Originally Posted by belizesailor
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
He sent me a whole presentation on the subject. PM me if you are interested and I will email
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
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.