Helmsman Ed Davies: 'This knot is for securing a rope, under strain, to a spar, ring on shackle. I would use this knot to attach a tow rope to the towing eye of a vessel such as a RIB or a jet ski, or to a strong point on a yacht such as a winch of keel-stepped mast (a mast that's directly fitted to the main hull.
'This knot is very useful for attaching a tow rope, because it can be untied while under strain.'
Step 1: Loop your rope around the post.
Step 2: Take the rope for a turn around the post, making sure you leave lots of rope on the working (tail) end.
Step 3: Pass the working end under the standing (long) end, then back over it and down through the loop.
Step 4: Repeat step 3 to make your second half hitch.
Step 5: Pull the standing end tight.