If the engine doesn't start, it drowns?

Discussion in 'Inboards' started by CaptainAB, May 8, 2010.

  1. CaptainAB
    Joined: Jul 2006
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    Location: South Australia

    CaptainAB Always Learning

    Conventional wisdom insists that all inboard wet exhausts should have a riser to stop water entering the exhaust from outside whilst going astern or in a following sea.

    However, while starting the engine, the starter motor is also turning the water pump, which in turn, is filling the exhaust system before the riser with cooling water.

    If the engine doesn't start reasonably promptly, then the exhaust system, and ultimately the engine, will fill with water - not good!

    I have I missed something here? What is the solution ?

    Cheers

    Alan
     
  2. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    Yes, you did!
    Cranking does turn the water pump, but at low rpm and the displacement of the pump is small. Besides, the point where water is injected is at the riser's exit, after the highest point.
     
  3. CaptainAB
    Joined: Jul 2006
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    Location: South Australia

    CaptainAB Always Learning

    More Information

    Hi CDK,

    Yes, I wondered that, but the engine is fitted with a Jabsco positive displacement pump. You can turn it by hand at the speed of a clock's second hand and it pumps very well. Perhaps a vane pump more like a normal car engine would be better?

    The attached file is from the current Vetus Catalogue and shows the riser immediately behind the transom and the water injection point way forward and at a lower level. How can it not fill with water if the engine doesn't start?

    Thanks again

    Alan
     

    Attached Files:

  4. baeckmo
    Joined: Jun 2009
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    Location: Sweden

    baeckmo Hydrodynamics

    When rotated, without starting, the engine is still "pumping" its volume of air along the waterflow from the pump. The idea is that the volume in the damper next to the engine will have an "air lock volume", that is big enough to swallow the backflow watervolume of the vertical pipe before the final damper, when the engine stops (or non-starting).

    If the first volume is substituted for a plain hose (=saving????), the system would be at risk. This is actually seen by most marinas yearly, giving a fair contribution to their yearly income........
     
  5. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    I was thinking about a different setup because you worried about the engine not starting. This Vetus exhaust is used with diesel engines: there is no restriction in their air intake so even while cranking, enough air is displaced to "cough" the water through the hose.

    Like Baeckmo wrote, the volumes of the lower silencer and vertical hose are critical, hence the min and max distances given in the drawing.
     
  6. mark775

    mark775 Guest

    If the engine doesn't start, it drowns?

    --------------------------------------------------------------------------------

    "Conventional wisdom insists that all inboard wet exhausts should have a riser to stop water entering the exhaust from outside whilst going astern or in a following sea.
    However, while starting the engine, the starter motor is also turning the water pump, which in turn, is filling the exhaust system before the riser with cooling water.
    If the engine doesn't start reasonably promptly, then the exhaust system, and ultimately the engine, will fill with water - not good!
    I have I missed something here? What is the solution ?"


    Pay absolutely no attention to that Vetus drawing if you want your engine to live. "Coughing" water uphill is preposterous.
    Your riser shall be dry. DO NOT EVER inject water until after the spillover of a dry riser (which you build to use all available engine room height). The Vetus drawing also shows 15cm above waterline. ******** - Who would trust their engines to what, 7"? If using a waterlift, the spillover needs to be BELOW the spillover of the dry riser. How I see Vetus exhaust engineering:

    F3640132_5599.jpg

    The plus side - You can make these components yourself out of used garbage cans!
     
  7. mark775

    mark775 Guest

    "I have I missed something here?" - The word "dry". You missed the word "dry" in "dry riser". Then you need a custom blanket around it - not that **** like an old-school cast.

    NS_Exhaust-System_20_300.jpg

    One Tony Athens did. I'm sorry, there is no substitute for doing it correctly.
     
  8. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Inboard engine manuals all say that after cranking the engine for more than about a minute, if it doesn't start you should drain the lift muffler.
    Many thousands of engines, gas and diesel, are fitted with lift mufflers with no problems.
     
  9. mark775

    mark775 Guest

    An anti-siphon system should be installed on the upward rise of the water injection system (make a riser to the highest available point in the engine room) and route water over the side BEFORE the water flows into the lift muffler filling the system. Crank the engine a while, drain, crank, drain...are you kidding? People like you keep mechanics busy.

    waterlift.gif
     
  10. capt littlelegs
    Joined: Apr 2010
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    Location: England

    capt littlelegs New Member

    Quite right and you don't need a muffler, get a water pump failure and the plastic ones melt which have been know to then flood the boat.
     
    1 person likes this.
  11. Submarine Tom

    Submarine Tom Previous Member

    Fix the motor starting issue and all else is solved...

    -Tom
     
  12. CaptainAB
    Joined: Jul 2006
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    Location: South Australia

    CaptainAB Always Learning

    A better idea....

    Yes, of course - Thanks Tom - I knew there was a solution !! :D

    Actually, the engine is really reliable and starts readily. While I'm the one driving it - no problem, but there are unidentified forces in the cosmos that some folk seem to radiate, called "mechanicas disruptus" waves. Nothing ever works properly for them especially around engines.

    I should probably change my friends........


    Alan
     
  13. Submarine Tom

    Submarine Tom Previous Member

    Ah yes, "Mechanical Disruptors".

    I feel for you.

    -Tom
     
  14. powerabout
    Joined: Nov 2007
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    Location: Melbourne/Singapore/Italy

    powerabout Senior Member

    I seriously doubt a cranking engine could pump enough air through it to lift the water out of a water lift.

    So yes you could fill one up and it has been done hence Gonzos mention that many manuals warn to be careful.

    Just hold your hand on the exhaust whilst its cranking versus it idling. I'l think you will find a difference. After all if the fire didnt expand the gas it wouldnt run would it?
     

  15. CaptainAB
    Joined: Jul 2006
    Posts: 13
    Likes: 1, Points: 3, Legacy Rep: 14
    Location: South Australia

    CaptainAB Always Learning

    I noticed recently that the local garden shop had 12v 1/2 inch solenoid taps in it's irrigation section that CLOSE when energised. (I can't think what you'd use them for in the garden.......)

    However, if you put one of those in the raw water feed line and connected it across the starter motor, it would stop the water flow whilst cranking. Seems like an idea?

    Alan
     
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