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Old 20-06-2013, 09:30   #46
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Re: Blue Hull Concerns

The dark blue, painted hull, docked next to us looks like crap. It's very faded and shows every bump and scrape. Except for the heat gain, a blue gel coated hull may not be so bad. I touch his hull from time to time to compare it's temp to our white hull, and you can rest assured it's very much hotter. I can lay my hand on ours for hours without discomfort, his about 10 secs.
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Old 20-06-2013, 09:54   #47
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Re: Blue Hull Concerns

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Mosquitoes think dark colors are more attractive too.
Only the discerning well-dressed ones
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Old 20-06-2013, 09:56   #48
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Re: Blue Hull Concerns

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Originally Posted by belizesailor View Post
This subject came up, and turns out there are paints suited for this application which absorb dramatically less heat.

These paints were originally developed for military use (think of a tank with same thermal signature as vegetation...didn't see that..oops...bang) and are now mostly being developed for the roofing industry. Also some which are suitable for marine applications.

He sent me a whole presentation on the subject. PM me if you are interested and I will email to you.
You need to be careful as there are two issues working together here in the heat transfer. The first is solar absorptivity, the fraction of solar energy absorbed by the surface. Dark surfaces absorb more solar energy than white surfaces. Even though the top sides are near vertical as the sun is coming up or getting lower in the sky the angles are significant for heating. Plus the water reflects a good fraction of solar energy at lower solar angles and this energy will impinge and be absorbed/reflected by the hull.

The second issue is emissivity. Emissivity is the fraction of thermal (long wave) radiation that is emitted by a surface as a function of it's temperature when compared to a perfect emitter (blackbody). In general, solar absorptivity does not depend on the emissivity of a surface.

This is somewhat related to the global warming issue because snow reflects a high amount of solar radiation back to space rather than having it stay on the planet. More snow cover more radiation reflected.

For example, a high absorptivity low emissivity surface captures more solar energy and does not let it radiate making the physical temperature of the surface very warm. These coating are used for systems that use solar energy to heat water. Not the way to go in the tropics.

Most coatings have a high emissivity and that is good, heat is radiated to a cooler environment (sky and water). White topsides have a low solar absorptivity and that is really good for the tropics because they reflect direct and scattered solar energy. Less is absorbed to heat the hull.

One can debate the high emissivity recommendation but overall that is where you want to be. Plus these low emissivity coatings are not made for marine applications.

Best for the tropics, topsides and decks, low solar absorptivity (white) and high emissivity (most coatings and non metallic surfaces).

A high solar day means that 900 watts per square meter of solar energy is making it to the earths surface. The solar absorptivity of a dark coating may be 0.7 to 0.8. A white coating the solar absorptivity may be near 0.5. Consequently, a dark hull or deck would absorb up to an additional 180 to 270 watts per square meter over a white hull.

Worst case in the example above is that the dark hull will absorb 60% more energy (heat) that a white hull.
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Old 20-06-2013, 11:44   #49
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Re: Blue Hull Concerns

One of the worst things about a dark hull is they show scratches and abrasion way more than white. Cruising boats get a lot of abuse from tieing up at commercial piers and others with not great buffering and the inevitable dinghy marks. There is a difference in interior temp with a dark hull as well. Cruising in the tropics, you don't need additional sources of heat in the cabin.

If possible, get a white boat with gel coat, not painted, topsides. Paint scratches and scrapes off with abuse that gel coat shrugs off.
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Old 20-06-2013, 12:08   #50
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Re: Blue Hull Concerns

I once read an article about the science behind how the Bedouin tents stayed cool. I searched but couldn't find anything online about it. If I remember correctly, the different colored tent segments and the pattern in which they were combined conspired to move air by causing warm air to rise on some sections but not on the other lighter colored sections. Even while the outside air was perfectly still, the inside contained its own manufactured whirlwind!

I wonder how this phenomenon might apply to differing colors and temp changes on a boat?
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Old 20-06-2013, 13:41   #51
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Re: Blue Hull Concerns

Our boat is glossy black (paint, not gelco) and we had no issues. However, only 2 ft of freeboard here - not much to heat up and not too much to keep clean and shiny.

If you get something like a Bene 54, blue gelco, the story may be different. Still, many newer designs have sandwich in topsides so there is no issue of the interior getting too hot this way.

Re maintenance - yes, gelco weathers and it shows on a dark hull more than on a white one. I do not mind this particular look but some people do.

So my say is if you like it in blue, get it in blue.

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Old 20-06-2013, 18:53   #52
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Re: Blue Hull Concerns

Thank you all very much for your very helpful comments. We have decided on a white oyster 66. Looking forward to hitting the high seas and meeting some of you.
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Old 24-06-2013, 16:00   #53
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Re: Blue Hull Concerns

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Originally Posted by virginia boy View Post
I once read an article about the science behind how the Bedouin tents stayed cool. I searched but couldn't find anything online about it. If I remember correctly, the different colored tent segments and the pattern in which they were combined conspired to move air by causing warm air to rise on some sections but not on the other lighter colored sections. Even while the outside air was perfectly still, the inside contained its own manufactured whirlwind!

I wonder how this phenomenon might apply to differing colors and temp changes on a boat?
Might be something in having different coloured sails then
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Old 24-06-2013, 16:09   #54
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Re: Blue Hull Concerns

Diva has a dark blue hull...its a Dupont LPU paint and it is exceptionally glossy....in 3 years in the south pacific, heat inside the boat was never an issue.
What did make a big difference was the ripping off the teak on the deck and cabin top, glassing it over and painting it white.
Dark blue certainly looks distinctive and we had many comments about the paint work when we were away.
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Old 24-06-2013, 17:14   #55
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Re: blue hull concerns

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Blue hulls are perfect for Newport, Rhode Island. Anywhere south of there, a blue hull is an abomination.
I guess that you are assuming that in Newport you would have someone to wax and buff.
IMHO dark blue hulls look great for about 60 days after the wax and buff.
I guess that translates to about 6 wax jobs a year
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Old 24-06-2013, 17:21   #56
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Re: Blue Hull Concerns

Once you wax it you'll be forever waxing it !
Diva's blue hull was painted in 2006 and NEVER waxed...only ever washed with fresh water and a mix of white vinegar and warm water for spots and marks that wouldn't come off with washing...it looks as good as new.
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Old 24-06-2013, 17:23   #57
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Re: blue hull concerns

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I guess that you are assuming that in Newport you would have someone to wax and buff.
IMHO dark blue hulls look great for about 60 days after the wax and buff.
I guess that translates to about 6 wax jobs a year
My guess is you never had one to know, right?

We wax ours annually primarily to protect the gloss. For purely appearance sake, it looks better after a year than most vanilla boats do immediately after buffing.

But what do I know...
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Old 06-03-2014, 15:23   #58
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Re: Blue Hull Concerns

On science base data a black car in the summertime in 95 degree heat is only 3 degrees hotter than a white car. A black boat is always the sexiest in the harbor!
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Old 06-03-2014, 18:25   #59
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Re: Blue Hull Concerns

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On science base data a black car in the summertime in 95 degree heat is only 3 degrees hotter than a white car. A black boat is always the sexiest in the harbor!

This seems a bit more reasonable. Are dark houses hotter than white ones? And why then are roofs usually black even in warm parts of the country??

And are boats with teak decks hotter than white ones?
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Old 06-03-2014, 19:06   #60
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Re: Blue Hull Concerns

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On science base data a black car in the summertime in 95 degree heat is only 3 degrees hotter than a white car. A black boat is always the sexiest in the harbor!
Can you give a reference or link to the "science base data" you refer to.

Temperature in a car is not quite the same as in a boat. There are several additional variables, most significant the large window areas that allow the suns rays to penetrate and heat the interior aka the greenhouse effect.

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.
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