"Doug Dotson" wrote:


"Rosalie B." wrote in message
.. .
"Doug Dotson" wrote:

Sorry Rosie. I stand by my emperical experience. I have been in
raftups during very hot summer days. Moving between a boat
with black canvas, burgundy (ours), boat blue, and cadet gray
(pretty light). I could not tell any difference. Not to say that there
was not any, but I couldn;t feel any.

I think it depends a bit on how much wind there is (you seem to
indicate that there was some). If there is a breeze the heat doesn't
accumulate. You would have to be able to measure the heat with no
ventilation in order to be able to feel the difference. Or you may
just not be very sensitive.

But this is exactly the typical situation. A bimini as shade. The situation
with an enclosure is different. Although I think that in an enclosure
with limited ventilation the darker the color, the faster it will heat up.
I suspect the actual temperature will not be that much different. We
just installed a full enclosere last week. It was very light gray. Hotter
than hell within minutes. I doubt that it could be much different with
darker canvas. Matter of fact, much of the enclosure was clear.

If the enclosure is clear, then it isn't shading the cockpit unless
the sun is directly overhead. And heating up faster is what we want
to avoid. If it heats up more slowly, then the heat can disperse
better. It may be true that it will get no hotter than a certain
temperature - but slowing the accumulations of heat is an achievable
goal.

This winter, the boat was at the dock and faced east. The rising sun
shown through the dodger down into the aft cabin and heated it up. So
I hung a towel down over the hatch. This blocked the sun and made the
cabin comfortable all day.

Anything that keeps the sun off will be cooler underneath. But I know
that having A/C in a building with a dark roof will be more expensive
to run to the same temp than a similar building with a light roof and
if you really want to bring the energy use down, run water over the
light colored roof. Also a building with a dark roof heats up in the
summer more than a building with a light colored roof. I do not see
how a bimini would be any different, absent lack of thru ventilation.

Correct. But this is not analagous. I believe we are talking about the
shade under a canopy open on the sides. Even with little wind, any
accumulated heat underneath will flow out a few inches below the
canopy.

I think it is quite analogous. I am talking about a bimini with sides
which can be rolled up or left down. In the winter when it is cold,
we leave the sides zipped up and the clear plastic enclosure is like a
sun room - the sun heats it up very nicely and we can be inside in our
shirt sleeves while those folks with aft cockpits and no bimini or
only a minimal one are in their foulies looking very miserable.

In the heat, we have at least one side (the back) stationary (we can
remove it, but then if it rains, the enclosure is open at the back),
and two pieces on each side of the dodger that aren't rolled up. If
we are sailing (or motoring into the wind), there is airflow unless
the wind is directly from the rear. If we are at anchor or at the
dock, we may not have any airflow. The heat may flow out below the
canopy but I will be miserable, and that little outflow won't really
help me a bit.

Actually, if it is really hot and the boat is stationary, a canvas
tent over the boom helps a lot to keep the cockpit and boat cooler.

I see no reason to think that the principles of heat transfer which
apply in other situations will not apply also to boats and biminis.
The fact that it will get hot under a bimini in the direct sun even if
it is light colored doesn't't disprove the principle that a lighter
color will reflect more of the light than a darker color. The
absorbed light becomes heat.


grandma Rosalie