Replacement head/holding tank design question

[tomwadbrook]

Hi All,


First post on these forums! I have a newly acquired 1986 Grand Soleil 39. The holding tank for the aft head is original stainless steel, leaking (and now removed), and is an odd shape that is less than 4 gallons. I'd like to relocate a bigger tank to the cabinet space above the head, which looks like it could fit almost 12 gallons tank. Proposed diagram attached.


This would put the top of the tank about 8 inches below the vent on deck - if the deck vent is outboard and I install the vent connection from the tank inboard, I don't think I will run the risk of spillage when heeling but I would like feedback from those more knowledgeable. Are there any rules about vertical length of vent line?



Also, the current manual head discharges to a Y valve that directs to holding tank or a thru-hull straight overboard. I think I should be deleting that Y valve, capping the thru-hull and going straight to the tank, right?


Thanks all!
Tom

[Mike OReilly]

Have you considered a composting head? Much simpler, safer and easier to install. Only really designed for small crews (2-3 person) for full-time use. And you do have to learn different skills to keep it running properly.

[tomwadbrook]

Thanks for the response. The Raritan head is barely going to fit in the space - I'm not seeing any composting toilets that have a small enough footprint to work, plus I would really like to make this one head be user friendly for non-sailors.



Which leads back to the original question about vent hose vertical rise - is 8 inches enough from tank to deck vent if I go from inboard side of tank to the outboard location of vent on deck? Are there any other issues with mounting the holding tank up high?


Thanks,
Tom

[Shrew]

Originally Posted by tomwadbrook

This would put the top of the tank about 8 inches below the vent on deck - if the deck vent is outboard and I install the vent connection from the tank inboard, I don't think I will run the risk of spillage when heeling but I would like feedback from those more knowledgeable. Are there any rules about vertical length of vent line?

Your vent line should have a loop that goes ABOVE the through-hull vent. This is to ensure that exterior water drains back out, rather than into the tank.

8" from the deck seems way too close. That is going to make for a small loop. You're also going to be lifting a column of fluid to raise it to the top of the tank. If the joker valve fails, how much fluid is in that column? Because it's going to flood back into the bowl.

All in all, I would personally not be entertaining this idea.

[pdx_sailing]

I believe your head can lift ~4ft vertically so that may limit the overall height of the tank. Pumping straight to the tank without a Y valve is fine, you can add another ball valve at the bottom of the tank to connect the through hull for direct discarge option. From discussion with Peggy Hall, she recomended the vent placement near the bow but not more than ~8 ft long with angles less than 45 deg. Now for an Aft head that won't work, but where is the vent located? That is likely relevent to the vent height / location decision.

[peghall]

Sorry shrew, but while a loop in a fuel or water tank vent line is a good idea, it defeats the purpose of a vent line on a waste holding tank: to allow the exchange of air with the gasses in the tank. A loop would only allow gasses to be forced out by incoming waste. And while it's definitely necessary to keep external water (sea water, rain etc) out of fuel and fresh water tanks, neither will have any negative impact on a waste holding tank.

Nor should a waste tank vent line be vertical because ambient air doesn't fall and the gasses in the tank don't rise. Ideally (which is rarely achieved), a waste tank vent line should be no longer than about 5' with a rise of less than 45 degrees.

I've sent him a PM offering to help him design a system that won't create more problems than it solves...hopefully he'll take me up on it.


--Peggie

[tomwadbrook]

Thank you so much Peggie - email sent!

[HopCar]

This is where I buy plastic tanks. They have close to 500 sizes and shapes. They will put what ever fittings you want anywhere you want them.
https://ronco-plastics.com/product/w...-holding-tank/

[Capt Clark]

Tom,

I’m with PDX - you can add another ball valve at the bottom of the tank to connect the through hull for direct discarge thus eliminating the macerator pump. I’ve seen this design on many cataramans where each head has a small tank behind the wall with short hose runs.

[Shrew]

Originally Posted by peghall

Sorry shrew, but while a loop in a fuel or water tank vent line is a good idea, it defeats the purpose of a vent line on a waste holding tank: to allow the exchange of air with the gasses in the tank. A loop would only allow gasses to be forced out by incoming waste. And while it's definitely necessary to keep external water (sea water, rain etc) out of fuel and fresh water tanks, neither will have any negative impact on a waste holding tank.

Nor should a waste tank vent line be vertical because ambient air doesn't fall and the gasses in the tank don't rise. Ideally (which is rarely achieved), a waste tank vent line should be no longer than about 5' with a rise of less than 45 degrees.

I've sent him a PM offering to help him design a system that won't create more problems than it solves...hopefully he'll take me up on it.


--Peggie

That is why the loop goes ABOVE the throug hull, rather than below it. If the loop drops below the through-hull, with the cross-over at the top of the loop, then this creates a trap. However, if the loop goes ABOVE, with the cross-over below the loop, then there is no trap. Contents on the vent hose side drain back into the tank. Contents on the through hull side drain back out to the through hull.

This concept applies to all loops in vent lines. Fuel vent lines won't tolerate a trap either.

[malbert73]

Originally Posted by Shrew

That is why the loop goes ABOVE the throug hull, rather than below it. If the loop drops below the through-hull, with the cross-over at the top of the loop, then this creates a trap. However, if the loop goes ABOVE, with the cross-over below the loop, then there is no trap. Contents on the vent hose side drain back into the tank. Contents on the through hull side drain back out to the through hull.

This concept applies to all loops in vent lines. Fuel vent lines won't tolerate a trap either.

Why do you want a loop? Like Peg said, will disrupt gas exchange...