Yanmar exhaust question

[Ted]

I have two questions regarding a Yanmar 2GM engine.

1. Is the "U" shaped mixing elbow the same part on both the raw and fresh water cooled engines?

2. Is there a reason/advantage why a boat builder would install an engine with the inverted "U" shaped mixing elbow as opposed to the straight mixing elbow? (assuming interference/clearance was not an issue)

 

[Ralph Johnstone]

Although I have a Yanmar 2GM20F, I have never seen a straight mixing elbow unless it was custom made for the purpose. Can you supply any details on the straight elbow or better yet, a web picture ?

 

[kloudie1]

Yanmar made both straight and inverted "U" mixing ells for that engine. The "u" is for engines that are a bit below the waterline or very close to it. The straight one is for engines that are at or slightly above the waterline.

 

[Ted]

Kloudie1, that makes sense. Thanks.

Ralph Johnstone, here are some photos of the two different types of mixing elbows. I have both and was wondering if there was a reason for swapping the straight mixing elbow for the inverted "U" shaped one.

 

[kloudie1]

The U has a bad habit of plugging, moreso than the straight (er) one.. but it would work in either application.. The straight one should not be applied to a below waterline application. To put a U on a formerly straight would require the cast adapter 128370-13610 and the left/right threaded joint 104214-13580 in addition to the elbow itself and the gaskets.. a fairly expensive conversion..

 

[Ralph Johnstone]

Ted,

Given my experience with the inverted "U" mixing elbow for the last 12 years, I would go with the straight elbow if at all possible. My greatest concern with the inverted "U" is that eventually the wall between the exhaust gasses and the sea water will corrode through and drain back into the exhaust manifold, through an open exhaust valve, and into a cylinder. This may be a little difficult to envision if you don’t have one in front of you. I have scratched
out the shape of this separation wall on your original picture of the inverted "U".

The straight elbow which your picture shows does not appear to suffer from that problem.

In order to sleep nights, I have religiously inspected and tried to clean the interior of the elbow every year and have replaced the elbow every three years. The last elbow I bought was $310.00. Carbon build up is not bad but the corrosion of the interior wall is
always good for a dose of diarrhea. In addition, the engine never runs less than 80% except for warm up.

When the "U" is in good shape, it runs cool to the touch and presents no exhaust back pressure. I'd hate to see
what happens if this item were ignored for several years.

If it were possible to change out my exhaust elbow from the inverted "U" to an arrangement where corrosion could never result in sea water leakage into a cylinder, I'd do it in a heart beat. Unfortunately, the engine is shoe-horned into a tiny little pod with no room for changes. Mind you, the mixing elbow is cool and adds no heat to the pod.

Best of luck with your choice.

 

[kloudie1]

Ralph, I am surprised to see that the hose between your heatex and the exhaust elbow does not have a vented loop in it. That would concern me because of the possibility of syphon into the engine.. The little silver loop and valve on the back wall of the engine box in the picture..

 

[Ralph Johnstone]

Kloudie,

Good eyes you have there.

I tore the #%$^%&*&^ vacuum breaker out of there four years ago when it started leaking sea water all over the engine. And as you can see, I have a bit of an obsessive compulsion disorder (OCD) about keeping my engine clean. I often eat off of the block as it's so clean. Spent a week cleaning it and repainted the discoloured/corroded areas.

If you'll notice the small black and white lamacoid label on the bulkhead just aft of the mixing elbow, it's the vessel's water level (fully loaded). The bottom of the water passage within the inverted "U" is 8 inches above this water level.

Now in theory, I should never get water rising this high as any momentary breakers creeping up aft and hitting the exhaust outlet would have their kinetic energy damped by the vanes of the raw water pump. Also, in theory, the cross sectional area of the water passage in the mixing elbow should never be able to fill completely and thus siphon due to the resistance of the water pump vanes. I also have a loop in the exhaust discharge which coils up to level of the deck.

However, due to my OCD affliction, I still wasn't happy with this arrangement. So I inserted a soft seated, spring loaded, check valve in the raw water line which added another 12 inches of water head resistence to any water trying to pass through the line by hydrostatic and/or velocity head. Total head of 20 inches.

Even with this added resistance plus the labeled flow switch, I can run the engine flat out at 3550 RPM (can't quite make 3600) without exceeding the thermostat specs.

Not exactly according to Hoyle, but given that the engine has no more than an inch or two clearance under the pod, it's the best we can do under these trying circumstances.

 

[kloudie1]

Laughing about block dinner.. Sounds like you've considered this for some time ! The only other thing would be the low pressure created between the lift muffler and the exhaust valves when ya shut down and the temp in the exhaust system drops and the combustion products condense. Shouldn't be enough volume to pull enough water downhill to cause any problem; the added resistance of the check valve probably has that under control.

 

[Ted]

Kloudie, Why would you want to have a vented loop on the hose between the heat exchanger and the exhaust mixing elbow? The water flowing through that hose is sea/raw water and will only go back into the heat exchanger not the fresh water loop of the engine. Am I missing the concept here? Did your boat come from the manufacturer with the vented loop or was it installed later?

Ralph, Why would you add a check valve to the sea/raw water hose before the pump? What harm could be done without it?

 

[kloudie1]

Ted, the main need for one is to keep the seawater from syphoning into the exhaust and filling it and then the engine. If a person were to leave the seawater valve open and the water pump has a little wear in it, then because the seawater outlet inside the elbow can be below the outside water level, the water could drool into the exhaust outlet until the muffler filled and the hose filled back to the engine.. This might happen on a long sail when the seacock is left open and the engine is not running. By putting a tall loop and a vacuum break between the heatex and the ell, the seawater can not syphon that way. A tall loop in the exhaust line after the muffler makes this possible because the engine floods before the water can get out of the outlet and overboard. If you've verified that the seawater outlet in the ell is always (even heeled) a good bit (maybe 8"-10 ") above the water level then you probably don't need one. Ralph's picture of the ell with the line drawn shows the approx location of the water mixing point on the left side.

 

[Ralph Johnstone]

More on Reduced Pressure in the Lift Muffler .......

Kloudie,

What you say about reduced pressure due to the cooling of exhaust gasses and partial condensation occurring in the lift muffler after shut down is, of course, something else to cause me sleepless nights.

What SHOULD, and I say "SHOULD" happen is that any water in the big aft exhaust loop will fall (from the top of the loop) forward into the voluminous muffler and aft into the sea, thus breaking any possible water seal which MAY have formed at the instant of engine shut down. I'm presuming, of course, that the muffler is mostly empty of water after gunning the engine just before shut down. This is my theory and it's all I've got to hang my hat on.

And further on the subject of water possibly entering a cylinder or two, did you ever consider the coming and going of old spent exhaust gasses (at 100% RH) into a cylinder through its open exhaust valve over the winter while the boat is laid up ? This, as the result of daily air temperature swings. I see nothing but cylinder wall corrosion in my nightmares. This is why I leave the mixing elbow off all winter after inspecting it. Remember, I'm riddled with OCD.

I'm so glad I took up sailing after a long stressful career ..... really carefree and helps me unwind. :cussing:

 

[Ralph Johnstone]

Reason for Check Valve in Raw Water Cooling ......

................. line is to add further resistance to the possibility of sea water being siphoned over the mixing elbow and flooding the exhaust system.

Ted, you have to remember that I've fixed my boat up to the point where it will very nicely withstand a nuclear attack.

The check valve is probably overkill in my situation but just a little more insurance against flooding. However, I have definitely broken Nigel Caulder's golden rule as well as many ABYC tenets in removing the vacuum breaker from the raw water line. This is something you don't want to do without a lot of very careful consideration as to elevations and any other etceteras involved.

Suffering from OCD is also a big help with these things.

 

[mitiempo]

Ted

Here is a Yanmar diagram showing the anti siphon and exhaust routing they suggest. Some run a hose from the anti siphon to the cockpit as a telltale that cooling water is flowing.

 

[Ted]

Thanks to all for your explanations. I hadn't thought about the possibility of the entire exhaust hose filling with water. It makes sense now.