Installation of a Dometic Sealand M28-711 Marine Gravity Toilet

[Nick F]

First - thanks to Peggy for making me aware of the existence of this type of toilet. I had recently purchased the boat (a 1976 CHB34) and the existing sanitation system was totally seized up and needed completely replacing. I threw out holding tanks, macerators and toilets and started again.

The target was to have one functional, legal head for the 2021 season. Here in Vancouver our yacht club has several outstations with shore facilities, so I did not need enormous holding capacity. Plus, the gravity toilet uses a minimal quantity of flush water compared to installations where the waste has to be transported horizontally to a separate holding tank. I also liked the ability to use fresh water for flushing, thereby reducing odours and corrosion.

I planned to be able to empty the built-in holding tank by pump-out and by overboarding via the original underwater thru-hull (when far enough from land). For this I decided on the attached schematic. Included in this proposed design was the plan to overboard by pressuring the tank vent with air. In this way, no pumping of the waste is required (no macerators, no diaphragm pumps).

I should mention that I do not put any paper into the sanitation systems on my boats. All paper goes into a tidy and gets disposed of ashore.

The new toilet was a tight fit in the space, but it was possible. One immediate problem was that the new tank covered the access to the old overboard thru-hull and this required some DIY to make a remote lever to operate the valve. An immediate 90 degree piping turn was also required.

I used two PVC swimming pool type valves to direct the waste to overboard or to the pump-out fitting and was able to install these under the bottom of the tiny cabinet directly behind the toilet. The valve handles are accessible inside the cabinet.

The tank vent was fitted with a garden hose type "Y" which has a small valve in each branch. This permits the vent to be isolated so that the tank can be pressured with air. For pressuring I used my dinghy bellows pump.

Results to date:
The toilet has had relatively light use so far, but has been totally satisfactory. It is nice to not wake up the whole marina when paying a nocturnal visit.

The overboarding with the dinghy pump seems a bit Rube Goldberg but does in fact work fine. I was concerned that the bowl valve might leak too much but this was not a problem. After overboarding I then flush the tank with a gallon or two of fresh water using the hand held shower and overboard again.

Although the final photo does not show the seat lid in place, I was indeed able to install this and a horizontal bungee cord holds it in the up position when required.

I have not suffered odour problems, although the Y fitting does somewhat restrict the vent hose at that point. If this becomes an issue I plan to add a second vent, connected to a fitting on the opposite side of the boat.

As for the PVC isolation valves, the system would work without them, although there would be longer dead legs. It was a lot of work to include them and I would be tempted to eliminate them on a future arrangement.
To date I have not used the pump-out.

Would I recommend this toilet? Yes, if it fits your cruising profile. For me it works as advertised. The bowl appears to be made of quality porcelain and the plastic tank base is very solid. (No affiliation!)

Nick, Jan 2022

Photos:
1 Original toilet
2 Original toilet removed - thru-hull valve visible
3 Trial fit of new toilet
4 Hosing from thru-hull to PVC valves
5 PVC valves mounted under cabinet
6 PVC valve handles inside cabinet
7 Extension for actuating thru-hull valve
8 Detail of thru-hull valve extension
9 New ply base in place
10 Ready for toilet connection
11 Toilet base connected
12 Toilet bowl mounted
13 Final view (lid and cabinet trim missing)

 

[Nick F]

Photos 11 - 13

Here are the remaining photos

 

[Nick F]

Schematic

Here is a schematic of the installation (freahwater flush connection not shown)

 

[HeadMistress]

The tank vent was fitted with a garden hose type "Y" which has a small valve in each branch. This permits the vent to be isolated so that the tank can be pressured with air. For pressuring I used my dinghy bellows pump.

WHY would you want to pressurize the tank??? It's not necessary and the valves in the vent line will create more problems that you think need solving. All that's needed is a y-valve in the pumpout line to direct tank contents to the deck pumpout fitting or a manual diaphragm pump to discharge overboard.

And as for never flushing TP... There are a number of "quick dissolve" TPs that are suitable for use in any marine toilet, readily available even in grocery stores. To check for suitability, put a sheet or two in a glass or mason jar full of water. Come back in an hour and stir the water or shake the jar. If the water is milky and all you see is "snow" it's safe to flush. But if the sheets are still intact or mostly intact, try another brand.

--Peggie

 

[Bacchus]

Interesting adaptation. I agree with Peggies comments.
If you have odor issues you might try using sodium percarbonate (SPC) and/ or a bubbler to add O2 with a shut off valve if you continue with pressurizing.
Pressurizing a waste tank would scare me and I would try hard to avoid it.
More info on SPC and bubbling available here on TF if interested.

 

[Nick F]

Thanks for the comments - I was expecting some controversy!

It actually takes very little pressure to lift the waste a few inches before it goes down to the thru-hull and, as I said, the tank is very solid.

Regards,
Nick

 

[O C Diver]

Thanks for the comments - I was expecting some controversy!

It actually takes very little pressure to lift the waste a few inches before it goes down to the thru-hull and, as I said, the tank is very solid.

Regards,
Nick

A tip of the hat for your ingenuity.

Plastics degrade over time. It's likely the manufacturer would have used a different type of material for a pressure tank. As previously mentioned, vacuum extraction would have been a safer long-term choice.

Just curious if the 90 degree fitting on top of the seacock / ball valve is galvanized steel?

Ted

 

[Nick F]

Thanks for the compliment. Time will tell!

Quote: Just curious if the 90 degree fitting on top of the seacock / ball valve is galvanized steel?

The elbow is 316SS and even then I keep the thru-hull closed except when overboarding.

Nick

 

[HeadMistress]

I have visions of a geyser if the pedal is depressed to open the dome in the bottom of the bowl when the tank is pressurized...and don't say it can't happen. It may be unlikely, but anything that can go wrong WILL eventually go wrong.

Far more likely is a geyser out the deck pumpout fitting if it's opened while the tank is a pressurized or a leaky bowl seal as the rubber ages.

And there is no product that can prevent odor in the anaerobic environment created by a blocked vent.

But...it's your boat so as long as you're not doing anything that violates any regulations (and I'm not 100% certain that blocking the vent even temporarily doesn't), whatever floats it for you is all that matters. Should be an interesting learning experience.

--Peggie

 

[DavidM]

So, it is basically an RV toilet on steroids.

I too am concerned about pressurizing any tank that isn't built for it and that OEM tank surely wasn't.

David

 

[Bill V]

Your marine head appears to be a Dometic Sealand M28-711 model, the same as I installed 10 years ago on my boat. Mine has served me very well although I installed it as the manufacturer suggested. I use both pump out and overboard discharge with no issues whatsoever and can't imagine adding an additional chore of hauling my TP off the boat rather then disposing it in the head. I have for 10 years and have had no issues. Your use of ball valves rather then a Y valve would not be legal within USCG regulations. They require a Y valve that can be "secured" in a position that forbids overboard discharge. Probably not a concern if you never cruise into US waters.

 

[O C Diver]

Here is a schematic of the installation (freahwater flush connection not shown)

One other point:
If you have either a blockage or one of the 2 valves for overboard discharge is closed, the pressure within the tank could reach critical mass before you know it.

Force can easily be calculated by measuring the length by the width of the tank and then multipling it by the pressure of the pump. Assuming the tank is 12" x 12" and the pump generated 2 PSI (Pounds per Square Inch), that would be 12 x 12 x 2 = 288 pounds of force. If the tank is larger or the pressure is higher....

What's your solution (probably a bad word choice) for a blockage from the tank draw tube through the through hull fitting?

Ted

 

[Nick F]

Hi Ted - thanks for the comments. My system is indeed somewhat unconventional but, so far, has worked well for me. Some discussion on your comments:

First, it would be quite an effort to reach 2 psi with a dinghy pump. We all know this from getting the dinghy hard at the end of inflation.

My experience to date is that there is very little resistance when overboarding and it requires very fast actuation oif the bellows to sense any resistance at all (obviously the 2 overboarding valves were open).

With a valve closed I dont think it would be easy to unknowingly overpressure the tank.

A further consideration is the danger of subjecting the holding tank to a high vacuum during pumpout. This danger exists with all holding tank systems. The tank on this toilet is definitely capable of withstanding a much higher pressure (or vacuum) than a typical flat-sided holding tank.

As regards a blockage, in the ultimate case the toilet would have to be disconnected from the short outlet hose for a mechanical solution. I think a blockage is highly unlikely due to the short distance of large ID hose. However anything is possible and unfortunately there are two 90 degree bends in this stretch (a result of the very tight geometry of the installation). The absence of paper also helps reduce the chance of blockage IMHO.

Regards,
Nick

 

[catalinajack]

Your marine head appears to be a Dometic Sealand M28-711 model, the same as I installed 10 years ago on my boat. Mine has served me very well although I installed it as the manufacturer suggested. I use both pump out and overboard discharge with no issues whatsoever and can't imagine adding an additional chore of hauling my TP off the boat rather then disposing it in the head. I have for 10 years and have had no issues. Your use of ball valves rather then a Y valve would not be legal within USCG regulations. They require a Y valve that can be "secured" in a position that forbids overboard discharge. Probably not a concern if you never cruise into US waters.

Please explain why the OP's ball valves are not compliant. All that is required is that a valve is closed and secured. A nylon wire tie doing so is compliant. So would be removing the valve handle.

 

[O C Diver]

Hi Ted - thanks for the comments. My system is indeed somewhat unconventional but, so far, has worked well for me. Some discussion on your comments:

First, it would be quite an effort to reach 2 psi with a dinghy pump. We all know this from getting the dinghy hard at the end of inflation.

My experience to date is that there is very little resistance when overboarding and it requires very fast actuation oif the bellows to sense any resistance at all (obviously the 2 overboarding valves were open).

With a valve closed I dont think it would be easy to unknowingly overpressure the tank.

A further consideration is the danger of subjecting the holding tank to a high vacuum during pumpout. This danger exists with all holding tank systems. The tank on this toilet is definitely capable of withstanding a much higher pressure (or vacuum) than a typical flat-sided holding tank.

As regards a blockage, in the ultimate case the toilet would have to be disconnected from the short outlet hose for a mechanical solution. I think a blockage is highly unlikely due to the short distance of large ID hose. However anything is possible and unfortunately there are two 90 degree bends in this stretch (a result of the very tight geometry of the installation). The absence of paper also helps reduce the chance of blockage IMHO.

Regards,
Nick

Regarding a blockage, I was also considering the thru-hull fitting as well. All sorts of organisms like a recess like that to make a home. Wouldn't take much to block 2 PSI.

The other thing to consider, with a full tank and liquid being essentially incomprehensible, it could take less than one stroke to reach maximum pressure.

Ted

 

[Nick F]

Please explain why the OP's ball valves are not compliant. All that is required is that a valve is closed and secured. A nylon wire tie doing so is compliant. So would be removing the valve handle.

Thanks Jack - I too am curious to know this.
Nick

 

[koliver]

First, it would be quite an effort to reach 2 psi with a dinghy pump. We all know this from getting the dinghy hard at the end of inflation.

Nick:

You don't say what sort of dinghy pump you have. My dinghy (Hypalon) is rated for 4 psi. The pump that I now use, presents a cylindrical design, with a piston of just over 4" diameter. That allows me, using body weight of under 160 lb, and putting less than my full weight on the pump handle, to put a theoretical 12 psi into the dinghy tubes. My previous dinghies had foot pumps, but with the added surface area of a foot pump, the theoretical maximum psi I could generate by standing on the pump was reduced to 4 psi.

My bicycle pump that I use to pump up my road bike tires will easily allow me to get those tires to 125 psi.

Don't assume the actual pressure you are using. Do the math and restrict your effort so that you don't generate more pressure than the equipment you are pressurizing is built to handle safely.

 

[Nick F]

Hi Keith - You are right - the piston type pump is capable of substantial pressure. I am using a bellows type pump and I pump it by hand which is quicker because I can pull it open more quickly than its own internal spring.

I trust our boats are safe in this cold weather - I am a long way from mine right now!

Nick

 

[FF]

There is no question to me ,the RV style toilet is the wave of the future for boats.5 to 10 times the waste tank capacity is hard to beat

The only hassle seems to be educating boat designers/assemblers to leave space for the holding tank below where ever the toilet is located.

WE built what I think is the simplest system possible..

The suction pipe was a straight shot from the tank to a deck to a pump out fitting on deck.

A solid Y fitting was mounted under the deck fitting.A poo pump was hooked to one side of the Y fitting.

I purchased a spare cap for the deck fitting and drilled a 1/4 inch hole in it for use where pump out was restricted. The overboard pump could not lift with the 1/4 hole as an air leak.

Offshore the solid cap was installed.

Most of these overboard pumps are easily made unhappy with a bit of paper or other debris , a pail of water poured down the deck pump out fitting could flood the pump, no effort to get it working again.

With the inshore cap the pump out was disabled , so should meet any Poo Police requirement .

The vent line was 1 inch so the dockside pump out would not harm the holding tank.

 

[Sea Q]

We had two gravity units into one tank. .They worked but if you let them sit for more than a week you would get some pretty funky smells on the first couple of flushes . Gut wrenching if longer .
The Sea Q holding tank was higher than the water line and direct flushed .
we replaced the two toilets with electrosan to the tank.
They had a grinder so the waste flowed better thru hoses and there was no chance of Floaty's.
One a older boat Loafer we had a main collection tank and when at sea we turned on a bellows pump to discharge. the switch for the pump had a oneshot timer that would run jus a little extra on the cycle to pump tank dry .
we did his when crossing the straight.

 

[ronobrien]

The tank vent was fitted with a garden hose type "Y" which has a small valve in each branch. This permits the vent to be isolated so that the tank can be pressured with air. For pressuring I used my dinghy bellows pump.

WHY would you want to pressurize the tank??? It's not necessary and the valves in the vent line will create more problems that you think need solving. All that's needed is a y-valve in the pumpout line to direct tank contents to the deck pumpout fitting or a manual diaphragm pump to discharge overboard.

And as for never flushing TP... There are a number of "quick dissolve" TPs that are suitable for use in any marine toilet, readily available even in grocery stores. To check for suitability, put a sheet or two in a glass or mason jar full of water. Come back in an hour and stir the water or shake the jar. If the water is milky and all you see is "snow" it's safe to flush. But if the sheets are still intact or mostly intact, try another brand.

--Peggie

My first thought when reading the original post was the same as Peggy's, but for a different reason,why pressurized the tank? I just saw the possible catastrophe of over pressurizing the tank and the resulting INCREDIBLE mess.

 

[Nick F]

Hi Ron - for me its a risk/reward trade-off.

The reward is a pump that only sees air - not poop, etc. This means simplicity and reliability.

The ultimate risk is, of course, something like what you describe.

I mitigate the risk by (1) having a pump that is very feeble and holding it in my hands like a concertina (2) knowing that the tank is incredibly robust (3) the fact (known from experience) that it takes minimal pressure to overboard the waste (4) operationally knowing that if the pumping gets difficult I should investigate before applying heavy pressure.

The risk/reward assessment is quite subjective and I understand that not everybody will follow my choice !

I promise to fess up if I do ever take a brown bath.

 

[ronobrien]

Hi Ron - for me its a risk/reward trade-off.

The reward is a pump that only sees air - not poop, etc. This means simplicity and reliability.

The ultimate risk is, of course, something like what you describe.

I mitigate the risk by (1) having a pump that is very feeble and holding it in my hands like a concertina (2) knowing that the tank is incredibly robust (3) the fact (known from experience) that it takes minimal pressure to overboard the waste (4) operationally knowing that if the pumping gets difficult I should investigate before applying heavy pressure.

The risk/reward assessment is quite subjective and I understand that not everybody will follow my choice !

I promise to fess up if I do ever take a brown bath.

I read in your post that it has already worked for you so I won't argue with success. It's just the thought of it would scare the crap out of me. Yes the pun was intended. Sorry for that!!