Your right, Mark, to remove the color factor from this test, my idea is to put the blocked thermometer also under the tarp. so that the air temp is the same for both thermometers, and the only possible cause for additional energy on the unblocked thermometer would be light thru the tarp. So the color of the tarp should should affect both thermometers the same.

I agree that you probably don't want to bake your boat. I'd probably lean toward a sun-shield that still allows enough air flow so that the temp of the boat is same as the temp under the shade of a tree.

obviously this would be less than scientifically definitive, but...

P.

> 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.