Two boats are on a collision course, one on starboard going downwind the other on port going upwind. The starboard boat sees the port hand boat in good time, the port hand boat does not see the starboard boat at all. Does the ROW boat on starboard have to hail to warn the port hand boat to keep clear or, perhaps upon realising that the port hand boat has not seen the ROW boat, just cross making a small glancing contact, quote rule 10 and protest.

Yes, it was me, I was the port hand boat.
Simon IF20 Fluffy UK 164