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Some interesting measurements with an IR remote-reading thermometer after making three changes: the forward deck shading tarp, isolating the return air from the compartment that the A/C unit is in, and adding a small circulating fan that hits the thermostat.
I've always suspected that the A/C breathes some outside air, and unnecessarily cools the inside surfaces of the cockpit walls and line lockers, since that compartment isn't closed to them. I built a thin plywood separator that better isolates the return cabin air from that compartment volume, essentially coupling the return grille to the evaporator better.
With a clear-sky south Florida July overhead sun, and air temperature of 87F and a slight breeze, the forward deck temperature was reduced about 13 degrees F with the blue-plastic tarp suspended eight inches above it. The exposed white deck was 117F while the deck under the tarp was 104. A white tarp or fabric would probably do better, I imagine.
The underside of the Bimini top (with side camper canvas and windows in place) was 124F, while the _topside_ was oddly cooler, at 104F. The cockpit air was 102F (with a 10" ventilator exhausting through a high rear side screen). The cockpit deck was 95F. The deck under the windshield ("Sand"-colored non-skid) was 170F! Surface water under the dock was 89F, and the vertical hull surfaces were the same.
I was surprised inside the cabin where the air was 75F at the floor, 72F in the mid-berth, and 79F at the A/C thermostat, mounted on the forward face of the breaker panel enclosure; the cabin ceiling (foam-backed fabric) directly under the 170F deck was 88F, and the head ceiling under the instrument cluster, door closed, was 95F. Most other internal surfaces were between 80F and 85F. The A/C return inlet was 72F while the vents were blowing 62F air. This was significantly better than I'd measured in the past.
In addition to shading the forward deck, I also ran a small desk fan on the galley surface that blew at the thermostat to determine if improved circulation would have an effect, and it seems it did. I've never seen 79F at the thermostat in the middle of a hot day before this. And, in fact, a few minutes after entering the cabin, the compressor cycled off at 78F; usually, it runs continuously all day and is never below 80F on a hot sunny day.
So, it appears that some combination of the additional deck shading, return air coupling and additional circulation has helped. I'll try relocating the thermostat, too. High in the cabin isn't the spot of choice, so maybe I can do without the additional fan.
FWIW.
Tom
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