Vented Loop for raw water exhaust.

[Scott T-Bird]

I'm getting around to replacing hoses as suggested in the survey on my boat. I'm not absolutely sure until I double-check but my Yanmar service manual indicates a hose directly from the heat exchanger to the mixing elbow. My boat has a vented loop with this run. There is approximately 22' of hose to make this happen (11' to the loop and 11' back to the mixing elbow). I'm not certain why this loop is necessary. The mixing elbow is a inverted U, quite high in the system (well above water line) and the stem for the raw water exhaust is on the exhaust side (underneath the hump). Assuming there is always air or exhaust gas in the mixing elbow, doesn't it provide the break and prevent any water from backing up and flooding the engine?

For additional info, I haven't yet found a riser at the end of the exhaust hose (between the water muffler and the exhaust port in the stern). I can't say that I've scoured every nook and cranny in the stern to find a riser, but I don't see a riser in the rear lockers. Is it feasible that the loop on the 5/8" hose serves that purpose?

I've already decided to replace the hoses and loop in kind ... but I'm still failing to understand completely.

 

[sail sfbay]

...........I'm not certain why this loop is necessary............

Vented loops are required to prevent back siphoning seawater into your engine when there is no exhaust pressure and destroying your engine (Vented Loops - MyBoatsGear.com). Same question came up on this thread: Engine vented loop | Sailboat Owners Forums

 

[FastOlson]

In our boat the exhaust hose is looped upward from its exit from the water-lift muffler. The top of the upward part probably adds a couple of feet to the total run. That does bring it up to just under the lip of the cockpit seat locker. I believe that this is needed to prevent possible back flow from the stern-mounted exhaust fitting. I have seen other boats with a similar hose routing scheme.
An approximation of the height, knowing the outside waterline would put this at least 2 feet above the DWL.

Running down 5' seas in the open ocean we have submerged the exhaust exit for short periods of time, altho the engine was not running at the time. Probably good to know that a surging bunch of water will not flood the muffler and then the engine.
(Some cruisers have traditionally installed a seacock at the exhaust outlet on their transom just to prevent such a catastrophe, albeit with a notice posted by the key to open this valve before starting the engine....)

 

[dlochner]

If the entire exhaust system and cooling water system is above the waterline, a vented loop is not necessary. If any of it is below the waterline a vented loop is necessary. Here's what Vetus says about their mufflers.

Wide collection Boat Exhaust systems at VETUS boat parts

VETUS is an internationally operating developer, manufacturer and trading company, selling marine engines, generators and technical equipment for recreational craft and small commercial vessels.

www.vetus.com

 

[sail sfbay]

From page 51 of your owner's manual showing the engine exhaust vented loop.

 

[Scott T-Bird]

Vented loops are required to prevent back siphoning seawater into your engine when there is no exhaust pressure and destroying your engine (Vented Loops - MyBoatsGear.com). Same question came up on this thread: Engine vented loop | Sailboat Owners Forums

Yes, I know why & where a v-loop is necessary. Looking at my installation, I think back-siphoning in this manner can't happen. The mixing elbow and the exhaust manifold is definitely above waterline and the raw water discharge can only run into the water-lift pot, which is below the engine. I'm sure there is a riser at the stern, I just haven't looked for it specifically.

Actually, that discussion never came to any conclusion. One person said he had 2 boats with Yanmar engine that never had a vented loop in the raw water system. One person said a vented loop was needed in the raw water hose but never explained why. Some people seemed to be confused between the exhaust riser in the stern and the need for a vented loop in the raw water system (I think Stu finally said he wasn't familiar with the loop for the raw water hose. The last person that responded said that he removed it because it isn't necessary, and another said it is not necessary for freshwater cooled system (as I have). So, to be honest, the link was completely inconclusive.

 

[dlochner]

If both ends of the raw water circuit are under the waterline at any level of heel, a vented loop is necessary in the event of any leak after the heat exchanger.

What kind of leak? A cracked hose, a rusted or compromised muffler, a failed gasket, filling up the exhaust and flooding the motor, or who knows what else.

This weekend I discovered a small leak in a drain cap in my muffler, wasn't much, but it probably on the way to being more significant, at which point I'd have a 1" hole in the muffler below the water line. I do have vent, fortunately it is right over the motor with a very short run.

 

[Scott T-Bird]

From page 51 of your owner's manual showing the engine exhaust vented loop.

View attachment 193667

Yes, that definitely shows the layout. That is Catalina's take on it and I won't fault any of their design for this. I plan to restore it as shown. I think that the raw water hose could only fill the muffler pot & eventually back feed to the engine only when the mixing elbow is below the waterline. There can be no siphon when the mixing elbow at the raw water hose discharge is above waterline, which is where it is. But there may be a rare circumstance, with extreme heeling only, when the mixing elbow could be below waterline (perhaps it isn't even possible - who knows for sure). It seems like the vented loop for the raw water hose is an added precaution - just in case there is a freak occurrence.

 

[dlochner]

If the water lift muffler is not drained, it will not allow air back into the system, so it can not break the siphon. The vacuum break is not necessary for normal operation, it is necessary when something goes wrong.

 

[Scott T-Bird]

If the water lift muffler is not drained, it will not allow air back into the system, so it can not break the siphon. The vacuum break is not necessary for normal operation, it is necessary when something goes wrong.

What do you mean by something goes wrong? What would have to happen to create a siphon when the mixing elbow is above the water line? The vented loop only prevents water from being picked up underneath the boat and discharging somewhere else below the waterline. If it discharges into the mixing elbow above the waterline, there can be no siphon. If the mixing elbow is below the waterline for any reason, then there needs to be a vented loop somewhere in that system and the vented loop in the raw water line isn't going to make any difference.

 

[dlochner]

The vented loop goes between the heat exchanger and the injection elbow. If the muffler is above the waterline then a vented loop is not necessary. If the muffler is below the waterline, then a vented loop should be included. Neither the injection elbow nor the muffler provide a good vent and the muffler effectively plugs the exhaust hose as it acts like a sink trap.

Conceivably, there line could be vented from air entering the air cleaner and then to the intake manifold, and through a cylinder if both valves were cracked open. Otherwise, the cylinders will block the airflow.

 

[Scott T-Bird]

The vented loop goes between the heat exchanger and the injection elbow. If the muffler is above the waterline then a vented loop is not necessary. If the muffler is below the waterline, then a vented loop should be included. Neither the injection elbow nor the muffler provide a good vent and the muffler effectively plugs the exhaust hose as it acts like a sink trap.

Conceivably, there line could be vented from air entering the air cleaner and then to the intake manifold, and through a cylinder if both valves were cracked open. Otherwise, the cylinders will block the airflow.

Good points. I think most important to remember is that the exhaust riser at the stern does not have a vent or does not even exist in all boats. The water lift muffler can't be above the manifold without a complicated system because you wouldn't want water draining from the muffler back to the manifold. The riser at the stern isn't vented so if the riser never had air introduced to its high point when the stern is submerged with high seas, the only introduction of air to break the vacuum at the elbow would be the small vent in the raw water hose.

 

[Calif. Ted]

On later boats the exhaust hose does a complete 360 degree loop before exiting instead of the configuration shown. Still use the vented loop on raw water.

 

[Tom J]

There can be no siphon when the mixing elbow at the raw water hose discharge is above waterline,

I think you are overlooking the fact that as the exhaust cools, it creates an area of low pressure (vacuum) that could pull existing raw water in the exhaust up into the engine if the vented loop is not there. Our exhaust system is above water level, so there is no loop after the muffler.

 

[Ralph Johnstone]

(I think Stu finally said he wasn't familiar with the loop for the raw water hose.

Are you sure about that ? ? ?

Can you give us a link to that particular conversation ? ? ?

 

[dlochner]

Good points. I think most important to remember is that the exhaust riser at the stern does not have a vent or does not even exist in all boats. The water lift muffler can't be above the manifold without a complicated system because you wouldn't want water draining from the muffler back to the manifold. The riser at the stern isn't vented so if the riser never had air introduced to its high point when the stern is submerged with high seas, the only introduction of air to break the vacuum at the elbow would be the small vent in the raw water hose.

There is no need for a vented loop after the muffler, only in the raw water system. There should be a raised loop near the end of the exhaust hose so that any water that would back up into the exhaust would have to run uphill. I can think of 2 situations in which this might happen, getting overtaken by a wave that rises above the exhaust outlet and backing down hard in to a wave, maybe a boat wake.

 

[Ralph Johnstone]

getting overtaken by a wave that rises above the exhaust outlet

Every wave (even flat water) rises above my exhaust outlet :



Hence the big exhaust loop which reaches up to the deck (as you mentioned).

 

[dlochner]

Every wave (even flat water) rises above my exhaust outlet :

View attachment 193672

Hence the big exhaust loop which reaches up to the deck (as you mentioned).

Makes you wonder what Hunter was thinking. I suppose with the exhaust exiting below the waterline it is quieter.

 

[Scott T-Bird]

Are you sure about that ? ? ?

Can you give us a link to that particular conversation ? ? ?

This from the link that @sail sfbay included in Post #2:


[IMG alt="Bill Roosa"]https://forums.sailboatowners.com/data/avatars/m/77/77182.jpg?1443871943[/IMG]
Bill Roosa
Think we are talking about two different things here. the raw water feed needs a vented loop. Only raw water in that circuit. The exhaust also needs a vented loop but there is raw water and exhaust in it. I believe the OP was concerned with the raw water feed and not with the exhaust loop.




[IMG alt="Bill Roosa"]https://forums.sailboatowners.com/data/avatars/m/77/77182.jpg?1443871943[/IMG]
Bill Roosa
the raw water vented loop (usually) has a valve that has a "cap" that holds the valve parts together. The valve is a standard mushroom type and after taking the cap off you can clean it then reinstall. no need to even take off the hoses. should be part of everybody's (that has one of course) yearly maintenance tasks




[IMG alt="Stu Jackson"]https://forums.sailboatowners.com/data/avatars/m/74/74093.jpg?1443871934[/IMG]
Stu Jackson

Bill Roosa said:
Think we are talking about two different things here. the raw water feed needs a vented loop. Only raw water in that circuit. ...

Bill, new to me. Where would that VL be?

_______________________________________________________________________________________________________________________________________

These comments were from the link that @sail sfbay linked in Post #2, which I read. I think this is in reference to the v-loop shown in the Catalina Owner's Manual. It wasn't easy to find. I had to take out a cabinet in the back of the galley and then remove the duct for the air conditioning to finally find it. I knew it was there somewhere. It was difficult to find, nevertheless.

Perhaps my interpretation is wrong, but it does seem to indicate that there are an awful lot of confusing opinions regarding this matter. For instance, Bill seems to indicate that the exhaust needs a v-loop - I'm assuming that would be at the stern riser, but I have yet to see a diagram showing one. Even though it is a loop, there never seems to be any vent portrayed at the top.

 

[Ralph Johnstone]

This from the link that @sail sfbay included in Post #2:

Maybe I'm missing something here, but after digging through your above reference to Post #2, I can find nothing in which @Stu Jackson confesses to a lack of familiarity with the loop for the raw water hose.

I see your refereneces to a number of quotes from @Bill Roosa, but nothing in this regard from @Stu Jackson . Any chance you've made a mistake here ?