1/ Get a thermometer to make sure what the temperature is where YOU are WITH the boat, not what some radio guy says it is outside his studio.
2/ Make sure you put the thermometer where you are going to apply the fiberglass or epoxy, not right in front of the heater blast.
3/ Bring LOTS of propane. If you think the keel of your boat looks like a huge heat sink, you are right. If you are applying resin to it, you will need to get it up to the right temperature (see dan above, or read the directions on your resin) AND KEEP IT THERE UNTIL THE RESIN SETS - probably many hours later. Many epoxies suggest overnight curing at a comfortably warm temperature, 70º F, for example. Colder = longer.
4/ Be there. Unattended propane heaters melt temporary plastic windbreaks and burn boats into black slagpiles.
If this sounds like a hassle... it is. That's why my outside fiberglassing project is waiting till spring, and I'm building a kayak in the garage this winter.