There is always lots of panic around this time of year about people worrying about thier engine blocks freezing and cracking. After having a boat all of my life in California and never winterizing I found myself a little concerned the other morning when I had to chip the ice off my truck windshield. So, last night I placed a few temperature recorders on my boat. One in the motor compartment, one protected from sun next to the fender, and one on top of the cover. You can see from the chart that the top of the cover got well down into the 20s, the side of the boat got down into the low 30s, but the motor compartment barely dropped below the 40s. You can also notice that there is about a 2 hour lag from when the temperature on the outside peaks/sags and when this happens on the inside of the motorbox.
Check it out.