Passive Solar Ventilation
Q: Spent a day working on my boat at the dock her in NW Florida and nearly died of heat stroke while being down in the cabin even with all hatches and vents open. I dont have too many fans because I have no connection for shore power. There was an obvious temp differential 'tween outside and inside that got me thinking. So..... Make a long fabric tube, say 10" diameter and 15' long with upper end held up with the jib halyard and lower end in the forward hatch with edges around it sealed. Upper half of the tube is black and lower half is white. Here is what happens: With a temp differential (hot inside, cooler outsside), you get a chimney effect with hot air rising out of the tube and pulling cooler air in from other ports. I can imagine getting a susbtantial flow this way. With little or no temp differential, sun still heats the black upper half and heats air inside causing it to rise causing air to be pulled through the tube giving air flow in the cabin. Steps could be taken to maximize solar heating of teh upper part of the air column such as better conducting baffles, passive solar fabric panels connected to the upper tube etc. I could even imagine a fabric reflector shining light onto the upper tube to maximize the effect. All of this works compared to not working in a house because the volume of the boat is fairly small compared to the volume of a house. Let's go further. Now that we can produce air flow, can we use such to actually produce cooling? Evaporation of water can produce a huge amount of cooling. Close all vents to the outside except for a 10" or so port and fill it with wetted baffles. Baffles can be wetted by osmosis from a reservoir. Evaporation will cool the air stream. Unfortunately, this will increase the humidity in the boat and reduce the air flow (conservation of energy). Any thoughts?
A: Do a search for "swamp cooler" - they're the evap coolers used in AZ, NZ etc before we all got rich and burn up electricity. They might work well in the Sea of Cortez in MX because of the very low humidity - but I wouldn't want a boat in FL to be any wetter inside than it normally is. Buy a solar panel and a fan. Your idea holds merit - just not to include more moisture, as another poster has observed. I used to go camping on an island off the coast of Portland (well, technically, Small Point, near Bath), Maine, with very fancy outhouses. They were called solar holers, and didn't smell. The way that was achieved was by having a "chimney" facing south, clear top (outside) and black face (inside), with the intake at the business end of the large (men's and women's, set up like rest rooms) houses. Never a smell. They also heated the water for the shower houses in the same fashion but with rooftop snakes (back and forth over the entirety of the south-facing rooftop of black pipe on black backgrounds). Almost never ran out of hot water, despite however many showered. Since this was in Maine, and the camping season was Memorial Day through Labor Day, there were *long* hours of sunshine fueling all this free energy accumulation! Why paint only the top half black? surely you'd want as much heat in there as possible. Most fabric doesn't conduct heat very well. If you have an alloy mast you have a chimny already, thermal coefficient is good (probably don't have to paint it), large surface area , height is also a positive factor (velocity). Only mod's req'd are to connect the bottom to the cabin airspace and the top to "outside" without letting rain in. Could go further if you don't mind a little extra windage up there: fit a weathercocking venturi at the masthead, any lurking breeze will add to the effect, although it's probably something you'd only consider if performance at anchor is more of a concern than racing.....
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