Phot,

Certainly in my day boarding officer was either a mid or a sub-lieutenant (least experienced officers on board i.e. most expendable should things go pear-shaped). That was a Navy with more than 200 ships in commission, not every ship carried a detachment of Royal Marines, I suppose now that the fleet has shrunk to present day levels they all do carry Marines to make up the numbers, I must remember to ask my son about that. Doesn't alter the fact Phot that as Bubblyrat and I stated it would not have happened in my day as the way we did it either the parent ship or the boat carrying the fire support group would have deterred the Iranian force.   

By the bye, junior officer by definition = Lt-Commander and below, senior officer by definition = Commander and above.

Promotion within the ranks of junior officer is automatic and rate of advancement is marked by seniority and time served.

Promotion to the ranks of senior officer by selection once in "the zone", your seniority determining when you enter the zone for promotion.