drill and tap holding tank

[a64pilot]

When I replaced my holding tank, I did the 1" vent, it is a short vent relatively and all up hill with no sags to a 1" mushroom type thru hull. Used to be that when someone flushed if the wind was right, the odor in the cockpit had to be experienced to be believed. I haven't noticed it since the new tank and vent.


I would think surely a 3/8" plastic tank is plenty thick enough to tap and hold a .25 NPT threaded nipple, I'd use a nylon nipple though as opposed to brass, and I'd wrap a layer of Teflon tape on it for lubrication more than anything.

[sailortype]

"replace the vent thru-hull with an open bulkhead thru-hull"
so Peggy... or anyone...
I am unclear what an ''open bulkhead thru-hull'' is : |
As in a giant hole on the outside of the hull with no screening..?
thanks

[a64pilot]

Originally Posted by sailortype

"replace the vent thru-hull with an open bulkhead thru-hull"
so Peggy... or anyone...
I am unclear what an ''open bulkhead thru-hull'' is : |
As in a giant hole on the outside of the hull with no screening..?
thanks

It is a thru hull like the one for your bilge pump

[peghall]

Originally Posted by a64pilot

It is a thru hull like the one for your bilge pump

One of these...any material except nylon (UV turns nylon into powder). This type of thru-hull not only allows much better air exchange with the gasses in the tank, you'll never have a vent blockage if you stick a hose nozzle up against it and backflush the vent line every time you wash the boat.

If your holding tank vent thru hull is one that has a screen, knock it out....'cuz screens actually create more problems than they prevent...they corrode and get clogged up with dust, pollen etc.

[sailortype]

thanks both :-)
was wondering how you stop it from getting blocked with webs etc but the hose idea is a good one, maybe with a bit of poking at the open end.
I have a 6" inspection hatch (never leaks btw) so I will drill for an outlet and add a backing plate with #10 bolts
Other question was... what does a 'uniseal' do that say a rubber or neoprene gasket would not do..?

[peghall]

Other question was... what does a 'uniseal' do that say a rubber or neoprene gasket would not do..?

The Uniseal isn't a gasket, it's a rubber grommet, so it seals the hole from both sides. A tight fitting smooth surface inserted into it won't leak. SeaLand uses it for their "diptube" retrofit kits. It's the easiest way to add or relocate tank fittings 'cuz you don't need access to the inside of the tank. Check out all the details on their website UNISEAL

[sailortype]

Thanks for the Uniseal info, had not heard of them.
I fear I am being a bit obtuse… but am not understanding what kind of vent fitting we are referring to on the tank itself. I am going to leave the current 5/8-3/4 NPT vent fitting on the one side (PE tank) and will enlarge the hose and use a larger thru hull, but would like to install a second ¾- 1” fitting on the other. If the tank wall is not thick enough to tap and drill, and one cannot access the rear of the fitting… how is it attached? (without taking it into a shop and having another boss welded on)
In my situation, I can in fact access the rear of the fitting but the only fittings I have seen with a 1” hose barb are pipe threads so could not get a locking nut on the back of it.

[GordMay]

The Uniseal is a single piece rubber grommet that pushes through from the outside. No need to access the back side (inside tank).
Uniseal Instructions ➥ How to Install a Uniseal

[sailortype]

ahah I see, said the blind man :-)
Thanks Gord... starting to make some sense now. So it is a piece of smooth-walled pipe and not a fitting at all that goes through the uniseal into the tank.
So... this would need to be rigid pipe vs hose, with 1" hose clamped to the pipe and leading to an open through hull as a vent in as short a route as possible. Seems to me the pipe would have to be securely clamped or blocked to restrict movement both side to side and in and out. CPVC pipe? copper or other metal would corrode and eventually leak

[a64pilot]

Take years to corrode a piece of aluminum fuel line, your only wanting a very small as in say 1/4" tube right? This is for aeration?
I would not trust a grommet for a 1" tube to make a vent myself, I'd want a piece welded on to double the tank thickness for that size a hole and tap it.

[sailortype]

trying to get a handle on this project and wading through the all the info and opinions. But- if wanting max ventilation and attaching a 1" hose to a 1" thru hull as a vent, seems to me you would want at least a 3/4" 'hole' in the tank to make it work. But- I do not know for sure

[sailortype]

and thinking about it, you may be right about the smaller pipe... it could be a Bernoulli thing and the size of the pipe does not matter. I just do not know the physics of the air exchange

[sailortype]

but having said that :-)
I think a 1/4" pipe would be prone to blockage from muck within the tank and also would be a choke point when trying to backwash from the upper 1" thru hull with a water hose. Seems to me a larger pipe would be better

[peghall]

Take years to corrode a piece of aluminum fuel line, your only wanting a very small as in say 1/4" tube right? This is for aeration?

Because only air is being pushed through the tube, it won't get clogged by any of the sludge at the bottom of the tank as long as the pump is running. However, tank contents can rise in the tube when it's not running...and the same animal fats in waste that clog tank level senders will clog up a tube that small pretty quickly. So I wouldn't use anything smaller than 1" on the aerator...and you'll still have to remove it to clean it at least annually.

I would not trust a grommet for a 1" tube to make a vent myself, I'd want a piece welded on to double the tank thickness for that size a hole and tap it.

Urine is far more corrosive than fuel...it'll turn a metal holding tank into a collander within just a few years.

A discharge fitting at the bottom of a waste tank is the only place I wouldn't recommend using a Uniseal...and that's only out of an abundance of caution. It's become widely accepted as the best way to relocate or add vent and inlet fittings because it not leak. As I mentioned in a previous post, Dometic/SeaLand uses it in their "diptube retrofit kit."

Peggie
"If you can't explain it to a six year old, you don't completely understand it yourself." --Albert Einstein
http://www.amazon.com/New-Get-Rid-Bo...dp/1892399784/

[mitiempo]

I agree with Peggie. When you have seen and used a Uniseal you will understand why they work so well. Incredibly simple and perfect at what they do.