Not really a question, more of a PSA. I think the subject of ventilation is not well understood. This is somewhat in response to another thread. It got me paying attention to what other people are doing, and noticing that there are some misunderstandings.
There are several facets to the subject of ventilation and condensation mitigation, and there is not a one-size-fits-all solution. But, cabin ventilation is part of everyone's life.
The #1 issue I wanted to offer some advice on has to do with using fans (in any location) to try to expel cabin air and replace it with air from outside (for any reason). The short version (since I tend to be really long-winded) is that you should look at how firefighters expel smoke from a building. It is essentially an identical problem.
The long version is there are two ways: positive-pressure ventilation (pushing air) or negative-pressure ventilation (sucking air). Theoretically, these could be equally effective, but negative pressure ventilation depends on more conditions that are harder to get right. A negative pressure fan works by creating low pressure on the interior side of the fan. That low-pressure area will be filled by air taking the past of least resistance. Typically, that means that air is drawn in from around the edged of the fan - creating just a local air flow. For this to be effective, the area around the fan needs to be more or less airtight and air needs to be easily drawn in from elsewhere (some distance from the fan).
PPV (positive pressure ventilation) is generally more effective because implementation is more forgiving. With PPV, you can create a high-pressure area over a large area by pointing a fan so that the cone of high pressure covers the entire exhaust area. So, for example, a fan can be pointed at the companionway (from inside) such that the entire companionway (with all of its cracks and vents) is pressurized slightly. (You cannot do this if the fan is too close to the companionway or built into it, somehow.)
Now, all that is left to do is allow air to be drawn in from somewhere else (like a forward hatch). This creates a path of least resistance whereby the high pressure air escapes by the nearest available vent.
So, why doesn't this just create another localized air flow between the fan and the companionway? Well, it could, but the fan is also creating a low-pressure area on the upwind side. If there is less resistance between this low-pressure area and your intake vent, then that is where replacement air will come from.
This means that, for most of us, the ideal setup is this:
Most folks incorrectly assume that the most efficient way to move air is to build the fan directly into a hatch. It seems counterintuitive that a fan just plunked down in the middle of the boat would do anything but slosh the air around the interior. If you doubt this, ask a firefighter or, better yet, try it out. You'd be amazed at how much better it works.
There are several facets to the subject of ventilation and condensation mitigation, and there is not a one-size-fits-all solution. But, cabin ventilation is part of everyone's life.
The #1 issue I wanted to offer some advice on has to do with using fans (in any location) to try to expel cabin air and replace it with air from outside (for any reason). The short version (since I tend to be really long-winded) is that you should look at how firefighters expel smoke from a building. It is essentially an identical problem.
The long version is there are two ways: positive-pressure ventilation (pushing air) or negative-pressure ventilation (sucking air). Theoretically, these could be equally effective, but negative pressure ventilation depends on more conditions that are harder to get right. A negative pressure fan works by creating low pressure on the interior side of the fan. That low-pressure area will be filled by air taking the past of least resistance. Typically, that means that air is drawn in from around the edged of the fan - creating just a local air flow. For this to be effective, the area around the fan needs to be more or less airtight and air needs to be easily drawn in from elsewhere (some distance from the fan).
PPV (positive pressure ventilation) is generally more effective because implementation is more forgiving. With PPV, you can create a high-pressure area over a large area by pointing a fan so that the cone of high pressure covers the entire exhaust area. So, for example, a fan can be pointed at the companionway (from inside) such that the entire companionway (with all of its cracks and vents) is pressurized slightly. (You cannot do this if the fan is too close to the companionway or built into it, somehow.)
Now, all that is left to do is allow air to be drawn in from somewhere else (like a forward hatch). This creates a path of least resistance whereby the high pressure air escapes by the nearest available vent.
So, why doesn't this just create another localized air flow between the fan and the companionway? Well, it could, but the fan is also creating a low-pressure area on the upwind side. If there is less resistance between this low-pressure area and your intake vent, then that is where replacement air will come from.
This means that, for most of us, the ideal setup is this:
- Front hatch is ventilated somehow
- Fan is positioned approximately amidships, pointed aft so that the cone of air completely covers the companionway
- The companionway is not covered (like with canvas that prevents airflow)
Most folks incorrectly assume that the most efficient way to move air is to build the fan directly into a hatch. It seems counterintuitive that a fan just plunked down in the middle of the boat would do anything but slosh the air around the interior. If you doubt this, ask a firefighter or, better yet, try it out. You'd be amazed at how much better it works.
