallons

So since 1000 cubic cm is 1 litre, to get 150K cubic cm the rough dimensions would be 100cm*15cm*10cm, which is workable.

What size tanks do other boats have?
Are there downsides to excessive tanks size?

Are folks happy or unhappy with your tank size?
 

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Discussion Starter #6 (Edited)
A Difficult Problem

Fiberglass is fine for a holding tank, provided it is properly constructed... after all fiberglass boats have lasted many years...
That is reassuring.
Are there any special advantages to HDPE?
Is it possible to fabricate your own HDPE tanks?

Long flat tanks have a problem in that they'll be harder to empty completely. If you leave much waste sitting in the bottom for extended periods of time, it can eventually build up and reduce tank capacity. A taller, more vertically oriented tank is usually a better idea IMHO.
That is what I feared. So a flat tank in the floor under the bowl would probably not work well. To get the right verticality I might have to rip out the sink and redo the vanity so a holding tank can go inside the cabinet. I am not at all keen to lose the hanging locker to a holding tank.

If the tank inlet is high on the tank, as it should be, having the tank higher is probably a good idea, since it may allow you to gravity drain the holding tank.
I think I will have a macerator pump (a requirement in may areas where discharge is allowed) to pump out the tank. As I understand this pump is positioned after the Y valve on the overboard seacock outlet. i.e. pumpout stations have their own pump.

The vent line should be at lest 5/8", preferably 1", and as short and direct as possible to maximize oxygen getting into the tank. Anaerobic bacteria is the stuff that makes the tank stink... Aerobic bacteria are good...
The vent is clearly very important. If the vent is too small or too long the tank will be anaerobic (bad), but if the vent is not positioned far enough away from the cockpit it could stink (also bad).
Would it work to have a small diam and short run inlet vent and a longer larger diam vent running up the wind gen pole?

I guess I am concerned that for a very short vent there is a risk of waste coming out of the vent when it is full and the boat is in rough sees and heeled. I suppose it is important that the vent exit is high enough above the tank that this cannot happen. (Which makes my planned config difficult, since the locker where the tank could go would be very close )

Where do folks position the vent outlets?
Anyone regretted their vent or tank installations or thinking of changing them?
What about using an air inlet vent in combination with an air outlet and a small fan to force air through?
Can folks ever smell the tank when outside on a calm day?

I guess what I am saying is that if there is any smell from the tank, I would consider the installation a failure and certainly my wife would justifiably freak out! So no matter what the cost I cannot have a smelly tank. So if holding tanks do lead to unpleasant odors, perhaps a Lectrasan or some other processing tank might be needed. So I really need to know, do ALL holding tank installations stink or just badly vented tanks?

I like the Lavac, as it is almost bulletproof and generally uses less water than traditional designs.
Does it use enough water to keep things moving so to speak?
 

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Discussion Starter #7 (Edited)
Thanks for the FRP Tank Links

Thanks for the links, very helpful!

Reading the MARKSTANK article (quoted below) I did wonder about how the tank is joined together again. Looks like it says create a rough scarfed joint and glue together using epoxy and chopped mat. But chopped mat would wick and cause osmosis and leakage. So I guess I would need to cut an access port in the top of the tank and sand down the internal join and coat it with epoxy, to make sure it is waterproof. Then I guess strengthen the join on the outside as well.

MARKSTANK Article said:
"Sand or grind (a 7-inch right-angle grinder does a great job here, but a belt sander works) a 3-inch scarf on the inside of the top and the outside of the bottom. Great
accuracy is not needed; just draw a reference line at 3 inches to start and taper to a
feather edge. Wet out the scarf with straight catalyzed resin. Make a glue by adding
chopped fibers (easily made by cutting your scraps into 1/4-inch pieces) to the resin,
apply this to the joint, and assemble the two halves of the tank, smoothing the
squeezed-out glue with a putty knife. Be sure to tap the top into place until it is parallel
with the bottom. Wrap another layer of biply around the sides of the tank. If you want a
non-tacky finish, you can either use finishing resin for this step or wrap the tank in
plastic wrap while it dries. The top and bottom can likewise be coated with finishing
resin or with plain resin and covered with plastic, but this is entirely optional."
Next step is probably to create a lid for the access port and screw it down using 5200 as the sealant. Probably best if the screws don't penetrate the inside of the tank, so the tank needs to be quite thick at the top where the access port will be.

Does this look ok?
 

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That is reassuring.
Are there any special advantages to HDPE?
Is it possible to fabricate your own HDPE tanks?
Not really, since most HDPE tanks are either rotomolded or blow molded in a single piece. The problem is that not much sticks well to HDPE, and IMHO it would be rather difficult to seal a homemade HDPE tank well.

That is what I feared. So a flat tank in the floor under the bowl would probably not work well. To get the right verticality I might have to rip out the sink and redo the vanity so a holding tank can go inside the cabinet. I am not at all keen to lose the hanging locker to a holding tank.
Even a cubical tank is workable... wide, flat tanks just are too difficult to drain completely, unless the bottom is fairly heavily sloped.

I think I will have a macerator pump (a requirement in may areas where discharge is allowed) to pump out the tank. As I understand this pump is positioned after the Y valve on the overboard seacock outlet. i.e. pumpout stations have their own pump.
I prefer using a non-electric diaphragm waste pump rather than a macerator. If you have a problem with the electrical macerator pump at sea, you're stuck with a full holding tank.

The vent is clearly very important. If the vent is too small or too long the tank will be anaerobic (bad), but if the vent is not positioned far enough away from the cockpit it could stink (also bad).
Would it work to have a small diam and short run inlet vent and a longer larger diam vent running up the wind gen pole?

I guess I am concerned that for a very short vent there is a risk of waste coming out of the vent when it is full and the boat is in rough sees and heeled. I suppose it is important that the vent exit is high enough above the tank that this cannot happen. (Which makes my planned config difficult, since the locker where the tank could go would be very close )

Where do folks position the vent outlets?
If the tank is mounted low, this is usually not an issue, if it is mounted higher, this can be an issue on a monohull.

Anyone regretted their vent or tank installations or thinking of changing them?
What about using an air inlet vent in combination with an air outlet and a small fan to force air through?
Can folks ever smell the tank when outside on a calm day?
If the tank is well ventilated, it shouldn't be too much of an issue... the cause of the holding tank really reeking is usually anaerobic bacteria in the tank, and having a well ventilated tank solves that. A solar powered fan and using two vents would be a great solution, but a bit overkill IMHO.

I guess what I am saying is that if there is any smell from the tank, I would consider the installation a failure and certainly my wife would justifiably freak out! So no matter what the cost I cannot have a smelly tank. So if holding tanks do lead to unpleasant odors, perhaps a Lectrasan or some other processing tank might be needed. So I really need to know, do ALL holding tank installations stink or just badly vented tanks?

Does it use enough water to keep things moving so to speak?[/
If you're serious about this, I'd highly recommend you sign up at the SBO forums and ask Peggie Hall directly. She's an expert on their forums. Why ask us hacks when you can ask Peggie??? :D It would help if you said what kind of boat you have and where the head is located in it. It sounds like you have a boat with an aft head.
 

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Discussion Starter #9
A Possible Solution

Even a cubical tank is workable... wide, flat tanks just are too difficult to drain completely, unless the bottom is fairly heavily sloped.
Thanks, your comment has given me an idea that I think will work. I am thinking I will cut a hole in the non-structural bulkhead between the hanging locker and the vanity and shape a tank with wider almost cubic base but with less height. I can get the volume and keep the top of the tank well below the vents. This will use up some space at the back of the vanity unit which is not that accessible anyway. It also means a smaller intrusion into the locker, so the locker will remain functional.

I prefer using a non-electric diaphragm waste pump rather than a macerator. If you have a problem with the electrical macerator pump at sea, you're stuck with a full holding tank.
Yes I know other experienced folks have also used this setup. Though in some areas it is a requirement to only discharge macerated waste. Whatever pump I use will have to be self priming.

If you're serious about this, I'd highly recommend you sign up at the SBO forums and ask Peggie Hall directly. She's an expert on their forums. Why ask us hacks when you can ask Peggie??? :D
Absolutely serious! What choice is there. Have to have a holding tank of some kind these days. Thanks, I will check out the suggested forum, but I must say I am very happy with the advice I have received here.
 

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Bender of Nails
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Just an addition here - it sounds like you've got the answers you're looking for.

Sealand makes a carbon filter that you plumb into the vent to kill odors. From the ones I've seen (smelled...), they are probably 75% effective vs no filter. No idea what they cost though - for us it's just another service item.

In powerboats up to about 45', the 10 gallon Sealand holding tank is pretty ubiquitous here - I don't think I've ever been asked to install a bigger one.

FWIW I plan to build my own as well from epoxy and ply and the inner coating of epoxy will probably be laced with graphite to make it harder and more 'slippery'. I'd caution against using epoxy with mat because mat requires styrene based resin to dissolve into a 'mush'. It will physically work to build up bulk, but it's pretty wasteful of epoxy IMHO.

I used to work for a shop that built sportfishers to about 30'. They used a solid 'glass tank made over a triangular plug mold that fit in the forepeak against the stem. I always wondered why they gelcoated the outside but not the inside of the tank...ewwww.....
 

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Glad to help... Post photos of the installation when you're done and let us know how it goes. :)
 

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Just an addition here - it sounds like you've got the answers you're looking for.

Sealand makes a carbon filter that you plumb into the vent to kill odors. From the ones I've seen (smelled...), they are probably 75% effective vs no filter. No idea what they cost though - for us it's just another service item.
Wouldn't recommend it, as it will restrict airflow drastically, leading to a stinky holding tank, leading to needing the filter... good ventilation is a better, and cheaper solution in the long run.

In powerboats up to about 45', the 10 gallon Sealand holding tank is pretty ubiquitous here - I don't think I've ever been asked to install a bigger one.

FWIW I plan to build my own as well from epoxy and ply and the inner coating of epoxy will probably be laced with graphite to make it harder and more 'slippery'. I'd caution against using epoxy with mat because mat requires styrene based resin to dissolve into a 'mush'. It will physically work to build up bulk, but it's pretty wasteful of epoxy IMHO.

I used to work for a shop that built sportfishers to about 30'. They used a solid 'glass tank made over a triangular plug mold that fit in the forepeak against the stem. I always wondered why they gelcoated the outside but not the inside of the tank...ewwww.....
Chopped Strand Mat is usually held together with a binder that dissolves in the styrene found in polyester/vinylester resins. However, there are now some "epoxy" friendly CSM mat materials available. I prefer to build thickness with cloth, because it is stronger than CSM is.
 

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Bender of Nails
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SD, you make a good point (as usual) about the use of vent filters. What I've seen in boats that come back in the fall that have seen use during the season though, is that the pumpout process draws air into the tank regularly enough that the filters seem to work pretty well. Most folks with newer boats seem to be pretty thorough about rinsing the tank after pumping out as well. Your post does remind me of a caveat I should have mentioned when suggesting the filter in the first place. The Sealand tanks we use have them already, but if I were fabricating a tank, I'd definitely add a tanksaver valve. If that filter ever jams up after overfilling the tank it probably won't clear itself...

I agree with you on building up with cloth. The whole point of epoxies are that they are stronger in laminate so you need less. In a conventional layup, mat is there primarily as a means to add density to the laminate, like filling the weave between structural cloth layers or reinforcing what will become outside corners within a female mold. To use the same technique with epoxy demonstrates an ignorance of the material's advantages.
JMHO of course.

Matt, you were asking about level indicators as well.
Again with the Sealand stuff (not a shill, it's just what I've used most), their Tankwatch system is pretty idiot-proof. It uses a series of coloured LEDs to give you an indication of the tank's level. Their are a couple things to watch when installing them, but they work pretty well once set up. If you leave a long enough pigtail of wire (or use disconnects), it can be used as an inspection port /cleanout as well.
The Heart TankTender system takes a bit on the part of the user and costs a fair bit more, but there's only a 1/8" air line going into the tank and the same guage can read multiple tanks containing different fluids. Oh, and it requires no electrical power to run...

Regarding your concerns about inspection plate fasteners penetrating the tanks: you could either add a plywood ring to the top of the tank, which is what it sounds like you were alluding to, or you could use anchor nuts (tee nuts) and glass right over the bottom of them.
 

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Tank size calculation

So for 2 people, 15 days between pump outs would make it
5*2*15=150 litres? -> divide by 3.8 to get 40 Gallons

So since 1000 cubic cm is 1 litre, to get 150K cubic cm the rough dimensions would be 100cm*15cm*10cm, which is workable.

What size tanks do other boats have?
Are there downsides to excessive tanks size?

Are folks happy or unhappy with your tank size?
100cm*15cm*10cm is only 15,000 cubic cm (15 litres). If you want 150 litres (150,000 cubic cm) you will need to increase the volume by ten times.
 

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Cal 9.2 SilverSwan
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My boat had a 10 gallon Vetus bladder tank that developed odor issues, I replaced it with a custom fabbed tank just behind the head to keep the hose length to a minimum. It is a 15 gallon epoxy and fiberglass tank. 12 layers thick with a slick coat on the inside and 45 degree fillets on all internal corners. It takes up 1/2 of the hanging locker, but the stowage that was consumed by the bladder is now usable. A HDPE tank would have been prefered, but I could not find on that fit the space and the cost of a new mold was too much $$$.
 

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