Vent hose on vented loop?

[Dougcole]

Hi All,


I'm trying to run down a mystery water leak in our lazerette/ER and came across something I haven't seen before. Mounted on the forward bulkhead of the laz, behind the genset on the port side, is a vented loop. I think it is in the discharge hose for the aft bilge pump. A piece of clear vinyl hose is pushed onto the vent flange of the vented loop. It is about 24" long and hangs down into the bilge, terminating below the boat's waterline.


I've never seen this before. Does anyone know why it would be set up like this? Maybe it was supposed to lead up and be attached to the underside of the deck and either fell off or was never attached (Mainship)?


Thanks.



Thanks.

 

[psneeld]

Should go overboard, but into another drain or just high enough will work.

 

[Cigatoo]

A venting loop should be only sucking air in via a check valve to eliminate siphoning effect. Not sure why a hose would be there?

 

[psneeld]

Vacuum break for anti siphoning, a check valve can malfunction so hoses are an alternative. For a bilge pump, the water was already in the boat so if a small percentage leaks back in, big deal, but if done right, none makes it back in.

 

[Dougcole]

The vent hose should terminate above the waterline, right?

 

[Cigatoo]

The vent hose should terminate above the waterline, right?

A picture might help. If it is a vented loop that you are talking about there is no vent “hose” necessary. A vented loop should be mounted above the water line. It is there exclusively to prevent sea water from siphoning back into the boat. A check valve will be on the high point of the loop. Normally a vented loop would be used on a discharge hose that terminates below the waterline.

 

[Dougcole]

A picture might help. If it is a vented loop that you are talking about there is no vent “hose” necessary. A vented loop should be mounted above the water line. It is there exclusively to prevent sea water from siphoning back into the boat. A check valve will be on the high point of the loop. Normally a vented loop would be used on a discharge hose that terminates below the waterline.

That's exactly how I have always understood their purpose. That's what is throwing me on the hose on the vent.



I won't be back at the boat for a few weeks, so I can't provide a pic. I'm pretty darn certain it's a vented loop though. I've installed them before on other boats, once on a head and once on a bilge pump discharge. I've just never seen anyone put a hose on the vent before.


The underside of the deck above the lazerette in my boat is barely above the waterline, perhaps eight inches. From eyeballing it, it appears to me that this vented loop is either right at or just below the waterline. That's what makes me wonder if they put the vent tube on to try to raise the vent above the water line.


It is, of course, in a very difficult spot to access.

 

[HeadMistress]

There should be an air valve in the nipple on the top of the vented loop, not a vent line. But if it is a bilge pump discharge line, no vented loop is needed because bilge pump thru-hulls are always above waterline. So make sure that line does come off a bilge pump.



-Peggie

 

[psneeld]

You can rig one either way, with a hose or check valve. Because check valves notoriously fail, some prefer hoses.

Whether a vented loop is necessary....I don't believe so either in this case but sometimes on sailboats with heeled waterlines they are. Might want to double check.

 

[danderer]

Standard Mainship setup. The clear tubing is to route any leakage through the duckbill to the bilge rather than have it spray around.

 

[oscar]

bilge pump thru-hulls are always above waterline

Never say "always" in this business......

 

[garmstro55]

Standard Mainship setup. The clear tubing is to route any leakage through the duckbill to the bilge rather than have it spray around.

What leakage might you be referring to?

 

[Fish53]

I have the have a vented loop on the hose connected to the mixing elbow on my sailboats exhaust. It's close enough to the waterline that I ran a hose vertically up two feet and placed the check valve there. You don't want it leading down or you may create a siphon.

 

[FF]

Many larger vessels push bilge water down and out , they do not waste energy lifting water.


But their bilge pumps are not $99.00 at the discount house.

 

[psneeld]

What leakage might you be referring to?

The check valves in some vented loops can fail in the open position. So when the bilge pump runs, it might spurt water.


Worse if the valve sticks closed, then it's no more a "anti-siphon" loop. Several of my older loops had duckbills as hard as rocks. One where the duckbill had turned to goo.... I wound up wrenching off the valve trying to get to the duckbill.

 

[FF]

"Worse if the valve sticks closed, then it's no more a "anti-siphon" loop. Several of my older loops had duckbills as hard as rocks. One where the duckbill had turned to goo.."


If there is only a very long tube instead of the air valve , the worst that happens is a squirt of water .

 

[HeadMistress]

Several of my older loops had duckbills as hard as rocks. One where the duckbill had turned to goo.... I wound up wrenching off the valve trying to get to the duckbill.

Like everything else on a boat, air valves require periodic maintenance (cleaning) and replacement, which obviously was never done.

If there is only a very long tube instead of the air valve , the worst that happens is a squirt of water.

The same salt and sea water minerals that can clog an air valve will also build up at the front end of that long tube, doing the same thing to it than does to an air valve: turn the loop into an UNvented loop that no longer has any ability to prevent a siphon.

--Peggie

 

[psneeld]

Thinking the size of the average hose used ....compared to the tiny duck bill that HAS to move to work.....

Sure it can happen to both.....I will let others guess which is more susceptible/ needs more frequent inspection.

 

[Steve DAntonio]

Remote hoses attached to vented loops are a recipe for creating a siphon, the very phenomenon a vented loop is designed to prevent. I have first-hand experience with failures of remote plumbed vented loops. In one case a new 40 foot sailing vessel used this approach on the engine raw water discharge, the vent hose was clear PVC, which was neatly wire tied for several feet down toward the bilge. After a few months of exposure to engine compartment heat the hose collapsed under the wire ties, converting the vented loop into an un-vented loop, the engine filled with water and bent a connecting rod when it was next started. The boat builder did pay to replace the engine.

In another case, a genset was installed by an experienced tech, he also dutifully and carefully plumbed the vent to the bilge, so that a leak would not drip water onto the genset. After running the genset one time, it would not restart. The vent hose was plumbed deep enough into the bilge so that it sucked water up when the genset was shut down, and water entered the genset's cylinders.

In addition to kinking, crushing or sucking up water, these small hoses can also easily become clogged with salt, and once again enable a siphon to occur.

Nothing should be plumbed to these vents, if they leak it's your notice that they need to be rebuilt. If they are over critical gear such as engines or electrical equipment, install a deflector shield. If you must remote plumb, you can remove the check valve all together and plumb the hose, it should be robust hose such as type A fuel line, not clear PVC, directly overboard. On some sailing vessels I've seen it plumbed into the cockpit, depending on its height above the WL water may or may not run out of this hose. But remember, if it becomes obstructed or submersed for any reason, you have the makings of a siphon.

More on anti-siphon valves here https://www.proboat.com/2017/06/antisiphon-valves/

 

[Maui]

I have recently seen two issues with vented loops on gensets leaking into the bulge. This was because the valve at the top failed. In both cases it was due to back pressure in the exhaust riser. One case had impeller parts stuck in the raw water elbow into the riser. The other was an almost complete clogging of the riser water jacket due to rust, salt buildup, and debris. A taking apart and physical inspection of your entire raw water cooling system ascertaining a clear and abundant flow of raw water through your system will save you thousand of dollars in repairs and disruption of you priceless summer cruising. I just did this on both my Cat 3208’s and my Koehler genset. I’m glad it was February and not July in the middle of Block Island Sound in 20 knots of wind. I had to replace the exhaust risers and gear coolers. I’m looking forward to a smooth running summer!