Hold tank gravity empty question

[nautical62]

On my current boat, (Beneteau) , the holding tank is above the water line, so simply opening the seacock empties the tank. What I like about this simple gravity fed system is there is no pumping, no pump to fail and the through-hull is closed most of the time. Everying goes into the holding tank and it's easy to empty where appropriate, with little danger of discharge when people are swimming due to leaving a y-valve in the overboard position. I've chartered several other boats with the same system and never enountered any problems. One thing I've noticed however, is the exit plumbing is a larger diameter than the typical 1 1/2 inch discharge on most boats. It looks like about a 2 inch diameter to me.

Now my question: A new boat I'm purchasing needs a plumbing upgrade and since the holding tank is also above waterline, I've though of converting it to the above mentioned system. The current through hull and seacock (which need replacing) are the standard 1 1/2. My question is while this is what is typical for most marine head systems where the sewage is pumped out, would this diameter likely cause issues with a gravity fed system? (having less pressure) My guess is there is a reason the Beneteaus go with a larger diameter, but I've never seen plumbing hardware larger than 1 1/2 carried anywhere.

Has anyone ever seen this system with stanard 1 1/2 plumbing and if so how well did it work?

Any thoughts on where to obatain marine plumibing fixtues of a larger diameter?

I have used a 1 1/2 inch system for this kind of dischage without any problem on my boat.

No solids going into your holding tank should be anywhere near that size once it goes through the system.

[nautical62]

Thanks GreatKetch - My thought was also that anything going through the head piston should gravity feed through a 1.5 discharge. I just don't want to chance finding I'm wrong the hard way on this one.

[gbanker]

same type system on our Hallberg-Rassy. The only issue we have had is that holding tank residue caking into a dried mud when stored for long periods. (I know, it should have all drained out.) So make sure you add a bit of holding tank cleaner around layup time.

[Boracay]

Boracay's PO (for some reason) installed a 1" seacock in the head area.

In pure desperation I connected the outlet of the holding tank to it.

Seems to work OK so far (only outside the harbour though).

[Pblais]

My question is while this is what is typical for most marine head systems where the sewage is pumped out, would this diameter likely cause issues with a gravity fed system?

I think gbanker makes a valid point. Lots of things work if they stay operable over a long period of time. Water makers love to work regularly too. Unrelated but the same idea. they sit around too long and you get problems. The other side is if you had a clogged vent it might drain out really quick.

[Cormorant]

Depending on where the through-hull is located, I would think you could get a strong sluicing action if you opened the seacock and drained the tank while in a seaway. If you ever get a clog, you might just need to open it up and go pound to weather for an hour or two. Depends, too, on how high your tank is above WL, and how long the connecting hose. . .

I've also wondered about the possibility of having some sort of fitting on top of the tank that would allow you to blast it out with a hose if necessary.

[nautical62]

I ask, because I have a ball valve and tail piece sitting here and it occured to me that the tail piece that will connect the through hull to the hose, has a much smaller interior diameter than the 1 1/2 inch ID hose and also will create a lip of sorts for solid matter to get hung up on. My concern is that with little pressure behind it, this constriction and lip might be enough to catch and gather solid matter and prevent the holding tank from draining. I thought of the sluicing action Cormorant brought up, but it also occurred to me this might just push an blockage up and down, not break it up.

I have to replace the seacock anyways, but it would be much less work and expense to stick with the same diameter plumbing than have to replace everything, especially the holding tank discharge fitting.