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Old 03-04-2013, 17:47   #31
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Re: Keeping the Cabin Cool

I'm not absolutely convinced teak decks make the cabin warmer. Granted the decks get hot to walk on with bare feet, but 3/4" of teak also has some insulative value. It would be interesting to see some real world comparisons with two similar boats side by side. Personally I think it's negligible...
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Old 03-04-2013, 17:49   #32
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Okay. I will move out of my air cooled cabin into the water cooled bilge.

Pseudo science, name calling, rudeness, fancy credentials, clever writing, aliases: hilarious. I'm not that easily duped.

To the kind poster above regarding humidity being a big part of the problem. So true. But there is a great amount of relief whenever a cloud blots out the sun ... until the cloud rains, making suffocating steam.
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Old 03-04-2013, 17:50   #33
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Re: Keeping the Cabin Cool

And just to deepen the scientific mystery - no one has mentioned that in the tropics where the sun is really hot it is also directly overhead (the definition of the tropics) so the rays of the sun are beating down on the TOP of the boat not the colored side.

If I remember correctly - the heat absorption of a vertical surface (the boat hull) is a function of the sine of the sun's rays on the the hull. When directly overhead (angle = 0 relative to the vertical hull) sine = 0 and NO infrared energy is absorbed by the dark or light hull.

When the sun is at 30 degrees to the vertical hull (60 degrees above the horizon) the sine=.5 so only 1/2 the maximum available energy is presented to the vertical hull.

And at 20 degrees to the vertical hull (70 degrees above the horizon) the sine is 0.34 so only 1/3 of the maximum available energy is presented to the vertical hull.

The tropical sun takes about 13 hours to traverse the sky during the hot season so that is about 14 degrees per hour - if we assume it does little to heat the hull when above 60 degrees that means that 1/3 of the day (180 - 30 - 30) the sun is too high in the sky to heat the SIDE of the hull.

So the color of the hull, if it makes any difference, only makes that difference when the sun is below an azimuth of 60 degrees or so which is only early morning (6:30 AM to 11 AM) and late afternoon (3:30 PM to 7:30 PM) in the tropics.

Just something to think about?
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Old 03-04-2013, 17:56   #34
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Re: Keeping the Cabin Cool

We had a beautiful Tayana 37 with teak decks and a dark blue hull. She was definitely hot hot in the summer.
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Old 03-04-2013, 17:59   #35
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Re: Keeping the Cabin Cool

A lot of people use windscoops on the hatches at anchor. Pretty easy to construct. Another option is an electric fan, I have a 12 volt one which works really well.

Another more complex idea is to suck fresh air in a large diameter pipe, have a mist of salt water, which dries the air, then a mist of fresh water to cool the air, then mist again with salt water which dries it again. Only downside is you need fresh water.

Maybe someone has tried some kind of system at anchor with a hose going overboard fairly deep to pump the deeper water through a heat exchanger to cool incoming air. This has potential as I recall the water temperature is usually cool until you are +- 10 degrees latitude.. and even then it would be cooler than the midday sun.
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Old 03-04-2013, 18:00   #36
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Re: Keeping the Cabin Cool

"Okay. I will move out of my air cooled cabin into the water cooled bilge. "

The scientists that responded here were careful to qualify their statements to IGNORE the effect of air circulation in the boat. That circulation is little effected by the color of the hull.

Once again - you are discounting the science by use of irrelevant issues, or issues that are specifically not included in the analysis.

And... Ambient Humidity can not conceivably be related to hull color

I guess I should stop here - my problem is that my graduate school training was in scientific methodology and the philosophy of science and it really irritates me to see modern society so carelessly dismiss scientific responses when they do not meet the personal beliefs or desires of the community.

Everyone loves science when it helps them (computers, Spectra Sails, fiberglass hulls, good weather forecasting) but dismisses the same science when it doesn't match their belief system.
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Old 03-04-2013, 18:33   #37
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Re: Keeping the Cabin Cool

I'd like to address jpm's summary above and the disagreements with it. jpm states "the internal temperature in the black box is ~2C warmer (on a daily average) than in the white box." He then adds "The biggest difference occurs during the day with nights being generally less than ~1C difference."

When you're on the boat you aren't concerned with 24 hour (daily) averages. You're concerned with how hot it is now. When is it usually hottest? Middle of the day. Again, per jpm, "During the summer in the Sea of Cotez that SW_rad term will peak around 500 W/m^2 at mid-day but average to about 30 W/m^@ over 24 hours." (W/m^2 means watts per square meter, by the way, ie a measure of the energy of the sunlight striking the hull surface). So, at mid day the energy from sunlight striking the hull is 500W/m^2 mid day peak/30 W/m^2 24 hour average = 16.67 times the average for a 24 hour period. That is a big difference.

So, while the black box is 2C warmer than the white box on a daily average, what you are going to really care about is the difference between the two at mid day. If, as per jpm's calculations, nights show less than 1C difference, and the total difference is 2C, then the average daytime temperature (sunrise to sunset) is about 3C higher in the black box. What will the peak temperature difference be between the black and white boxes at mid day? He doesn't say. I'd guess at least double the 3C daytime average. And if this is close to the mark, 6C is very noticeable when you're in that box, mid afternoon.

As lots of other people have said, if you've got awnings then it doesn't make much difference because they block the sun. And other factors - ventilation and insulation - can make enough difference that the boat color can become insignificant. Neverthless, it is unarguable that the darker boat has a larger heat load to deal with than the lighter one. Eliminating the use of averages and displaying the temperatures on a 24 hour diurnal chart would give a more accurate display of the differences, over time, of the black and white boxes.
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Old 03-04-2013, 19:12   #38
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Re: Keeping the Cabin Cool

I forget the line but something about "sailors are the only people who will spend hours pontificating the argument points about unrolling toilet paper from the bottom or top."
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Old 03-04-2013, 19:33   #39
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Re: Keeping the Cabin Cool

[QUOTE=TacomaSailor;1202002]"To refute the experience of many, and simple observations of others, you will need to work harder. "

AH! My favorite kind of forum poster - a member of the George Bush / FOX News science community which would NEVER let well established science for which there is no dispute at all get in the way of personal dogma and magic belief.

"To refute the experience of many, and simple observations of others"

Oh, we should believe the Obama/Reed/MSNBC lovers because they listen to those oracles of truth. After all, the anointed one says it's so.

Went on board one of those horrible slab sided Columbias that they painted a broad darker colored stripe down the middle of the topsides so no one would realize they were horribly ugly slab sided boats. Putting your hand on or close to the hull, you could feel where the white ended and the darker color began. The heat coming through the darker painted area was significant. FWIW, The dark color was that medium blue Columbia loved, not dark flag blue.

The way to keep a boat comfortable is to keep the sun off the boat. Way more important is having awnings to block the sun entirely. We had white canvas awnings that went from bow to stern and boat was comfortatble through a Tahiti Summer. Not only were most days above 90 degrees but the humidity gauge pegged. Awnings will keep the decks cool whether they are fiberglass or teak. Still, teak will act as a heat sink, soaking up heat in the daytime and radiating it out after the sun goes down. Awnings will only be partially effective in keeping the sun off the topsides.

If you have a question about teak decks being hot when exposed to direct sun, try walking on them around noon in bare feet. It will improve your dance moves.

We used to get great enjoyment laughing at the boats that were 'engineered' by their engineer owners. Never saw more stupid ideas tried that may have had some proper engineering principles behind them but didn't work worth a dam in the real world. If you want to be uncomfortable in the tropics, paint your un-insulated boat black and put on teak decks.
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Old 03-04-2013, 19:41   #40
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Re: Keeping the Cabin Cool

Quote:
Originally Posted by roverhi View Post
Oh, we should believe the Obama/Reed/MSNBC lovers because they listen to those oracles of truth. After all, the anointed one says it's so.
You had to bring politics into this? Come on man. I'm on the other side of the fence from nearly every person on this forum but you don't see me bad mouthing anyone's ideology or democratic choices.
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Old 04-04-2013, 19:46   #41
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Re: Keeping the Cabin Cool

Sounds like a good Mythbusters experiment.
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Old 04-04-2013, 20:12   #42
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Re: Keeping the Cabin Cool

Visited a new catamaran at a marina near Brisbane, Australia last December at the height of our summer in middle of the day. Temp around 35C with little breeze.

Vessel topsides were white and all anti-slip surfaces were an off white colour.

Expected to be jumping around barefoot when I walked on saloon roof/cockpit hardtop. Materials used were thick, resin infused foam sandwich.

I could not believe how cool the surface was to walk on barefoot. The builder indicated the off white colour was a major factor with the cool deck surface.
Teak decks as earlier poster indicated retain heat.
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Old 04-04-2013, 20:35   #43
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Re: Keeping the Cabin Cool

Okay not to dispute Bill the science guy and all you other geeks on here... but I live in Miami. My boat was blue I painted it white and it was cooler inside, sailed to Mexico it was hot put up a 20 dollar tarp for a sun shade and it kept sun off the top decks and it was even cooler. Get off the computer and go sailing!
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Old 04-04-2013, 22:43   #44
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a simple thought ...
teak is a hart and solid. Heat is nothing else than vibration of material. As more solid the material, as better it vibrates and transfers the heat in that case to the underlying fiber, and so to the cabin air.
So balsa sounds like the best choice for the material.
Dark surfaces attract a stronger vibrating wavelength from sun, probably one, which transfers again its potential vibration better to the type and size of material we use.
A different color = a smaller wavelength, white, stays cooler.
Am I right so far?
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Old 05-04-2013, 00:09   #45
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Re: Keeping the Cabin Cool

albedo = reflectivity 1 = perfect reflection (white) 0 = no reflection (black)

The LOWER the albedo the less energy the surface reflects back to sky or sea and most likely, depending on the conductance of the material, the cooler will feel the surface.

You can draw almost no conclusions about the reflective properties (albedo) of a surface solely based on color. For example:

"white" asphalt shingles have an albedo of 0.21 and "white" cement concrete is about 0.7 and "white" acrylic paint on cement is 0.8 so the color white tells us nothing about the reflection of energy.!

The "white" shingles absorb almost four times as much infrared energy (long wave solar energy) than does "white" acrylic paint on concrete.

Other experiments show that the albedo (absorbtion of solar energy) of a textured surface can decrease by almost 100% when soiled or dirty. For example the albedo of any color non-painted concrete will change from 0.25 to 0.45 when soiled or dirty thus absorbing almost twice as much infrared (heat) energy.


Likewise calling a surface "wood" tells you nothing as shown by the following albedos:
Red Gum = .48
Red Oak = .62
Spruce = .72
US Forest Service Research Paper May 1966

We can see that a NEW spruce deck would reflect 50% more energy than a NEW red gum deck but a DIRTY spruce deck would absorb MORE energy and feel warmer than a NEW red gum deck.

Therefore, how can anyone conclude a teak deck absorbs less energy than a fiberglass deck without knowing the state and age of the teak, the anti-skid and age of the fiberglass, and the color of the fiberglass?

Everyone has their personal experience but none of that is science.

My personal experience, earned over 300 days in blistering hot desert sunshine between 23N and 27N, is that my WHITE fiberglass decks were so hot they blistered my feet. I measured deck temperatures of over 130 degrees F with my IR remote sensor. Others report that their white fiberglass non-skid decks were so cool they could walk on them in bare feet.

Just goes to show how little personal experience can tell us unless we specify a LOT of other important details.
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