I just went through the same struggle this winter.  I went with a kerosene heater designed to be used inside.  The propane heater will generate alot of moisture, not good, in my opinion, for building a wooden kayak. My heater easily heats my 14' x 24' shop space. Good luck.   

I used two little ceramic heaters out of the motorhome that easily kept garage above 60, was in the 30's outside. I am a fan of these little heaters due to their safety and ease of use and most RV'ers use them.  They run on 110 but I do have my shop on two 20 amp circuits.  My work area is a 22' square garage.  I insulated all the walls and the garage door, and put the cardboard from the dory shipping cartons on the floor..  The ceiling is closed in, but not insulated.  My electric bill was (guessing) $30-40 more in the coldest months.

  I draped 4 mil plastic from the ceiling in front of some storage shelves to keep the dust off when sanding but it also had the benefit of reducing the heated portion of garage.  Have one of those inexpensive digital thermometers that has a wireless remote transmitter.  I put the remote unit in the garage so I could watch the garage temperature from inside the house in the evenings...let it go down into the 50's at night and turned them back up every morning.

Curt 830/997-8120

https://picasaweb.google.com/114717787929554738224/BuildingWoodenBoatCLCNortheasterDory?authkey=Gv1sRgCKCBzczm_aKs2wE#

  I live in mid-Michigan and agree with Moonchaser, electric heaters are the way to go.  My shop is 20 x 24, well insulated, and i use one ceramic block plus one of those oil-filled radiator type heaters.   They are on full time (temperature swings aren't good for my cast iron machine tops) and keep it pretty comfortable out there.  Cost is about $45/month.