neet to change my holding tank thru hull vents

[duality]

they are looking a little rough and since i am renewing my stripe i would prefer to change them now. my question is they are currently small clam shell type, and facing aft slightly. i was wondering if i could use these (hopefully pictured below) my vent lines are looped in case that would be a factor

thanks mike

 

[Peggie Hall HeadMistress]

You can use that type of thru-hull

In fact, I recommend that type over so-called "vent" thru-hulls because they make it so much easier to backflush the waste vent line to keep it clear. However, while stainless is a good choice for a water or fuel vent thru-hull, you can expect to see a considerable amount of corrosion if you use stainless for the waste tank...I recommend you use marelon or bronze.

Put clamshell covers over'em...opening facing aft. And take the loop out of the waste tank vent lines...I can't tell from your post whether you have two tanks or two vent lines on the same tank...if one tank, two vent lines, the loops are defeating whole purpose for having two. If two tanks, the loops are still impeding the free flow of air necessary to keep the tank aerobic--and therefore odor-free.

 

[duality]

Re: You can use that type of thru-hull

thanks peggy, i do have two tanks. how would i put clam shell covers over them, i thought these would be all i needed?

also should i keep the vent loop in the water tank vents (two tanks) or remove the loop as well.

it came from the catalina factory looped



thanks
mike

 

[Tim R.]

I saw a boat a couple of years ago that had a hose adapter that screwed onto the vent line. He would connect a hose and run water through it to fill the tank after pumping. He would do this a couple of times alternating between pumping and filling just like you normally would. No more hose swapping.

This kept the vent line clean and also did an awesome job cleaning the tank.

Maybe overkill but do you see any problems with a setup like this Peggy?

 

[Peggie Hall HeadMistress]

Tim and Mike

Tim. that's a great idea. Starting the water during pumpout would make it quicker and easier.

Mike...

"how would i put clam shell covers over them?"

Most clamshells attach to the hull with screws

"i thought these would be all i needed?"

Not if you want to prevent water from being forced up the vent into the tank when your rails are in the water.

Keep the loops in the fuel and water vents. It won't matter if a little sea water gets into a waste tank, but it will matter if it gets into your fuel or fresh water.

Btw...you might want to consider checking out the link in my signature.

 

[sailingdog]

What one of my friends used on their boat for the vent lines are these:

Image courtesy of WestMarine.com

Ditch the cover and chain... and cover with a clamshell. It allows you to attach a hose and flush it pretty easily. The only problem with it is that it is only 1/2 NPT on the inside.

 

[Roger Long]

Tim. that's a great idea.

Unless he spaces out and starts the water flowing before opening the deck plate. I've seen structural steel tanks on boats bowed out by city water pressure. Can you imagine blowing up your plastic tank. Hard hose connections are dangerous on boats unless used with more care than sun addled people can muster 100% of the time. You only have to mess up once.

 

[Roger Long]

I have one like that. No clam shell added. This has the advantage that, when you forget and overfill the tank, the discharge shoots several feet out and makes a splashing noise you can hear instead of running down the side of the boat. Just be sure to dock with that side outboard and not tie the dinghy up under it

My original clamshell was punched into the fiberglass by a PO's clumsy docking. They also punch fenders. The smooth thru hull spreads any impact out and doesn't damage anything it comes in contact with.

 

[sailingdog]

I'm curious...but did you learn this the hard way??? If so, that must have been an awful mess..

I have one like that. No clam shell added. This has the advantage that, when you forget and overfill the tank, the discharge shoots several feet out and makes a splashing noise you can hear instead of running down the side of the boat. Just be sure to dock with that side outboard and not tie the dinghy up under it

 

[Dan Johnson]

Down here...

on the Chesapeake you need to have a screen in the outboard end of any vent or spiders will find a home in there and clog the airflow with its nest, which leads to nasty smells in the boat each time you flush the head. The screen doesn't keep them from building a nest, but makes it a whole lot easier to clean the buggers out when they do! This is why the clam shell head fuel line vents are the common solution.

 

[Peggie Hall HeadMistress]

Can't happen. Roger...and Dan...

Unless he spaces out and starts the water flowing before opening the deck plate. I've seen structural steel tanks on boats bowed out by city water pressure. Can you imagine blowing up your plastic tank.

The hose would have to be connected to a fire hydrant. No faucet has that much pressure 'cuz that much pressure coming out of any faucet would destroy household appliances and take the skin off your hands. You wouldn't be able to fill a water tank, much less connect a dock water system. The tank vent should be adequate to prevent the tank from pressurizing. And you should know better than to let your tank overflow out the vent!

Dan, screens cause more problems than they solve...they corrode and become clogged with dust and pollen--and any waste that may spill out the vent when the boat is heeled...and a clogged vent will cause a much bigger problem than just odor when you flush. If air displaced by incoming waste can't get out the vent, the tank becomes pressurized, resulting in anything from just a geyser when the deck pumpout cap is opened to an eruption in the toilet...even a burst tank. If air can't get in through the vent to replace contents as they're sucked out by a pumpout, the pump will pull a vacuum that prevents it from removing anything...and can result in an imploded tank.

It's simple matter to keep an vent with an open thru-hull clean...all ya gotta do is stick the hose up against it and backflush it every time you wash the boat and/or pump out.

 

[Bob S]

Just curious where on the side of the boat you'd cut that in? Catalina uses the stanchion as the vent which doesn't allow you to flush it. How high up would you recommend cutting that in and is it better forward or aft? Love to see a picture if anyone has one.

Thanks
Bob

 

[Roger Long]

The hose would have to be connected to a fire hydrant. No faucet has that much pressure 'cuz that much pressure coming out of any faucet would destroy household appliances and take the skin off your hands.

Not with tanks. It's a matter of hydraulics and the principle of surface area ratios that are analagous to lever arm lengths. The pressure on a closed tank is always equal 5.33 psi (sea water) x the distance in inches from the point where the pressure is measured to the top surface of the water. This is true even if the top surface is a small diameter tube.

The old Goudy and Stevens shipyard was mostly finished with a fishing dragger in which the foscle floor was also the top of the fresh water tank. They wanted to pressure test if for leaks but knew enough not to hook it up directly. There was a tank of water up the hill in the shed so they ran a hose down from that. Just gravity draining over the 20 - 30 foot vertical elevation difference bowed up the floor and it had to be all cut out and re-welded.

The hose had no problem with the pressure but the large flat surfaces of the tank did. The basic calculation factor in designing tanks on boats is the distance from the bottom to the overflow.

 

[Roger Long]

I'm curious...but did you learn this the hard way???

No. The tank vent on my boat is right next to the head right outside the port that is usually open. Not the best location but it has only caused an occasional whif of odor. When you hear the stream splashing, you know to say, "Oh shit", and head for the nearest pump out or Colregs line.

 

[Peggie Hall HeadMistress]

Stanchion is a horrible location for a vent

Just curious where on the side of the boat you'd cut that in? Catalina uses the stanchion as the vent which doesn't allow you to flush it. How high up would you recommend cutting that in and is it better forward or aft? Love to see a picture if anyone has one.Thanks Bob

The opening is too small to allow any air exchange and it doesn't take much more than a speck of dust to clog it.

Install your new thru-hull directly below the toe rail. The vent line should be as short and straight as possible, and ideally 1" instead of the 5/8" "standard" vent size for all tanks. So if the tank is forward, run the vent line forward...if the tank is aft, it really doesn't matter whether you run it forward or aft as long as it's <5" long.

Before tackling this project, you too might want to check out the link in my signature. It goes to a comprehensive "marine toilets and holding tanks 101" manual that will help you learn how to vent and maintain your system to keep it odor-free and prevent problems instead of fixing 'em...'cuz prevention is always cheaper and easier than cure.

Btw, Roger...the sheet metal had to be thin, the tank inadequately vented...and gravity can generate a LOT more pressure than any faucet. Besides, holding tanks on sailboats aren't big enough for surface area hydraulic ratios to come into play.

 

[Roger Long]

Btw, Roger...the sheet metal had to be thin, the tank inadequately vented...

1/4" or 5/16" steel plate in that size boat. I was working for the designers at the time and it would have been one of those thicknesses. They had capped the vent though because they wanted to leak test it beyond the working pressure. They just over did it.

If you run a city hose from a high wharf like we have in downeast Maine and the faucet is up the hill a bit, that gravity pressure gets added to the utility pressure. You want to be sure you have a vent or an overflow that's working before you make hard connections.

This is also an issue that got another boat in trouble. It had a steel mast so someone had the bright idea of running the vent up to the top of the mizzen mast. When they forgot to pump out, the electric heads filled the vent line to the top. Nothing broke but there was a manual head in the crew's accomodations and I think the joker valve inverted. Anyway, something gave out with dramatic results.

 

[Dan Johnson]

Thanks for the comments, Peggy...

I can attest to how strong a Todd holding tank is when the vent hose became clogged when sailing hard heeled over to the tank's side with a near full tank. It was a brand new tank or the worst may have happened.

Hunter ran the vent hose downward before taking it through a bulkhead and up to the vent fitting. The overpressure lifted the seat board above it or it may not have been noticed. It is now a straight run out throught the bulkhead and upward to the vent fitting. A nasty problem to solve without creating a bio-hazard mess.

Your point on screens getting clogged is noted. Periodically the vent hose gets a shot of compressed air from a tire inflator from the tank side to keep it free and the outside gets a regular pressure spray everytime the boat gets washed as you recommend.

The spiders don't get to be a problem until mid season, but are held at bay now with periodic bug spraying of the vent and frequent applications of hose water pressure when washing. The pointy end of a tie wrap makes a nice clean-out tool.