Keeping the Cabin Cool

[somedayy]

oops, had a few. I meant wear black. LOL

[boatman61]

LOL... working on some science one would think that wood being a poor conductor of heat would keep the boat a tad cooler.. and.. if you douse it well at dawn and dusk it may have some effect on cooling the boat... along with a good Boom Tent from mast to cockpit and some wind scoops for the hatches... if you have opening ports little scoops for them are great..
Personally I don't think it matters... apart from the if it looks cooler it is cooler mind trick we play on ourselves... heat rises chaps..
My experience in the tropics from the Carib to Darwin were... its not the sun that makes you suffer its the Humidity... and you can't do SFA about that.. I dripped just as fast on a white boat as on my blue with teak deck 54ftr delivery.. and walking into an air conditioned shop led to a Tsunami effect from my pores...
Suck it up chaps... great for slimming/weight loss...

[Miniyot]

Originally Posted by TacomaSailor

Anytime from mid-May to late September in the Sea of Cortez ( 24 N to 29 N) I could not walk in bare feet on our white decks during mid-day - I literally blistered them several times.

I guess I have tender feet?

I have heard you can walk on a teak deck barefoot in the summer with no problem....AND they are a better no-skid deck than a "no skid" fiberglass deck....Wood also has better insulating qualities than fiberglass.

But, I have to admit that I do not have 1st hand knowledge of this...

[Kettlewell]

I had a boat that was painted dark green with a light grey deck and after a winter in Florida and the Bahamas we changed the hull color to white with a light tan deck. The boat was much cooler after we painted her. We left a wide dark green stripe along the sheer line (which looked really nice), but you could feel a distinct difference inside the hull where that paint line was--hot above, cool below. And no you can't walk on teak decks at noon in the tropics without burning your feet--at least I can't.

[zeehag]

teak decks can be walked on in sun only after wetting em down with sea water.
yes interior is cooler with a layer of insulation on them. i wish my formosa still had her teak decks, if for nothing else, for safety as in teak is best non skid ever found. as long as it is not varnished, oiled or cetoled.
without teak decks, tarps rock for cooling off boat.

[TeddyDiver]

About teak decks there's some difference having them grey vs shiny and spoiled.. err oiled

[jpmckay]

This is actually a non-trivial question but it is not open to opinion, it is simply a matter of creating a heat budget. First you need to throw out all data from cars, a car in the atmosphere is in a completely different environment than a boat in the water.

Lets start by simplifying and considering only a pure black boat (hull and deck) and a pure white one. We'll further assume that the black hull has an albedo of 0, that is that it absorbs all solar radiation and reflects none (a perfect black) while the white one has an albedo of 1, it reflects all radiation which reaches it and absorbs none. We'll further assume that both boats are sealed boxes with no air exchange with the atmosphere.

There are five primary mechanisms by which heat is exchanged with the boat. The first is short wave solar radiation (sunshine). The second is long wave radiation (heat radiating away from the boat as IR waves). The third is evaporative cooling (latent heat exchange) off the skin. The fourth is conduction between the boat and the water. The fifth is conduction between the boat and the air. We will ignore convective exchanges, rain cooling and such.

During the day, SW_rad dominates over LW_rad while that reverses at night. 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. Evaporative cooling is negligible. LW_rad will depend on boat temperature. Conduction will vary depending on the relative temperatures of the boat, the air and the water.

Letting both boats start at the same temperature in the same conditions, the white boat gains no heat from SW_rad while the black gains 30 W/m^2 on average. LW_rad is initially the same but increases linearly as the black boat increases in temperature. Thus as the black boat gets hotter it also radiates heat away faster. Temperature exchange with the air and water scale the same, as the black boat heats up more from the sun it exchanges heat with the air and water at a rate that scales with the temperature difference.

So if the air and water are hotter than either boat and the black boat is hotter than the white boat, it will gain heat from the air and water more slowly. the converse applies as well. If the air and water are cooler than the boats and the black boat is hotter than the white boat, it will cool off faster.

I've run a quick simulation assuming air and water conditions as taken from 1 AUG 2012 to 1 SEP 2012 from the COAMPS reanalysis and find that for totally sealed boxes the internal temperature in the black box is ~2C warmer (on a daily average) than in the white box. The biggest difference occurs during the day with nights being generally less than ~1C difference.

I did not run the numbers assuming airflow but it is reasonable to assume that a well ventilated boat will see little difference in interior temperature, surely not enough to dictate your paint color.

Take with a grain of salt, just some first order calculations from a sailor with a Ph.D. in ocean physics and some time to kill.

Paul
New Orleans

[zeehag]

Originally Posted by TeddyDiver

About teak decks there's some difference having them grey vs shiny and spoiled.. err oiled

yes, there is that--first of all--oiled or shiney decks means fall overboard or fall down, slip and slide. always good for a laugh , as this can be entertaining, when boat is at dock. could be fatal at sea. is why decks are not oiled nor varnished.

grey decks means no one at all is standing on them nor using them nor pouring the water over them daily, which is the only treatment teak wood decking requires.


properly wetted teak decks are pretty and wood colored, not slick nor shiney nor grey and disintigrating. they are very easy care, as all they require is water from ocean daily. they also donot leak when properly cared for. the problem with 30 yr old teak decking is obvious--all the previous owners let them sit, drying them out to point of cracking and disintigration. this is not reversible.


as for the topic at hand--i have sailed dark hulled boats and white blinding topsides boats and every color in between except purple (no one had one) there is no difference in heat inside cabin due to hull color when water and air are flowing by and thru.
when ventilation ceases, even bright blinding white boats are ovens without protection from sun.
btdt. buy what you want. buy tarps if you plan on sailing tropical waters. the ocean will be 85 in winter and above 90 in summer. plan accordingly. butter separates.
as far as ambient heat--is the humidity that gets ye..is like walking thru water.
in summer i try to be in marinas so i can use an airconditioner that de humidifies the interior beautifully, and i use on lowest settings, just to rid air of wet.
i also place tarps so a/c doesnt have to be always in use.

[TeddyDiver]

Well 'real' teak decks are so rare.. read teak deck as GRP covered with thin teak strips..

[TacomaSailor]

jpmckay - CAUTION:

Your post has violated the standards of much of what this board stands for
- you used hard math
- you used real science
- you stated demonstrable facts
- you refused to state your opinion

And most important
- you killed the topic because there is nothing left to say after your post!

[daddle]

Originally Posted by TacomaSailor

jpmckay - CAUTION:

Your post has violated the standards of much of what this board stands for
- you used hard math
- you used real science
- you stated demonstrable facts
- you refused to state your opinion

And most important
- you killed the topic because there is nothing left to say after your post!

Nor does it relate in any way to the maximum temperature of a dark boat. Or a dark box. Fancy science words and a PhD won't sell your confusion about what is going on in a hot boat.

[jpmckay]

Can't argue with that. Years of experience, multiple publications about oceanic heat budgets, degrees in ocean physics and marine engineering and I am defeated by a "neener-neener".

If you do not believe the math then try this. Put a thermometer in a white boat and a black boat and see what happens.

For my own self, I live in a seriously hot part of the US (New Orleans), I have done the math and I am planning to paint my hull a dark blue. I work in topical seas on large research vessels, many of them are painted dark colors. This is not an issue.

[daddle]

Originally Posted by jpmckay

Can't argue with that. Years of experience, multiple publications about oceanic heat budgets, degrees in ocean physics and marine engineering and I am defeated by a "neener-neener".

If you do not believe the math then try this. Put a thermometer in a white boat and a black boat and see what happens.

For my own self, I live in a seriously hot part of the US (New Orleans), I have done the math and I am planning to paint my hull a dark blue. I work in topical seas on large research vessels, many of them are painted dark colors. This is not an issue.

No need to be rude. Your credentials are worthless in explaining the evidence. You did not present any math. To refute the experience of many, and simple observations of others, you will need to work harder.

Your only numbers are for average daily temperature. That's plain deceptive.

Why are solar water heaters painted dark colors? Because they look better that way? Because the stuff inside gets hotter that way? Or because some misguided PhD was not consulted about the color?

[jpmckay]

Solar heaters are in air. Air is not a good conductor of heat. The hot water bag gains its heat through solar radiation. Boat hulls are in water. Water is a good conductor of heat. Note my earlier mention that studies with cars are not relevant since the cars are in air, not water.

The relevant fact here is that boat hulls gain and lose heat through their contact with water far more rapidly than they do through their contact with air. The color of the interface between the air and the water is irrelevant to the conduction of heat. It only matters with radiant heat.

[TacomaSailor]

"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"

SO - my observations based on three years of living full time aboard a boat in one of the hottest marine environments in the world (did I mention day time air temps were above 90 for 100 days in a row and there was hardly a cloud in the sky) are of no value?

We anchored for months at a time off of desert islands that did not see a drop of rain for years at a time. We anchored where the afternoon winds were always over 95 degrees. But - what I experienced is not valid?

Typical of the scientific non-believer - they deny facts and then when someone presents an inconvenient empirical observation they claim only their experience is valid.