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Old 03-10-2010, 05:30   #1
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Dark or Light Hull - Is the Rumor that it Makes a Difference True ?

We're in the process of repainting our hull, and are leaning towards Flag Blue. I've heard that darker hulls tend to be warmer, though. Is there anyone out there who has experience with both dark and light hulls and can confirm or deny this rumor?
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Old 03-10-2010, 05:35   #2
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Rumor????? LOL

Dark hulls soak up the sun's heat, and make a boat warmer. Not a great idea for tropical climates, as you'll be sleeping on deck. Probably a terrific idea for northern latitudes.
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Old 03-10-2010, 05:48   #3
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Originally Posted by Tropic Cat View Post
Rumor????? LOL

Dark hulls soak up the sun's heat, and make a boat warmer. Not a great idea for tropical climates, as you'll be sleeping on deck. Probably a terrific idea for northern latitudes.
True, that's the theory, but theoretically, dark surfaces would also then radiate more heat at night, which would cool the boat.

I know how conventional wisdom permeates among sailors (we're all guilty!! ) and was wondering if anyone who has sailed with a dark hull and a light hull in the tropics has actually noticed any difference.
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Old 03-10-2010, 06:04   #4
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you misunderstood me. I meant what I typed literally. We have a dark hulled boat next to us in the marina.

Paint your boat dark if you head north, paint it white if you're planning a trip south.
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Old 03-10-2010, 06:06   #5
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Originally Posted by heatherbrie View Post
We're in the process of repainting our hull, and are leaning towards Flag Blue. I've heard that darker hulls tend to be warmer, though. Is there anyone out there who has experience with both dark and light hulls and can confirm or deny this rumor?
I wish I had sunlight right now, if I did I'd take some pictures for you to look at. I painted my decks with a basic gray epoxy undercoat, and topped it with an epoxy white. There wasn't a problem for about three months, until the paint yellowed a bit, and now it's a rather pleasant and less-glaring (but much hotter) creme-white. Just for fun, I painted an acrylic white around my chain locker and it's stayed pure, bright, blinding white. The difference just between those two locations on the deck is easily 15-20 degrees F during the day.

This has led me to toy with the idea of re-painting the entire hull white, also. That's how big of a deal it seems like it might be, to me. Of course, I'm in the Philippines, so the problem is maximized.
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Old 03-10-2010, 06:07   #6
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Spent a couple of decade owning and diving from rigid hull inflatables. Even in the UK black tubes need over pressure valves to stop them going pop in the sun. Plus on the one day a year the sun does shine dark tubes get so hot you couldn't sit on them wearing shorts or a bikini. The solution was to glue light grey wear patches on the top of the tubes.

Next boat came with light grey tubes

No idea what it would be like south of 50.N with dark tubes and can't imagine how hot a small yacht would be in the Carib or Med during July / Aug. Why not paint up a plastic box, white on top and blue round the sides, stick it in the sun for a day with a thermometer inside.

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Old 03-10-2010, 06:20   #7
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[QUOTE=heatherbrie;532873]True, that's the theory, but theoretically, dark surfaces would also then radiate more heat at night, which would cool the boat.

I'm a little confused. What is the source of heat that the boat is radiating at night.
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Old 03-10-2010, 06:21   #8
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Spent a couple of decade owning and diving from rigid hull inflatables. Even in the UK black tubes need over pressure valves to stop them going pop in the sun. Plus on the one day a year the sun does shine dark tubes get so hot you couldn't sit on them wearing shorts or a bikini. The solution was to glue light grey wear patches on the top of the tubes.

Next boat came with light grey tubes

No idea what it would be like south of 50.N with dark tubes and can't imagine how hot a small yacht would be in the Carib or Med during July / Aug. Why not paint up a plastic box, white on top and blue round the sides, stick it in the sun for a day with a thermometer inside.

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I stand corrected..... dark colors north of 40 degrees lat are apparently an equally bad idea.
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Old 03-10-2010, 06:26   #9
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I'm a little confused. What is the source of heat that the boat is radiating at night.
The roasting bodies of the crew
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Old 03-10-2010, 06:36   #10
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Dark hulls show abrasion and salt more too. White or off-white is easier to maintain an acceptable finish.
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Old 03-10-2010, 07:02   #11
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[QUOTE=Blue Stocking;532890]
Quote:
Originally Posted by heatherbrie View Post
True, that's the theory, but
1. theoretically, dark surfaces would also then radiate more heat at night, which would cool the boat.
2. I'm a little confused. What is the source of heat that the boat is radiating at night.
1. There may be not enough difference (in emissivity) to matter*.
2. The heat absorbed during daylight gets re-radiated at night, as the ambient air cools to lower temp. than boat structure.

* Excerpted from Wikepedia:
“... Lighter colors and also whites and metallic substances absorb less illuminating light, and thus heat up less; but otherwise color makes small difference as regards heat transfer between an object at everyday temperatures and its surroundings, since the dominant emitted wavelengths are nowhere near the visible spectrum, but rather in the far infrared. Emissivities at those wavelengths have little to do with visual emissivities (visible colors); in the far infrared, most objects have high emissivities. Thus, except in sunlight, the color of clothing makes little difference as regards warmth; likewise, paint color of houses makes little difference to warmth except when the painted part is sunlit.
The main exception to this is shiny metal surfaces, which have low emissivities both in the visible wavelengths and in the far infrared. Such surfaces can be used to reduce heat transfer in both directions; an example of this is the multi-layer insulation used to insulate spacecraft...”
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Old 03-10-2010, 07:16   #12
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Here is my direct experience - not conventional wisdom or theory. I owned a green hulled boat that I painted white after 4 years. The difference in heat inside the boat was tremendous. You couldn't touch the green hull on the inside and you definitely couldn't take a nap or read in a bunk next to the hull. The white was always cool. This was at 45* latitude - I can't imagine what it would have been like in the tropics.

Unless your boat is well-insulated, the dark hull will not only cause a hotter cabin, but will make your refrigeration much less efficient if the box is mounted next to the hull as most are. You may use twice as much power running it double the time with a dark hull, even if it is well-insulated.

I don't understand your radiating heat at night point. Fiberglass doesn't have much heat capacity, so it doesn't store heat - it passes it through. And if it did store heat, it would be radiating it all night INTO the boat as well as out of. Thermal transfer is a property of the material and is independent of the color of that material. For example, an aluminum boat would transfer heat better than a wood boat. The color only comes into play as a heat collector from a direct source (IR from sunlight in this case).

I love the look of dark colored hulls, but they just aren't practical unless they are insulated very well.

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Old 03-10-2010, 07:20   #13
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I have a blue-hulled boat in the tropics. Most of the time in the trade-wind belt your bow or stern will be pointing towards the sun when it isn't high above the horizon. I can feel a big the temperature difference between sunny and shady side from the inside when I put my hand against the hull; but it isn't enough to be truly bothersome. Note that I am usually at anchor, I think if I were at a dock I'd notice the blue hull temperature increase quite a bit more.
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Old 03-10-2010, 08:02   #14
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We live in the subtropics. I can only tell you that my husband's white car is regularly 10 degrees cooler than my black car, as noted on our thermometer.

I love the look of dark hulls- they make every boat look beautiful. We have a white hull and because we plan on sailing in warmer climates, we will never go to a darker color.
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Old 03-10-2010, 08:10   #15
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... Fiberglass doesn't have much heat capacity, so it doesn't store heat - it passes it through. And if it did store heat, it would be radiating it all night INTO the boat as well as out of. Thermal transfer is a property of the material and is independent of the color of that material. For example, an aluminum boat would transfer heat better than a wood boat. The color only comes into play as a heat collector from a direct source (IR from sunlight in this case)...
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