Need to cut a hole in my boat (holding tank vent)

[rebel heart]

Hey everyone. I'm on a mission to seek out and destory everything related to a stinky head on my boat. I've redone all the sanitation hoses, and put a Lavac in. The only thing left is the holding tank, which structurally seems okay.

I've got a single holding tank vent, and the hose is maybe 15' long. I read online that it shouldn't be longer than 3', optimally, and that having cross ventilation is a huge advantage.

My plan, before reading too much, is to do this:
- Double check 10 times over again that I'm drilling above the water line.
- Size out my hose, keeping the run short as possible.
- My desired location is in the v berth, about 2' above the matress, to starboard, all the way forward. It wont be visible looking in because of some cabinets, but is still in a very accessible place.
- Put something like this outside:
Clamshell Midget Ventilator*

What I'm wondering about is:

- What kind of fitting do I install in the hole I drill? I know it's not a thru-hull, or should it be, even though it's above the waterline?
- How do I bed this fitting into the hole?
- Anything else I'm missing?

[Pura Vida]

I did this not too long ago and used a perko gas tank vent. I drilled the hole with a standard paddle bit but if you have a small enough hole saw that would be better. Remember to wear a long sleve shirt and long pants or painters coveralls. Getting the GRP chips off of you skin is a bummer (packing tape will do the job but why mess with it.) I bedded it with mmm 5200 but some consider that product outdated.

Because of the nature of the vent I put mind on deck to avoid submerging the vent. That meant drilling the hold and sealing the core with epoxy.

[rebel heart]

Okay, so you're talking about something like this:
Perko Chrome Fuel Tank Vent, 5/8in(#85-3530/#509DP4CHR)

With that, it's:
- Measure, measure, measure.
- Cut hole.
- I wont need to epoxy because I'm just going through the fiberglass.
- Bed with 5200 or something equally brutal. If it's on the bow, on the topsides, it's going to get pounded pretty hard, so make sure it's a tight seal.

And then I need to remember that I'm working with fiberglass and that it's going to be itchy and get everywhere.

[Boracay]

I finally got my holding tank completely plumbed last week.

It differs from yours in that it is quite close to the toilet and quite large (160 litres).

I made a mistake with the vents in that I only used 1/2", but as you can see from the photos I tried to use very short hoses.

I located one outlet in the cabin top, and one in the cabin side, trying for one to always be in a high presssure zone and one always in a low pressure zone, using el-cheapo skin fittings..

1/2" is very easy to plumb.

[GordMay]

Originally Posted by rebel heart

... What I'm wondering about is:
- What kind of fitting do I install in the hole I drill? I know it's not a thru-hull, or should it be, even though it's above the waterline?
- How do I bed this fitting into the hole?
- Anything else I'm missing?

Install a Vent Fitting, such as pictured below (not a clamshell cap).
The vent is installed above the heeled waterline, and does NOT require a seacock.
Toilet intake and overboard discharge lines should be equipped with seacocks (valves) to stop the inflow of seawater in the event of a hose failure. A seacock is operated by a lever-type handle that gives clear indication whether the valve is open or closed. These seacocks should be readily accessible for maintenance and oriented so that their handles are easy to operate.

When drilling, maintain a relatively slow drill speed, and advance the drill bit very slowly. Let the drill bit do all the work. Where practical, use a hole saw, or a shallow angle tip drill bit.

Seal the raw exposed fibreglass edges of the hole with epoxy resin. All FRP working tends to leave a rough, shattered or chipped edge, where water can penetrate into the laminate.

I like to chamfer (countersink a conical recess) the outside surface of holes. This relieves the stress on the gellcoat, mitigating cracking. The chamfer also provides a cavity (think o-ring) for the sealant to bed the fitting.

Bed the fitting with a flexible polyurethane sealant, such as 3M “5200" or “4200". Don’t use silicone. Prior to installation, clean the hull area and fitting, ensuring no wax, oil, or release agents remain to contaminate the bond joint.

The holding tank vent fitting should be located above the holding tank, and above the waterline (at all angles of heel). It should also be no less than 24 inches away from any opening (hatch/portlight) into an accommodation space.

Ensure that the holding tank vent line has a constant rise from the top of the tank to the vent fitting. Otherwise low spots may fill with sewage and act as a "U" trap, effectively plugging the vent. Avoid bends, as far as practicable.

Vent line should be as short as possible.

If fitting is to be located in hull topsides, select a protected spot where it will not be subject to damage from pilings, etc.

Keep vent fitting opening(s) free of corrosion, insect nests or other material that may result in a clogged vent line. If the vent pipe is not clear, it is possible to build up pressure in the tank and either rupture the tank or blow the hoses off. Do not use the cheap clear vinyl tubing; but rather a high grade, thick wall, reinforced tubing. All tubing should be opaque. Letting sunlight get into the lines allows the algae to grow that can cause a most unpleasant odor.

[rebel heart]

Okay, so I'm getting closer to doing this project. I have a bit of a problem, however.

The tank has two prebuilt vent holes. One is up top, on the side. That one is plumbed to a deck vent, but it's a very long run, and I want another one to create a cross vent. The other hole that's available is on the top of the tank, but I don't think I can plumb it because the matress sits nearly on top of it. I'll look into putting an elbow on it, but can't I just cut a new hole?

Obviously this isn't a place you want to mess up, but I would just need to cut a hole, smooth the edge, put a plug in with some bedding, and whalla! I should be able to not only put a hole in, but I can put a 1" hole in, which is what I want.

[rebel heart]

The vent through-hull should not be the same type as a fuel vent through-hull (a cap with a slit in it), but should be a should be a straight open type through-hull.

Straight open? So we're talking about something along these lines:

Thru-Hull With Nut*

[rebel heart]

I can use a plastic thru hull plug of 1&1/2" to put the hole into the holding tank. That's perfect, because then I can have a nice big hole, and it's plastic on plastic thru hull fitting.

Thru-Hull Tailpipe*

I suppose I'll need to open up the inspection port on top of the holding tank, which I would wonder how you go about making sure it's sealed once you're done with it.

[Vasco]

The only problem with these vents in the topsides, especially if they're up forward is that in heavy weather when they're underwater it'll fill up the holding tank. If you have two vents it'll fill up all the easier as it'll vent the air as the water comes in. I found this out the hard way and always plug the vents with silicone sealer in heavy weather.

[rebel heart]

Hmmm... maybe I'll put a sea cock on it.

Apollo Shut-Off Valve*

When the weather gets a little rough, I can go into the v berth and close it. I suppose it's a little weird knowing that I'm going to have a 1 1/2" hole going right into the holding tank, so maybe just as a peace of mind I have the ability to shut it off if I need to.