Vented Loop or Dirrect Discharge

Discussion in 'Fiberglass and Composite Boat Building' started by Stevens47, Feb 12, 2009.

  1. Stevens47
    Joined: Jun 2007
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    Stevens47 Junior Member

    I am not sure this is in the correct section but here goes, if not then suggest what section to pose the question.

    I have a large volume evacuation bilge pump to install. There is a small volume every day working pump lower in the bilge, this large volume pump should not see much use. The delivery hose is 2" ID. The delivery location is port side, mid section of the boat. The highest that I can get a vented loop is tucked just under the deck, about 3 1/2' above level water line.

    My question. What is the difference if locate a vented loop or just placing the discharge through hole at the same location which is about 3" below the deck level. I assume that water will egress into both installations on a starboard tack in a large sea. I was considering using a SS exhaust through hole with a rubber flapper on the direct discharge.

    The direct discharge allows for a higher volume of water to discharge with the extra 3 feet of hose plus the vented loop it adds about 1 foot of head or about 200 gph in volume reduction.

    Voice your opinions on the merits of this installation.
     
  2. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    Don't rely on flapper valve only as a mean of preventing a water flow-back from a through-hull hole. Valves can break or might not close properly.
    In the attached pic you can see why is a vented siphon necessary.
     

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  3. Stevens47
    Joined: Jun 2007
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    Stevens47 Junior Member

    Daiquiri,

    Yes I can see the value with the vented loop and do not question its use. My point is if the direct discharge is at the same level as the vented loop on a installation what is the difference. They will both back flow with water if the water rises above their level. The diagram shows a discharge in both cases that is below the level of the vented loop. It would seem that the only way for the vented loop to out perform the discharge would be to place it somewhere in the center of the boat well above the discharge. This is a difficult thing to do on most sail boats. I do agree that one should not rely on a flapper or one valve to stop water from backing into the boat. What I am looking for is reasoning that a vented loop will perform a direct discharge that is located at the same level. If the vented loop is at a level that is higher than the discharge, no question that is better.
     
  4. robherc
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    robherc Designer/Hobbyist

    Stevens-

    I think that the whole point of the vented loop is that you're placing the vent, and the top of the loop above the level where you expect water to ever reach (or higher than the boat is likely to recover from without capsize). With that in mind, I don't think it would be all that realistic to compare vented loop to direct discharge if you're wanting to put the direct discharge at the same level as the top of the vented loop, or of the vent.


    ...just my $0.02, take it or leave it.
     
  5. Stevens47
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    Stevens47 Junior Member

    Robjerc

    On the installation that is in consideration. At some time in the normal operation (large seas) the vessel will have water above the discharge or vented loop, allowing for the possibility of water to back flow. All be it not for a continuous period of time.

    I do not understand your response, can you elaborate.
     
  6. TeddyDiver
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    TeddyDiver Gollywobbler

    So whats the best you can do with the loop is to place it so close to centerline as you can below deck height (make sure it's above 35deg heeled waterline). This way it's even possible to place the loop lower and thus lower static head and lower the discharge closer down to water line.
     

  7. robherc
    Joined: Dec 2008
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    robherc Designer/Hobbyist

    OK, looking back on it... I read through the thread, thought for about 5 minutes, reached the "that's a dum idea" thought, and then started typing from there with absolutely no explanation for how I got there...oops.

    Anywise, I think my point was that direct discharge was developed for a different purpose than vented loop.

    Vented loop:
    Low-maintenance, using a vented loop allows you to not use valves, which require frequent maintenance, in your bilge-pump discharge line.

    Direct discharge:
    High-flow, using a direct discharge method with (preferably) 2 valves in the discharge line allows you to run your bilge-pump discharge out a through-hole that is at or below the water line. This way you eliminate ALL unnecessary head (you'll always have to pump against the outside water level...that's unavoidable), and use a MUCH shorter tube.

    With that in mind, the "ideal" vented-loop system would run the loop up ABOVE the deck a bit, with the vent covered by an overlapping "cap" to avoid water splashing in the vent from a wave breaking over the deck. The discharge, then can be located anywhere along the hull, so long as it is belowdeck. BIGGEST CON: You're sacrificing flow by running the loop so high above the bilge & may need to source a stronger pump to overcome the increased head.
    The "ideal" direct discharge system would run the discharge hose nearly level from the bilge pump to the nearest hull, where it would discharge through an anti-siphon valve directly overboard. This would save you materials in that the discharge line is FAR shorter, and yield increased flow rates as the amount of head is minimal. BIGGEST CON: You're adding maintenance (2 anti-siphon valves that must be periodically checked/replaced) as a sacrifice to gain a higher flow rate.

    ...that is the reasoning that leads me to ask: Why are you trying to make a direct-discharge system pump against unnecessary head, or a vented-loop system function with the loop lower than intended?


    I hope that I have been more succinct (and perhaps a bit less rude) this time. ;)
    Rob
     
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