Make sure you have a good auto bilge pump
or are out of the water
before taking any drastic measures.
As a last resort I've had to use a hammer and flat chisel in tight spaces a few times. If the box is mounted to a rubber tube as is most common, chances of cracking the stern tube are low, as the rubber cutlass bearing cushions
the shaft, and the rubber hose limits the forces transmitted to the stern tube. Make sure you are going the right way, then try the chisel on the locknut first. The end of the chisel should be around 1/4" in from a corner of the nut. For the first couple blows I keep the chisel nearly perpendicular to the surface, then can angle it more in the direction I want the nut to turn. THIS WILL LEAVE A DENT IN THE BRONZE NUT. And as such only use it as a last resort. I prefer a small sledge hammer swung lightly over high speed beatings with a lighter hammer. If the locknut won't loosen, try going the other way with the main packing nut. If you can get any to loosen a little it sometimes unlocks enough that the other can be freed, followed by thorough cleaning
, wire brushes
, etc so it all fits back together freely. If you have to do this always check the integrity of the rubber tube, shaft, and related hardware
. Keep in mind also that with this method is is possible to bend the shaft, though I have never had a problem with that.
If still doesn't move, you might be able to make up an extended wrench. Have a long piece of pipe welded to a properly sized open-end wrench, x2, and pull the handles towards each other just like with the pipe wrenches but with far more leverage. Eventually either the nuts will come free, the wrenches will break, or the stuffing box will break. One way or another something exciting will happen. (keep appropriate beverages on hand for all possible outcomes)
With heat, though it won't do a lot while in the water
, you can get the nut(s) to warm enough to help break them free from the stuffing box. Actually the nuts will get far warmer than the box/threaded tube which is cooled by the water, which causes the nuts to expand more than the box, which increases the chances of increasing diameter clearances and breaking the 2 free from each other from what I've ran into.