Macerator Through Hull and backflow question

Peggie Hall HeadMistress · Oct 16, 2018

jonelli said:
Theoretically... >IF< I were to leave that through-hull open all day, every day for weeks, is there anything keeping backflow of seawater going through the macerator?

Without a vented loop in the line (which needs to be the same 6-8" above waterline *at any angle of heel,* same as the ones in the toilet inlet and discharge lines) between the pump and the thru-hull, nothing is preventing it. The macerator pump will slow it down, so you don't have to close it in a panic...but it'll allow seepage that'll rise at least to the boat's waterline--which from what you describe would come close to filling it with sea water.

With or without a vented loop in the line, that thru-hull should be kept closed except when dumping the tank...for several reasons: 1. it's illegal to have it open when you're inside the "3 mile limit." That could result in an expensive citation if you're ever inspected. 2. thru-hulls left open while no one is aboard is a leading cause of boats sinking in their slips. 3. seacocks that are never "exercised" are never lubricated either...so they have a nasty habit of seizing, making it impossible to close 'em when you need to. So keep it closed except when you're dumping the tank.
--Peggie
"If you can't explain it to a six year old, you don't completely understand it yourself." --Albert Einstein

weekendrken · Oct 16, 2018

I’m not familiar with that particular macerator, but in general they are impeller pumps. The impeller seals against the body but not 100%, so you can experience some backflow. I would never leave the thru hull open, but you won’t have a problem flooding if you don’t close it immediately.

SG · Oct 16, 2018

Is there room to have the macerator pump located so that is is above the water line. I personally don't like the idea of a any pump being submerged in salt water with a load of effluent and urine above it if you could avoid it.

The pump can suck-out the holding tank, it doesn't need to be simply gravity feed, does it?

Ward H · Oct 16, 2018

On my 93 there was what looked like a check valve between the macerator and thru hull. Macerator was leaking so it along with the hose and check valve went into the trash. We have easy access to a pump out.

SG · Oct 16, 2018

Check valves wouldn't be something that I think you depend on. Aside for general lack of reliablity, they depend on being clean and having a clear "back flow" in terms of pressure and volume.

A submerged check valve which you've been pushing sh** thru would not like keep water from coming back up.

The other issue is that on a sailboat, it rises and falls as you move through the seaway. That creates a "pattern" that really would be challenging for a check valve. It might restrict flow for a while, but...

Peggie Hall HeadMistress · Oct 16, 2018

SG said:
Is there room to have the macerator pump located so that is is above the water line. I personally don't like the idea of a any pump being submerged in salt water with a load of effluent and urine above it if you could avoid it.

If there's water in the bilge deep enough to even partially submerge a macerator pump, whether it is submerged would be the least of your problems! But if you're only concerned about it being below the waterline, bilge pumps and sump pumps are installed below waterline.. Toilet pumps are at least partially below the waterline on most boats.
A macerator pump can lift up to 4', so it can be mounted above the tank if there's room for it. That would eliminate the need for a vented loop in the tank discharge line.
--Peggie
"If you can't explain it to a six year old, you don't completely understand it yourself." --Albert Einstein

Ward H · Oct 16, 2018

SG said:
Check valves wouldn't be something that I think you depend on.

I don't disagree.
Here's a photo of it. It's to the top right corner of the aqua lift muffler.

Is it something else?

Peggie Hall HeadMistress · Oct 16, 2018

That wasn't the only bizarre thing about your plumbing when you bought the boat, Ward...I couldn't believe anyone could have done what I saw in your photographs! You've done an excellent job of getting rid of all of it and doing it right.
--Peggie

Ward H · Oct 22, 2018

Interesting timing. I was talking to a surveyor who had just finished surveying a 89 C30 in my marina this past weekend. He asked about how my macerator discharge was plumbed because on that 89 the discharge hose went direct from the macerator to the discharge thru hull. (no check valve on this one) He couldn't believe it was plumbed without a three way valve. He advised the new owners to wire the thru hull valve closed so they would be compliant with no discharge requirements and they wouldn't be going off shore.

Peggie Hall HeadMistress · Oct 22, 2018

A lot of boat builders use tees instead of y-valves...which I don't like either because it doesn't provide any way to shut off the flow of tank contents to the macerator pump when (not IF, definitely WHEN) the macerator pump needs a new impeller or other work.

If you know you're never gonna dump a tank, I'd eliminate the plumbing to the thru-hull instead of just wiring it closed. If/when circumstances change and you want to be able to do that, it's a simple matter to cut the hose and install it. The macerator won't remain in working condition if it sits unused more than a year or two anyway.

--Peggie