Hello,

I am wondering if the vented loop on the Onan, (MDKAV), generators are all mounted up against the ceiling of the engine room as seen in the attache photo ?

This is how the vented loop is installed in a 2004 Super Maramu 2000. 

Is this the normal way that Amel installed the venue loop ?

Thank you.

Paul 
ARAMIS
SM 444
Comox, BC, Canada

Thanks Bill,

I have been told by three mechanics and two surveyors that:

1. The vented loop should be vertical not horizontal.

2. The way the vented loop is plumbed into the exhaust hose is not a good idea.

All five said the way it is set up can potentially lead to problems with water working its way back to the engine causing mechanical problems or worse.

We just spent a few days cleaning out the exhaust elbow, exhaust manifold and replacing a two valves, (that prevented the Kubota from working). It looked like there was slight water or moisture causing corrosion in the area of the valves. The mechanic who was helping me said he believed the current vented loop set up is most likely what caused the issue. We now have the generator working again, but I have been strongly advised to modify the current vented loop system.

1. Change the vented loop so it is vertical, (not horizontal).

2. Remove the drain of vented loop from where it is now, (plumbed into the Onan wet exhaust hose), and move it to a place like or even beside the one for the main engine.

With the upmost respect to the designers of my Amel,  I am wondering if there have been any problems on other Amels with the way the original vented loop system has been set up on their Onan-Kubota gen-sets ?

Has anyone changed or modified the vented loop coming of the the Onan-Kubota gen-set ?

Thank you for your time.

Paul
ARAMIS
SM - 444
Comox, BC, Canada

My Amel 55 has the same arrangement for the gen set, the loop is on the ceiling.

Thank you Micheal, Danny and Billy for your replies, I really appreciate your comments.

I have been advised to do as Micheal did when he installed his new genset, switch over to a Vetus vented loop and mount it vertically. This would also mean not using the the hose currently connected to the top of the vented loop that drains into the exhaust hose just prior to exiting the boat. As Micheal was told, you can not tell if the horizontal vented loop is working or not, ( with it draining back into the wet exhaust).

The way the vented loop on the main engine is set up, vertical and then draining into the cockpit drain I can hear that it is working. 

Danny, the exhaust elbow did have some rust and a little corrosion so I cleaned it up and reinstalled it. The exhaust elbow that I have does not have a tube inside but a series of vents from an inner sleeve that does travel about three inches before the mixing occurs. After cleaning it up I did blow through the water inlet nipple and I did not feel any resistance which would have indicated blockage). I am not sure if the exhaust elbow that is installed here is the original spec installed or not ? 

I have been advised that with the connection of the vented loop top, (now mounted horizontally), into the wet exhaust hose,  there is a chance it could allow water to back flow up the vented loop and then back towards the engine. This could happen, as Danny stated, when the engine stops, giving a little cough, sucking exhaust gases back, and possibly a little water from the exhaust hose, particular if the boat is healed over to port while sailing on a passage. It would be kind of like drinking and breathing in at the same time, aspirating. The corrosion on the exhaust valve and the area closest to the wet exhaust did not look like a one time event. It looked like a build up over time. A couple drops of water or perhaps salty steam over time. Due to circumstances beyond my control, I had not run the Onan for over a year, I am sure this exasperated to problem, allowing more corrosion.

This genset has less than 2000 hours on it now and is running fine again. To be safe, I will order a new wet exhaust elbow. I will install the new elbow when it arrives,(before running the genset again). I will remove the current horizontal vented loop and make sure it is good order and does not have any blockage. I will check the angle of the horizontal vented loop and and do as Danny has suggested, make sure it is draining properly.  I will also start looking into the Vetus vented loop.

I wonder if there have been many premature failures of the genset engines with the horizontal mounting of the vented loop and the way it drains into the exhaust hose ? 

I appreciate that you took time to read the post and responded.

Paul
ARAMIS
SM 444
Comox, BC, Canada

We need to distinguish between failure of the syphon break and the routine corrosion and missed maintenance of the exhaust elbow.

In any saltwater service wet-exhaust engine, the exhaust mixing elbow is a maintenance item that needs to be inspected regularly and replaced, before it fails. They corrode, and collect soot and carbon.  It is the nature of the beast.  That behavior has nothing to do with the design of the syphon break.  You have hot and corrosive salt water mixing with hot and corrosive exhaust gases.  If (when!) it corrodes internally, or clogs with soot, water can short circuit the correct flow path and damage the engine.  This is what I see described.

if the syphon break fails to work, then right after shutdown, the water in the exhaust system flows back and overflows the water-lift muffler, and fills the engine with water. This is a catastrophic failure. The engine will water lock and fail to crank, possibly bending piston rods. There are other system design and operational issues that can flood the engine, but that is the only one that the syphon break is there to prevent.

There is no reason a horizontal syphon break should function ANY differently than a vertical one.  The physics are the same for either one.  Amel did it this for a very good reason:  To get the high point of the system as high as possible, while also keeping the hose runs as short as possible.  If you change this system, do NOT lower the vent point for the syphon break.  Every inch matters.  Be aware, that if you end up with longer hoses (especially from the syphon break back down to the exhaust elbow), you will put more water into the water-lift muffler when you shut down the generator.  You will need to evaluate that volume and be SURE the water lift is still of sufficient size.

Most surveyors do not really understand the details of an exhaust system. They know: "This is the way I see them, it works, and I am not going to risk approving anything else."  There are MANY good and functional ways to design a wet-exhaust system, and an infinite number of WRONG ways.  Amel's system is a good and functional one.  Be aware, that changing it, even with the best of intentions, risks putting you into the space of one of the wrong ways.  In an exhaust system, details matter, a lot.  

Remember... hundreds of Amels have been traveling around the world with exactly this system for many decades.  My own boat has been aroud the world twice, and is pushing 200,000 sea miles.  The idea that this system needs to be changed to work needs more than an opinion--even an expert's--to convince me.  When we replaced our old Onan, we kept the Amel syphon break system, because it works.  Ironically, one of the things that forced us to dump the Onan was we could no longer get replacement exhaust elbows for the one that had corroded and failed...

Bill Kinney
SM160, Harmonie
Port Louis, Grenada
http://www.cruisingconsulting.com