If you have a large rock that’s more or less round and smooth, is there a good way to tie a rope to it? A simple loop slips off too easy.
What you essentially want to do is make a basket out of the rope. Picture three circles at right angles to each other. The intersections of the circles should be two ropes make a right angle turn and passing over and under each other.
This would be much simpler to explain if we could post pictures.
Any of these parcel lashings might work, though you might need to have someone hold the lines in place.
What they said, but cheat and use nylon zip-ties where the ropes cross.
When looking for a rock to use as an improvised fishing boat anchor, we chose one more or less like a cube only half height. That shape you can wrap just two loops (like a christmass package) and not have the rope come off.
I think I know what you mean- so that it looks sort of like an octahedron. I’m not sure how you’d tie it to begin with though.
ETA: I’m surprised there isn’t a sailor’s knot for lifting large cannon balls.
For a more complicated but pretty knot that can be used as an improvised anchor is a Monkey’s Fist It is generally tied around a marble but with enough rope and patience it could be tied around a cannonball.