There's a problem with this method though. Blue is a dark color, which will absorb light and become hot. The temperature under the blu tarp may be hotter than no tarp. Infrared is more closely associated with heat. You could have a white tarp and a black tarp in an experiment where the black tarp blocks much more UV than the white tarp, but still causes higher temperatures under it.

I don't know all of the aplicable science very well, but I think the suppositions are reasonable.

SPF 15 means 1/15th of the exposure, which should help. You can always stretch the tarp over a frame and roll on some latex paint. This would keep the tarp away from the boat, which someone else suggested was important.

> I have learned out here in Hawaii, where the sun can rip the
> hide off of you, that even beach umbrellas come with SPF ratings now.
> The cheaper ones with material that reminds me of those blue tarps,
> have only a 5-15 spf rating. That was good to know, as I promptly
> went out and bought a 1000 spf beach umbrella!

> I guess my point is that I doubt that those blue tarps are really
> sun-proof.

> You could do the same experiment that was done hundreds of years
> ago to prove the existance of non-visible frequencies of light...
> (recall, I work for an astronomical observatory). What the guy did
> (can't remember his name) was to put two thermometers next to each
> other. One was just past the violet strip of the output of a prism
> (where UV light waves would hit) and the other was adjacent to it
> under a truely opaque (spelling?) cover. He discovered that the thermometer
> that was near the prism output, had a higher temperature than the
> one with the opaque cover, proving that even though he couldn't see
> light, that there was energy of some sort being deposited there.

> So, anyway... If you put a thermometer under your blue tarp,
> and another thermometer adjacent to the first but with a truely opaque
> cover (scrap of wood, high enough above it to not affect air flow.
> If the temperatures are different, then i'd say the blue tarp is leaking
> some frequency of sunlight thru.

> Chances are no-one wants to know the answer bad enough to do
> this experiment, but I think It would be interesting.

> Just another of my screwy ideas... :-)

> Aloha! Paul