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Old 06-03-2014, 19:47   #61
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Re: Blue Hull Concerns

Had a friend with a high sided ketch painted dark green. He decided to repaint it white one summer in the keys. After the initial prep was through the painter shot a coat of white primer. The next day the thermometer inside the boat showed a temperature reduction of 9 degrees with the same outside temps.
That's enough science for me.

As an aside, when I was in the canvas business I always tried to convince my customers to use dark shades of Sunbrella when doing biminis, dodgers, and awnings. Why? Because no matter how much heat radiated down with the darker colors, the average human was more comfortable due to the reduced glare.
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Old 06-03-2014, 19:55   #62
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Re: Blue Hull Concerns

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Originally Posted by skipmac View Post
All I can say is based on my personal experience of buying a black boat and painting it white, the difference was dramatic. Same boat, same cruising area. In fact, when we painted the boat white we left a black trim stripe on the side of the hull because it looked good. But on a sunny day on the inside of the boat you could hold your hand a foot away from the side of the hull and feel the heat radiating from the stripe like an electric space heater. Hokd your hand next to the white area of the hull and the difference was dramatic.
I believe there would be a difference, but seems a little hard to believe it would be that dramatic. I googled several studies and they all show a slight difference only. I can't help thinking the difference in your case could have been something else going on. The only time the sun would hit the side of the hulls directly would be sunrise and sunset, when the temperature was the coolest. During the hottest part of the day, the sun would hit the white top of the hulls only.
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Old 06-03-2014, 20:02   #63
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Re: Blue Hull Concerns

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Originally Posted by Rohan View Post

1. Are dark houses hotter than white ones?

2. And why then are roofs usually black even in warm parts of the country??

3. And are boats with teak decks hotter than white ones?
1. Yes.

2. Dumb ideas.

3. Yes. See #1.
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Old 06-03-2014, 20:53   #64
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Re: Blue Hull Concerns

Quote:
Originally Posted by Rohan View Post
I believe there would be a difference, but seems a little hard to believe it would be that dramatic. I googled several studies and they all show a slight difference only. I can't help thinking the difference in your case could have been something else going on. The only time the sun would hit the side of the hulls directly would be sunrise and sunset, when the temperature was the coolest. During the hottest part of the day, the sun would hit the white top of the hulls only.
So what studies? Post a link? Studies of boats or cars or something else?

All I can say is I owned the boat for 6-7 years and lived onboard off and on for about 3 years total, half that time it was black, half the time white with a stripe. It was very obvious to me, my wife and friends that sailed with us the heat radiating inside the boat where the outside had the black stripe. I'm pretty sure there was no radiant heater embedded in the hull sides, no difference in the fiberglass layup from one area to the next, no hidden insulation so nothing weird going on that I can imagine.

Have to disagree about the sun only shining on the hull at sunrise and sunset. No the suns rays may not be perfectly perpendicular to sides of the hull in the afternoon but will certainly be shining on the hull.

Only if you are at the exact latitude of the sun and it is exactly local noon will the sun shine only on the deck. Well I guess if you had a boat where the entire hull flared out at the deck level it might shade the hull sides a little longer.

If you are north of the Tropic of Cancer even exactly at local noon the sun will be south of your position and not directly overhead so some part of the suns energy will hit the side of the boat. By 3-4 pm the boat's sides will be well lit and receiving a lot of energy.
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Old 06-03-2014, 20:59   #65
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Re: Blue Hull Concerns

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Originally Posted by Stu Jackson View Post
1. Yes.

2. Dumb ideas.

3. Yes. See #1.
1. Yes.

2. For looks and they don't show dirt and stains as much.

3. Had a boat with teak decks and on a sunny day it would actually burn the bottoms of my feat and I went barefoot all the time and had pretty tough feet. Would walk along the white cabin top and it was just fine.
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Old 06-03-2014, 21:26   #66
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Re: Blue Hull Concerns

Please see avatar...that boat is HOT! You cannot walk on the decks in the Texas summer sun, period. And the inside walls are crazy hot. The "addition" AKA shanty house we're almost finished building across the bridgedeck of the same boat, has tan (primer) walls right now and a white roof. Pick a sunny day and walk around feeling the inside of these three wood surfaces, only difference is the color outside, the difference is DRAMATIC! Last summer a 900BTU AC was not able to cool 1/2 of the port hull where my bed was during the day, and I sleep during the day, so not good.

I HATE white boats (because everyone has one) and yes, that whole boat will be white when I'm done the transformation.

Are people really going to continue to PooPoo this many personal anecdotal examples?
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Old 07-03-2014, 05:43   #67
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Re: Blue Hull Concerns

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Originally Posted by skipmac View Post
So what studies? Post a link? Studies of boats or cars or something else?

One of them was here, on cars:

How much hotter is a black car than a white car in the sun?

This guy noticed that while the paint temperature on the surface of the car varied greatly between light and dark colors, the interior temperature (measured on the glass) didn't vary much. And cars are made of metal, which holds heat more easily than wood or fiberglass.

I guess the point here is that people hoping their dark hulled boat will keep them warm in wintertime would probably be disappointed.

Quote:
All I can say is I owned the boat for 6-7 years and lived onboard off and on for about 3 years total, half that time it was black, half the time white with a stripe. It was very obvious to me, my wife and friends that sailed with us the heat radiating inside the boat where the outside had the black stripe.
In my limited experience, that's just something I never noticed, although admittedly I wasn't looking for it either.

I wonder too if its the difference between having insulation or not. Certainly if you can feel the bare fiberglass hull from inside your boat, you will feel the sun more easily. However, most production boats today have some kind of insulation or lining inside them.
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Old 07-03-2014, 06:37   #68
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Re: Blue Hull Concerns

In this study;
"... the silver Honda Civic (shell SR 0.57) had a cabin air temperature of about 5-6°C (9-11°F) lower than an identical black car (shell SR 0.05)..."


Silver and white cars are cooler, says study

In this amateur test;
"... the glass temperature (which I assume to be a proxy for interior temperature) varied only 5 to 6 degrees between the black and white cars on average. So I conclude that the interior temperature only varied somewhere in the neighbourhood of 5 to 6 degrees. The paint temperature, however, varied by about 55 degrees, which is ten times as much..."


How much hotter is a black car than a white car in the sun?
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Old 07-03-2014, 06:40   #69
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Re: Blue Hull Concerns

Quote:
Originally Posted by Rohan View Post
One of them was here, on cars:

How much hotter is a black car than a white car in the sun?

This guy noticed that while the paint temperature on the surface of the car varied greatly between light and dark colors, the interior temperature (measured on the glass) didn't vary much. And cars are made of metal, which holds heat more easily than wood or fiberglass.

I guess the point here is that people hoping their dark hulled boat will keep them warm in wintertime would probably be disappointed.



In my limited experience, that's just something I never noticed, although admittedly I wasn't looking for it either.

I wonder too if its the difference between having insulation or not. Certainly if you can feel the bare fiberglass hull from inside your boat, you will feel the sun more easily. However, most production boats today have some kind of insulation or lining inside them.


A little knowledge is a dangerous thing. The quoted "scentific" study relied on an infrared thermometer to measure surface temperatures in the sun. That study was less scientific than the one every kid does on the first day of summer--walking barefoot across the street, and finding that the white lines are a hell of a lot cooler than the black asphalt. The problem with infrared thermometers is outlined at

Pyrometer - solving emissivity problems in infrared thermometer temperature measurements

"Every object radiates thermal energy at temperatures above absolute zero. Measuring the temperature of an object using an optical pyrometer is based on the principle that the thermal radiation from the object being measured is a function of its temperature, and measuring thermal radiation sounds like a very straightforward engineering problem.
But the real world is more complicated. For any particular temperature and wavelength, the energy radiated by a surface is directly proportional to the spectral emissivity of the object. Emissivity is the value associated with a surface’s ability to get rid of heat by radiating thermal energy, and different substances have different emissivities. The value of a substance’s spectral emissivity is a number in the range of from 0 to 1.0, which is the ratio of the energy radiated by object’s surface to the energy radiated by a perfect blackbody at the same temperature. The higher the value, the better the surface is at emitting energy."
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Old 07-03-2014, 07:16   #70
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Re: Blue Hull Concerns

Additional proble with the dark hull is colour degradation due to UV.

On my present boat I use the A-Glaze, for preventing this, to a lot of success. It is easy to apply. In Med You need to reapply it every second year. in tropics annually.

Sealant for GRP vessels - Sealant - Exterior Cleaning Products - Shop
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Old 07-03-2014, 07:21   #71
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Re: Blue Hull Concerns

Quote:
Originally Posted by Rohan View Post
One of them was here, on cars:

How much hotter is a black car than a white car in the sun?

This guy noticed that while the paint temperature on the surface of the car varied greatly between light and dark colors, the interior temperature (measured on the glass) didn't vary much. And cars are made of metal, which holds heat more easily than wood or fiberglass.

I guess the point here is that people hoping their dark hulled boat will keep them warm in wintertime would probably be disappointed.



In my limited experience, that's just something I never noticed, although admittedly I wasn't looking for it either.

I wonder too if its the difference between having insulation or not. Certainly if you can feel the bare fiberglass hull from inside your boat, you will feel the sun more easily. However, most production boats today have some kind of insulation or lining inside them.
Without running my own test this point is just speculation on my part but with the large windshield and window area on cars I think there is a very different dynamic there compared to a boat. The car will experience significant greenhouse effect which would to some degree overwhelm some of the difference produced by the different paint colors.

On my boat we weren't really looking for the effect it made itself known to us just being inside on a sunny day. Any part of the interior of the boat that didn't have cabinets or furniture the inside of the hull was lined with carpeting so was somewhat insulated. But I am not exaggerating at all. You could easily feel heat radiating from the hull on the inside of the boat. The first time I felt it I didn't know what it was. Thought it was a reflection or part of the hull was shadowed or something. Looked outside and it was obvious that the hot spot was inside exactly where the stripes were on the outside.
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Old 07-03-2014, 07:25   #72
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Re: Blue Hull Concerns

Quote:
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A little knowledge is a dangerous thing.

"Every object radiates thermal energy at temperatures above absolute zero. Measuring the temperature of an object using an optical pyrometer is based on the principle that the thermal radiation from the object being measured is a function of its temperature, and measuring thermal radiation sounds like a very straightforward engineering problem.
But the real world is more complicated. For any particular temperature and wavelength, the energy radiated by a surface is directly proportional to the spectral emissivity of the object. Emissivity is the value associated with a surface’s ability to get rid of heat by radiating thermal energy, and different substances have different emissivities. The value of a substance’s spectral emissivity is a number in the range of from 0 to 1.0, which is the ratio of the energy radiated by object’s surface to the energy radiated by a perfect blackbody at the same temperature. The higher the value, the better the surface is at emitting energy."
The emissivity of the surface is not relevant to this problem. That is because the emissivity of the painted white and painted blue (darker) surface are approximately equal.

The relevant parameter for this problem is the solar absorptivity. A portion of the solar energy incident on a surface is absorbed (converted to heat). The remaining portion is reflected (some may be transmitted through but not relevant here).

The fraction of the solar energy absorbed is the "solar absorbtivity" of the surface. So white surfaces reflect more solar energy than dark surfaces. White surface therefore have a lower solar absorptivity than dark surfaces for normal coatings (paint).

I did a little calculation to look at how the temperature of a vertical flat plate would change by painting a white surface black. The only change in my calculation between the dark surface and white surface is the amount of solar energy that is absorbed by the surface. All other physical parameters are held constant.

I assumed a wind speed of 5 meters/second, air temp of 27 deg C, and a background radiation temperature of 17 deg C (a temp that a vertical surface would see). There was no heat exchange on the backside of the plate which would be somewhat consistent with an already hot interior cabin. Used a long wave emissivity of 1.0.
I also used a high value for the solar energy incident on the vertical plate, 600 watts per square meter.

By changing the solar absorptivity from 0.97 (black surface) to 0.6 (white surface the steady-state temperature of the plate was reduced by 12 deg C (about 24 deg F).

Even allowing a small amount of heat transfer on the cabin side of the plate would not change the result by much.

This 12 deg C change in temperature will be significant wrt human comfort in warm or tropical climates.
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Old 07-03-2014, 07:37   #73
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Re: Blue Hull Concerns

Additionally, typical hull insulation will not help much. It will slow the heat transfer in and out of the cabin. However, over the course of the day a closed cabin may end up just as warm as the uninsulated hull.

Why you may ask? The typical hull insulation is thin and inefficient. It is more designed to prevent condensation (and mold) in colder (than air temp.) water, which is a different kind of problem. Consequently, a significant amount of heat flow is realized through this kind of insulation and so the cabin heats up.

Part of the human comfort equation is the surrounding radiation temperature or temperature of the interior cabin walls. The walls and surfaces are radiating heat to you. Hot walls make for less comfort than cooler walls if the air temperature remains the same in a warm environment.
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Old 07-03-2014, 15:31   #74
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Re: Blue Hull Concerns

I am REALLY looking forward to the day when hull colors can be changed by the press of a button. Wouldn't that be fantastic.

I predict this to be available in 15 years, 20 tops.
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Old 07-03-2014, 15:51   #75
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Re: Blue Hull Concerns

"2. And why then are roofs usually black even in warm parts of the country??"

Ah, because the most common roofing material is asphalt shingles, and the color of asphalt is...wait for it...BLACK.

There are other materials and in places like Florida and Texas where the sun kills everything, you'll see ads for white and silver roofs for larger buildings expressly to cut the heat and insulate them. Not applicable to shingled roofs, which often are terra cotta tile in hot places--not the hot black asphalt either.

Asphalt shingles are quick and cheap, that's why they dominate so many markets. Black just comes with the package.

Most important reason not to go to sea in a blue boat: As any Greek can tell you, blue is Neptune's color and Neptune can be a jealous god. Go to sea in a blue boat, and you are just daring him to take it.
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