Vented Loop on 3QM30 exhaust?

Discussion in 'Main Morgan 38 Sailboat Forum' started by MDHazel, Jan 4, 2019.

  1. MDHazel

    MDHazel New Member

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    I'm going through my (new to me) Yanmar 3QM30 by checking all maintenance items and not assuming that anything is correct.

    The raw water hose that injects cooling water into the exhaust elbow doesn't have a vented loop and I'm wondering if that is the case with other 3QM30s in the Morgan 382 fleet.

    Does your 3QM30 have a vented loop? Anyone have problems with the raw water siphoning into the exhaust system, and then the exhaust valves?

    Many thanks,

    Marc
     
  2. scupper

    scupper Vern Gliot

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    Yes! There needs to be a vented loop in the line. Easy to do. Mount in engine bay high on Stb. side bulkhead.
     
  3. jimcleary

    jimcleary James M. Cleary

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    Marc
    If you are talking about the small hose from the aft end of the exhaust manifold to the mixing elbow in the 2" exhaust hose, I don't think a vented loop is necessary. The mixing elbow is made so that the raw water entering it can only flow aft away from the engine where it is carried to the water lift muffler by the pressure of the exhaust gases. You could put a vented loop there but it isn't needed. Our Yanmar 3QM30H has never had a vented loop there. When the engine is running the raw water goes aft and out. When the engine shuts down the flow of raw water stops at the same time as the pressure of the exhaust gases so the water can't back flow into the engine.

    Jim
     
  4. bluesbyrd

    bluesbyrd Chris Langton

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    I agree with Jim.

    The conditions for back-siphoning cannot occur in this circuit, so there is no need for a vented loop.

    Chris
     
  5. scupper

    scupper Vern Gliot

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    Sorry I miss understood the question. Not needed at entrance to elbow. I do however have one between the pump and heat exchanger. as that hose rises from the pump and drops down to exchanger . FWIW.
     
  6. jimcleary

    jimcleary James M. Cleary

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    Could a back-siphon be created in that raw water intake line, between the pump and the heat exchanger, when the engine isn't running? The blades of the centrifugal pump impeller should stop any flow when the pump isn't running. Unless a number of the blades are broken away to allow the water to pass by the pump. If that happens there will be other troubles to worry about. So I just don't see the need for a vented loop anywhere in the raw water supply.

    I can see the need for a high loop or a vented loop in the 2" exhaust hose. If the engine isn't running (under sail) and a wave slaps the hull at the exhaust port, water could be forced back up the exhaust piping, through the muffler, and into the exhaust manifold and the cylinders. A rare but possible event.

    I am not a fan of vented loop fittings because they are known to clog easy, require frequent maintenance, and may not be able to work when you need them.

    Jim
     
  7. scupper

    scupper Vern Gliot

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    My vented loop was put in because I did have a minor impeller issue (one missing blade). I guess it ended up in just the right position when I shut down the engine and allowed water to siphon into the exhaust hose with expected results. There was no sign of the engine getting hot, and the impeller was less than one year old. I never had this problem with any other boat. Not any design issue as I can see. Maybe just a freak event, but it did happen. One less point of failure to worry about and inexpensive to do. FWIW.
     
  8. jimcleary

    jimcleary James M. Cleary

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    Vern

    What does FWIW mean?

    Jim
     
  9. mpearson

    mpearson Mark Pearson Staff Member

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    Jim - it means For What It’s Worth
    ;)
     
  10. jimcleary

    jimcleary James M. Cleary

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    Mark
    Thanks! I'm not very good at this use of abbreviations as a means to save wear on the thumbs. My 9 year old Granddaughter has been trying to teach me but I constantly have to ask her to explain it to me again. The one that baffles me is: LOL. Is it Lots Of Luck or Laugh Out Loud?

    Jim
     
  11. MDHazel

    MDHazel New Member

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    This is the scenario that I have also heard about. I've also heard of raw water sneaking past a worn face plate on the pump. I was curious if Morgan installed the vented loop on the 3QM30. It's cheap enough to add one, but they are worthless if one lets the vent get clogged with salt, etc.
     
  12. jimcleary

    jimcleary James M. Cleary

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    We still have the 3QM30H that came with the boat. There has never been a vented loop anywhere on the engine at all. There was one of the vented fittings on the head overboard line, but that was constantly clogged and never worked right.

    Jim
     
  13. john Gonzalez

    john Gonzalez Member

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    Jim, do you think there should be a vented loop on the head water intake? My Morgan did not have one and I wonder if that would be a good idea. The thought is that if you forget to close the valve after use you can flood the head. This may be for a different thread but I would be interested in your opinion.
     
  14. jimcleary

    jimcleary James M. Cleary

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    John
    You are right. If the valve in the intake line isn't properly closed, it can start a siphon and flood the head. We've had that happen when a guest on board didn't totally understand the instructions. A vented loop might work in that instance, but, in my opinion, the failure rate of the vented loop fittings is too high to put my faith in one of them. As you can tell, I'm not a vented loop fan. I do run the head discharge line and the engine exhaust hose up as high as they can be run in the boat, just without the vent fittings.

    Jim
     
  15. john Gonzalez

    john Gonzalez Member

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    Jim, I have seen exhaust hoses on the Web showing the hose from the manifold to exhaust with a large loop, almost appearing to be in the way but not necessarily being attached to the top of the engine room but having a hose with a lot of inches for the water to travel. Do you recommend that? I shortened my hose with a prebend made into the hose so that it is not pinched off when bent to attach to the exhaust but it is shorter than most I have seen. I can understand the concept of the head discharge hose with back siphon but I don't seen the engine exhaust hose having that problem. But I must admit that I am not savy mechanically, particularly with regard to marine engines. Let me know your thoughts and if I should put in a large loop going from manifold to exhaust.
     
  16. jimcleary

    jimcleary James M. Cleary

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    John
    You are right about the high loop in the hose from the exhaust manifold to the exhaust elbow. It doesn't need to have a loop but because the two fittings are so close together, the only way to get a straight piece of hose to work is with a loop. If you have or can find a hose with a prebend manufactured in it, it would work just fine. When I replaced the short hose from the heat exchanger on the 3QM30H to the exhaust manifold, I found the perfect hose with a prebend from a Volvo car engine. Works fine.

    Jim
     
  17. john Gonzalez

    john Gonzalez Member

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    That's good news as I did the same thing with autozone folks who went through their stock to find the right bend and then I cut off the remaining hose that also left me with a spare the same length with another bend. So good news. Thanks Jim.
     
  18. stnick

    stnick lee nicholas

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    John , Im not tolerant of any one on my boat Forgetting to empty the head valves, and returning them to the proper dierction. . I have Printed out a "How to use this Head" Sign Posted on the bulkhead above the head.! I feel its dangerious , And if you cant read than dont use my head !
     
  19. john Gonzalez

    john Gonzalez Member

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    We all have a vented loop on the pump out side from the head as the boat was designed for that. But I'm puzzled by the fact that the intake of seawater for the head doesn't have vented loop when the head is clearly below waterline. The Morgan manual does not show one either. Wouldn't a vented loop be necessary to keep water from coming into the head if the water intake valve is left open?
     
  20. 1lostbouy

    1lostbouy Member

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    If you had a vented loop on the head or engine intake it would not work it would just suck air in the line
     
  21. bluesbyrd

    bluesbyrd Chris Langton

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    For the head, the vented loop goes on the hose between the pump and the bowl, not on the hose between the intake and the pump.

    In that position it works as intended and is essential.

    C
     
  22. bluesbyrd

    bluesbyrd Chris Langton

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    Here’s an illustration that may help:

    (Pump to the right, bowel to the left)

    upload_2019-1-21_10-15-0.jpeg

    (Source: BoatUS)
     
  23. john Gonzalez

    john Gonzalez Member

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    So, if the intake valve is left open would the vented loop on the hose from pump and bowl, prevent raw water from over flowing the toilet which is where the hazard is?
     
  24. 1lostbouy

    1lostbouy Member

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    Yes as long as it is on the discharge side of the pump
    You can push water through the vented loop (discharge side of pump)
     
  25. bluesbyrd

    bluesbyrd Chris Langton

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    Yes!

    I had exactly that problem, and a vented loop on that hose fixed it.

    Note: The vented loop has to be run *above* the waterline.

    C
     
  26. john Gonzalez

    john Gonzalez Member

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    Thank you for this information. Let me look at my layout and i'll try to attach a picture of the way it is now for advice. thanks again.
     

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