Re: [bolger] reflective coatings > Micro in Noosa
- Once I read an interesting study on reflective coatings, and I'm sorry I have no memory of where I read it now. They had 3 pictures each of several different kinds of materials, one taken with infrared light, one taken with visible light, and one taken with ultraviolet light.For a coating to be effective in keeping a structure cool when its in direct sunlight, it needs to be highly reflective from the low infrared end of the spectrum to the high end of the ultraviolet spectrum.Some of the coatings, that looked very bright white under visible light, were nearly black under infrared or ultraviolet light, and so they absorbed quite a bit of heat from sunlight.Its not easy to create a coating that is highly reflective across such a broad spectrum. So the best bet is to get a reflective coating from a reputable company who worked hard on their research to come up with a highly reflective coating across all of these wavelengths. They will have to make compromises in some sections of the spectrum, and so ideally they would come up with a compromise that gives the greatest net reduction in heat absorption. They will also have to make compromises on materials so there is pretty good reflection at a pretty reasonable price.Another factor to keep in mind that the angle of the sun effects the percentages of different wavelengths that actually reach the ground. So the sunlight on the equator isn't the same sunlight at 60 degrees north. The sunlight isn't just weaker at 60 degrees north, it has different percentages of radiation than at the equator. I don't know if this is taken into account when formulating reflective coatings.I'm sure there are price differences between companies that don't exactly correlate to exactly how reflective they are, and how effective they are in reducing heat absorption.I'm sure comparative studies have been done, but the trick is to find free competent results on the internet. Since I'm in a cool damp cloudy climate, I never made any notes on this issue, like the source of the study or what materials worked best.The flip sides is if you want to absorb heat from the sun effectively, the coating needs to be black across the whole spectrum from infrared to ultraviolet.If its really important to cool the boat, maybe the simplest thing to do is get a few highly recommended brands of coatings from a few different reputable local naval suppliers, coat sizeable test patches (at least a couple feet square), all on the same kind of material (probably plywood so you are more measuring the temperature of the coating than the material), and use a digital thermometer with a probe (just lay the probe on the surface and wait till the temperature stabilizes) and see if there are any significant temperature differences between the coatings under different conditions.Keep in mind that moving air will cool the coated surfaces, so for the results to be comparable, the air movement needs to be the same for all of the measurements. Probably the easiest thing to do is do all this behind a windbreak so there isn't any air movement when making all of the temperature measurements when comparing different coatings.If anybody does this, please share the results and be sure to mention the location the testing was done and the material the coating was put on.Of course, if somebody finds a well documented study like this on the internet, that would be even better.Sincerely,Roger----- Original Message -----From: "Peter" <pvanderwaart@...>To: <firstname.lastname@example.org>Sent: Thursday, November 03, 2011 7:07 AMSubject: [bolger] Re: Micro in Noosa>> Problem is that it absorbs heat to the
>> pointthat you do not want to lay your bare hand on it
>> when in thesun.
>What you describe is also true of unpainted aluminum. You would think the highly reflective surfaces would stay cool, but they don't.
> Yeah, there is a phenomenon here that I don't understand.
>the metal just transfers heat to your hand faster than a painted wood surface.
> On maybe,
> We need some science here.
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> If its really important to cool the boat,Or take an entirely different approach, and put up an awning so the boat stays in shade.
> maybe the simplest thing to do is get a few
> highly recommended brands of coatings...