Thank you everyone for your responses!
Jonfreeman, your comments got me researching the weather/temperature patterns in Nfld and I learned something new. Average winter temps are -2 C. Here in southern Ontario we're well below that at -20 C. I gotta move there, seriously.
What the heck, I thought it would be colder there. Instead I find out you guys are melting the icebergs!
Anyway, I like the BTU calculator but I do wonder if having a hull sitting in cold water alters the variables a bit. On the other hand, isn't unfrozen subsurface water usually warmer than surface air temperatures in winter? Maybe iceaters and bubblers actually affect boat temperature? So much to consider!
Seems like we need a calculator specifically for marine use, taking into consideration more varied weather regions. Until then, experience is our guide.