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Old 03-08-2014, 07:45   #1
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Keeping Cool Under the Bimini

I found myself sitting more and more inside in the raised saloon area. It seemed cooler even though there was less breeze than outside in the cockpit under the bimini, so I set about investigating why. The saloon has a lot of windows, all open and they are shaded externally.

This is what I measured by pointing the IR thermometer at the same cushion after allowing the temperature to stabilise. I picked a cream coloured cushion as close to skin tone I could find to try to simulate how the body might heat up.

In the saloon with a light breeze: 30 deg C
Under the cockpit table with a light breeze: 30 deg C
Under the cockpit bimini with a good breeze: 34 deg C
Under the cockpit bimini, behind the windshield with no breeze: 36 deg C
Under full sun, good breeze: 49 deg C (Who doesn't need a bimini...)

So clearly a lot of radiation is getting through the bimini. The breeze also convects a good bit of heat away. The key results from these measurements is that the breeze in the cockpit compared to no breeze reduces the temperature by 2 deg C and full shade i.e. under the table or inside compared to being under the bimini - reduces temperature by a further 4 deg C or probably 5 deg C as a guess. - I'm guessing the difference in temperature between inside and outside would be a degree lower outside than inside as the breeze is better there, but I couldn't find a way to measure that directly.

How much heat is getting through is surprising to me as the bimini is light in colour - a cream Sunbrella, so should reflect a lot of energy. I would expect a dark one would block more IR, but would also absorb more and radiate the heat out again, some of which would be upwards. I wonder if it would be overall better or worse?

I would like to win that 4 deg C of temperature reduction outside. It should improve comfort quite a bit. One option is a fixed white fiberglass bimini. Or I could insulate the current one and use an alu foil.

Anyone have any experience or ideas?
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Old 04-08-2014, 07:03   #2
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Re: Keeping Cool Under the Bimini

Interesting study. I have two large solar panels mounted above my bimini with about a 2 1/2" air space between, so I wonder how this setup would read? I would think it would be close to the 30C reading thinking that the air space acts as an insulator.
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Old 04-08-2014, 07:18   #3
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Re: Keeping Cool Under the Bimini

I suspect to a thermometer the breeze is meaningless, as our evaporating perspiration makes us "feel" cooler in a breeze. Now if you wet the pillow slightly your experiment may be more accurate.
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Old 04-08-2014, 07:20   #4
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Re: Keeping Cool Under the Bimini

I am having a boom tarp made (soon) and am planning to have it as a bi-level. The two levels will be joined at the seam (over the boom) and the flies will attach at two levels to shrouds and bimini arch.

The idea of course is to get shade on shade with airflow between.
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Old 04-08-2014, 07:39   #5
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Re: Keeping Cool Under the Bimini

If your trying to reflect the heat you might want to get a couple of survival blankets and try them to see if the concept is worth pursuing.
I know my Bimini absorbs and radiates a lot of heat on mine, I can only imagine if it were two ply, with a reflective layer in between, then it would block almost all of the heat as opposed to radiating on it's bottom side.
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Old 04-08-2014, 07:41   #6
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Re: Keeping Cool Under the Bimini

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Originally Posted by Ex-Calif View Post
I am having a boom tarp made (soon) and am planning to have it as a bi-level. The two levels will be joined at the seam (over the boom) and the flies will attach at two levels to shrouds and bimini arch.

The idea of course is to get shade on shade with airflow between.
That works very well on military tents. We called them tent liners, primarily made to help keep the heat in, in Winter, but also helped keep the heat out in Summer
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Old 04-08-2014, 07:44   #7
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Re: Keeping Cool Under the Bimini

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I suspect to a thermometer the breeze is meaningless, as our evaporating perspiration makes us "feel" cooler in a breeze. Now if you wet the pillow slightly your experiment may be more accurate.
Your re-inventing the WBGT

Wet Bulb Globe Temperature - Wikipedia, the free encyclopedia
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Old 04-08-2014, 07:48   #8
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Re: Keeping Cool Under the Bimini

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That works very well on military tents. We called them tent liners, primarily made to help keep the heat in, in Winter, but also helped keep the heat out in Summer
Exactly! I am also planning to make it out of my old genoa. I've sat under the heavy blue or green canvas and it's oppressively dark. I am hoping it will allow light in yet provide the shade.
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Old 04-08-2014, 08:47   #9
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Re: Keeping Cool Under the Bimini

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Originally Posted by Tortuga's Lie View Post
Interesting study. I have two large solar panels mounted above my bimini with about a 2 1/2" air space between, so I wonder how this setup would read? I would think it would be close to the 30C reading thinking that the air space acts as an insulator.
That is something I would like to know. I know the bimini gets warm to the touch, so there is some radiated heat and I know it lets light and IR energy through as it is bright to look at.
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Old 04-08-2014, 08:53   #10
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Re: Keeping Cool Under the Bimini

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Originally Posted by capngeo View Post
I suspect to a thermometer the breeze is meaningless, as our evaporating perspiration makes us "feel" cooler in a breeze. Now if you wet the pillow slightly your experiment may be more accurate.
I am trying not to measure the effect of perspiration, just essentially to get an idea how much a person is heating up because of a) radiated heat from the bimini and b) transmitted heat through the bimini. The ultimate objective is to help make a better bimini.

The evaporation question is vital to staying cool, but not in answering these questions.
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Old 04-08-2014, 08:56   #11
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Re: Keeping Cool Under the Bimini

This is funny. I built my awning out of dark brown Sunbrella. I had observed that the level of shade under Biminis depends on the opacity of the "roof". My shade is darker than your shade. Light indicates the presence of energy. Shade has less heat energy. Better shade has the lowest temperature. Now, if there were no air movement laterally, then there might be a higher temperature due to absorbtion by the darker surface, but after many years, I've found there to be no higher temperatures, and generally much lower ambient temperatures under the awning. I suppose the best combination would be a Reflectex awning, combining the opacity of the mylar coating with the foam insulation.

Prove it to yourself: Hold a dark towel over your head, then do the same with a teeshirt or white towel. You will feel a difference.
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Old 04-08-2014, 08:58   #12
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Re: Keeping Cool Under the Bimini

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Originally Posted by a64pilot View Post
If your trying to reflect the heat you might want to get a couple of survival blankets and try them to see if the concept is worth pursuing.
I know my Bimini absorbs and radiates a lot of heat on mine, I can only imagine if it were two ply, with a reflective layer in between, then it would block almost all of the heat as opposed to radiating on it's bottom side.
Thanks for the suggestion. It would be useful to test as they are good at reflecting radiated energy. It would be also interesting to compare the results with a light coloured one.
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Old 04-08-2014, 09:14   #13
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Re: Keeping Cool Under the Bimini

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This is funny. I built my awning out of dark brown Sunbrella. I had observed that the level of shade under Biminis depends on the opacity of the "roof". My shade is darker than your shade. Light indicates the presence of energy. Shade has less heat energy. Better shade has the lowest temperature. Now, if there were no air movement laterally, then there might be a higher temperature due to absorbtion by the darker surface, but after many years, I've found there to be no higher temperatures, and generally much lower ambient temperatures under the awning. I suppose the best combination would be a Reflectex awning, combining the opacity of the mylar coating with the foam insulation.

Prove it to yourself: Hold a dark towel over your head, then do the same with a teeshirt or white towel. You will feel a difference.
I haven't tried the suggestion, it does depend on how transparent the shirt is of course, but the question of dark or light shading has long intrigued me. I personally wear white and definitely feel hotter in dark clothing. I remember reading some time ago that paradoxically the Bedouins like to wear black as it is cooler for them, but a quick internet search seems to disprove this:

Why do Bedouins wear black robes in hot deserts?
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Old 04-08-2014, 09:19   #14
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Re: Keeping Cool Under the Bimini

When my old (blue) bimini gave up the ghost I replaced it with white 10mm sheet PVC, product name Komocel. I was pleasantly surprised how much cooler it made the cockpit. I also changed the dodger and occasionally use Suncloth between the two on especially hot days.
https://www.flickr.com/photos/brazen...e/11104062065/
https://www.flickr.com/photos/brazen...e/11104062065/
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Old 04-08-2014, 09:57   #15
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Re: Keeping Cool Under the Bimini

I think if you put a "space blanket" or survival blanket on top of your Bimini, you'll get some funny looks, but I bet it would be cooler.
Best I think as one poster suggested, put Solar panels up there
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