Locating new waste tank vent mushroom fitting

[smokey73]

I want to install a new 1" waste tank vent with a mushroom fitting per Peggy's recommendation. The old fitting has a clamshell with a screen and is only 5/8 inch id.
Per the attached picture you can see that the clamshell vent fitting inside the boat (the metal one to the rear) apparently screws onto the thruhull above the waterline. I'm considering just plugging that line and installing a new mushroom fitting in the area between the white thruhull (for the shower drain) and the electrical panel (just forward of the shower drain thruhull. I plan on using 1" Shields White Sanitation VAC Heavy Duty Hose. Anybody see a problem with that?

 

[Justin_NSA]

You want to make another hole? Why aren't you using the hole that's already in the hull?

 

[smokey73]

Its very tight and very hard to get to the area behind the clamshell vent to get it off and consequently very hard to get into there to tighten the mushroom thruhull, screw on a 90 degree low profile adapter to a barb fitting, get a hose on it and thetefor hard to make sure there are no leaks. The proposed location has good access to make sure everything is tight and to inspect for leaks. All the holes in the hull are way above the waterline. I'd still have to enlarge the existing hole if I used the same location.

 

[Parsons]

That Shields stuff looks like good smooth-wall hose at a reasonable price ($2-3 / foot). From other research I've done, you do not really need sanitation-grade hose for the vent to prevent odor as material is not normally sitting inside the hose, just a gas. Even fiber-reinforced PVC hose like used for water would be smell-proof. Using the higher-grade sanitation hose you're suggesting would be extra protection. Of course, @Peggie Hall HeadMistress is the final arbiter of all things smelly! I'm with @Justin_NSA , don't drill more holes and count on patching. Use a Dremmel to expand the existing through-hull. Trying to balance a drill or hole saw on an existing large hole is impossible. Bed your new mushroom with good butyl, or 3M 4200.

 

[Peggie Hall HeadMistress]

I was with Justin till I read how difficult it would be to get to the thru-hull to enlarge the hole. If you move the thru-hull you can just leave the old one place with a plug in it.
However...I DO hope you're replacing more hoses than just the vent! Corrugated hose especially has to go, but all of 'em really need to go. And while it's ok to use the Shields flex PVC #148 for a vent line, but you don't want to use it anywhere else if you only want to do that job once. There are a lot places on a boat where it's ok to skimp on the cost, but sanitation hose isn't one of 'em! (Parson, whatever gave you the idea that fiber reinforced water hose is odor-proof. It's what some boat builders use because it's cheap, but it takes just about long enough for the boat's warranty to expire to starting stinking to high heaven!) Raritan SaniFlex RaritanSaniFlex hose is your best choice (has a list pricee of about $15, but is available for about $10/ft)...it's not only proven to be 100% odor permeation resistant, but it's also so flexible it can be bent like a hairpin without kinking...which makes it a lot easier to use than Shields PolyX or Trident 101/102.
--Peggie

 

[Blitz]

The corrogated hose look like water tank fills.

 

[smokey73]

The problem is access to the existing fitting to get it out, enlarge the hole then reattach the new hose.

Peggy, the existing hoses were replaced about 5 years ago with Trident Sanitation hoses when I put in a new poly tank. The vent hose was only 5/8 inch with a Clamshell fitting with a screen. I just want to go to a 1" VENT hose.

The corrugated hose is the pump out line for the aft shower sump.

 

[smokey73]

Peggy, there is currently a loop for the waste tank vent but no vacuum breaker. The opening is above the waterline but could be below when the toerail is in the water. Do I need a vacuum breaker fitting at the high point.

 

[Peggie Hall HeadMistress]

If you replaced the hoses with Trident 102, they should remain odor-free for at least another 5 years (10 years is the average working life of any hose). So what is that corrugated hose connected to?

A loop can be a good idea in a water or fuel tank vent line--to protect the contents from sea water contamination, but no vacuum breaker is needed in either one. But NO loop OR vacuum breaker in a waste tank vent line....you want that one to be as short, straight and as close to horizontal as possible--only high enough to keep the thru-hull out of the water when heeled...which, judging from your description is a lot of the time.
So let's look for the best place to put the new vent thru-hull. Where's the tank located in the boat?
If you'd like to go one-on-one to sort out this out, send me a PM that includes your email address.
--Peggie

 

[smokey73]

Peggy, thanks for the reply. First, the corrugated hose is NOT for the waste tank vent, it is for the overboard discharge of shower sump.

It is Trident 102 for the existing waste hose about 5 years old. The long run from the forward head is almost never used and when it is I flush fresh water down it at the end of the sail. I'll bet it hasn't been used more than 5 times on 5 years and then mostly to exercise the pump.

The location I have proposed is as close as you can get to the waste tank. It is about 1 to 2 ft aft of the tank and on the same side of the boat, effectively the shortest distance possible. I could put it a few inches higher to minimize any times it might be under water with the toerail in the water but as you said, that is few and far between as this is a cruising boat. I'll leave out the loop.

 

[Dalliance]

If you must relocate an above waterline through-hull, you can replace the abandoned through-hull fitting with a mock through-hull fitting rather than plugging the old one or patching the hole. The company that sells them is called Marine East.

www.marineeast.com

 

[Peggie Hall HeadMistress]

The location I have proposed is as close as you can get to the waste tank. It is about 1 to 2 ft aft of the tank and on the same side of the boat, effectively the shortest distance possible.

Again...where is the tank located in the boat? Depending on how far forward, it may make a lot more sense to run slightly longer vent line forward to catch more air.
--Peggie

 

[smokey73]

Peggy, I hadn't thought about "catching more air." However, because of the hull liner location, this is the only place that I can see where I have reasonable access to put in the mushroom fitting close to the tank to minimize run length. Any further forward would require tearing out cabinet woodwork and or hull liner and would then be nearly impossible to inspect and or install. The tank is on a Hunter 40.5 and is about 1/2 of the way aft from the bow. I possibly could run it forward to near the forward V-berth compartment locker which looks like it may have good access. However, that would add an additional 10 to 12 feet to the vent line making if go from about 6 feet to about 16 feet. Not sure that is a good idea.

Attached is a pdf file showing the waste tank. The vent locations aren't exactly as shown on the drawing. the current waste tank vent and shower sump overboard are immediately behind the aft toilet and have a hull liner obstructing access to them. The proposed location is a little forward just outboard of the Nav Station seat which is immediately forward of the aft head and directly above the waste tank.