I have 2 magnetic heaters I leave on the bottom of the pan all winter long. They definitely do their job, are easily removable, cheap, and actually seem to heat the whole engine room a little more. Bought them from Northern Hydraulics
Right on. Exact reason a couple of the larger draggers in my harbor leave the aux engines on for days at a time often...to keep the moisture levels in check in the engine rooms. To them, it's worth the fuel burn if the boats are going to sit all winter wet.
My 671 is still a bear to start on the colder days however, even with 2 heaters on the pan. Just the way these older Detroits are. Friend put a fresh completely rebuilt 871 in his clam boat last year and has the same trouble. Such rough engines but I love them...run forever.
I had a J&T 6-71 in my lobster boat years ago, in the winter i could get it to start easily by just misting the air intake with a squirt of starting fluid, i'd hold the can 3 feet away from the breather and just touch the button on the can.