Bilge ventilation- how to? - thermodynamics genius wanted

Discussion in 'Boat Design' started by mydauphin, Sep 19, 2010.

  1. mydauphin
    Joined: Apr 2007
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    Location: Florida

    mydauphin Senior Member

    It seems no matter what I do, there is a little water left in bilge. I was thinking of getting 3 inch duct fan, kind used for ventilating engine compartments in inboards and getting air from ceiling and pumping it down into bilge, then opening little holes in floors to let cooler air out. Air would travel over water and help evaporate it and at same time cool it. The fan would be solar power and run all day.

    The question is this; should I get air from above floor which is already cooler and pump it into bilge and exhaust it at top, or get warmer air from top and pump it into bilge.

    Which is better????
  2. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Get drier air, from where ever. In Florida, evaporation can be a difficult thing to manage. Heat helps, but it's the amount of moisture vapor all ready in the air that determines how effective evaporative arrangements will be.

    Generally, you want to draw air out of the bilge, not push air into it. This is because you'll get the smell of the bilge if it's pressurized, but not if you're drawing off the air. So, place the 3" or 4" in line fan hose in the bilge and send it up to a clam shell or something. This will draw air through the ceiling and down into the bilge, across the standing water and out through the hose and fan.

    In winter months this will be very effective in Florida. In the summer less so, but still to a degree effective.
  3. missinginaction
    Joined: Aug 2007
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    Location: New York

    missinginaction Senior Member

    PAR, do you think that adding a second fan in the bilge area to encourage air movement before it's actually exhausted from the boat would help? This would encourage evaporation in the bilge and then the primary ventilation fan would exhaust the moisture laden air from the boat.

    I'm thinking that he has a condensation issue. If so that liquid water must be all over everything in the bilge area before it accumulates and then runs down into the bottom of the boat.

    I've had some luck with running a ventilation fan that simply circulates air in the usually stagnant bilge area. This process evaporates the condensation off the inside of the hull before it accumulates enough to break the water's surface tension and run into the bottom of the bilge.

    This might not be a perfect solution but it seems to help. Small ignition protected fans are pretty cheap.

    Of course here in the northeast US the condensation may be easier to deal with as the humidity generally drops during the day.



  4. mcollins07
    Joined: Jan 2006
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    Location: Texas

    mcollins07 Senior Member

    You need low humidity air to pass over the bilge. Low humidity air is hard to come by naturally in Florida. To get the effect you want, there are a couple of approaches. 1) change the air flow based on the humidity of the naturally available air, 2) artificially reduce the humidity.

    A common small window air conditioner unit cools the warm humid air, reducing the amount of water the air can hold. These A/C units typically reduce the water content and temperature of the air coming out of the front of the unit, and have water leaking out the back of the unit. They artificially reduce the humidity of the air.

    In my marina in texas, we usually run a small air conditioner in the summer and a small heater during the winter to dry the interior. In the winter, by taking in cool air and heating it, the air is made relatively dry. The moisture content is primarily limited by the cool temperature of the outside air which comes in the cabin. Heating the air in the cabin increases its capacity to hold water. So, the warm air in the cabin has a relatively low humidity and absorbs moisture from the interior. This warm moist air will find a way out because of a slight positive pressure created when heating the cool air.

    I think the first step in drying your bilge is to decide where to get low humidity air. If you have low humidity air in the cabin and still have moisture in the bilge, I’d consider placing a 3” or 4” hose in the bilge with a fan on the upper end of the hose, blowing air outside, creating kind of a vacuum cleaner for the bilge. If this vacuum is pulling low humidity air from the cabin, across the bilge then venting it outside, it will dry the bilge.

    I see Par posted while I was typing and I said about the same thing, but I'll post in case the different wording helps.
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