end on to the other; in other words, to cases in which by day each vessel sees the masts of the other in a line or nearly in a line with her own, and by night to cases in which each vessel is in such a position as to see both the side lights of the other.

It does not apply by day to cases in which a vessel sees another ahead crossing her own course, or by night to cases where the red light of one vessel is opposed to the red light of the other, or where the green light of one vessel is opposed to the green light of the other, or where a red light without a green light or a green light without a red light is seen ahead, or where both green and red lights are seen anywhere but ahead.

  ART. 19.  When two steam vessels are crossing, so as to involve risk of
  collision, the vessel which has the other on her own starboard side
  shall keep out of the way of the other.

  ART. 20.  When a steam vessel and a sailing vessel are proceeding in such
  directions as to involve risk of collision, the steam vessel shall keep
  out of the way of the sailing vessel.

  ART. 21.  Where by any of these rules one of two vessels is to keep out
  of the way, the other shall keep her course and speed.

  ART. 22.  Every vessel which is directed by these rules to keep out of
  the way of another vessel shall, if the circumstances of the case admit,
  avoid crossing ahead of the other.

  ART. 23.  Every steam vessel which is directed by these rules to keep out
  of the way of another vessel shall on approaching her, if necessary,
  slacken her speed or stop or reverse.

  ART. 24.  Notwithstanding anything contained in these rules every vessel
  overtaking any other shall keep out of the way of the overtaken vessel.

Every vessel coming up with another vessel from any direction more than 2 points abaft her beam—­that is, in such a position with reference to the vessel which she is overtaking, that at night she would be unable to see either of that vessel’s side lights—­shall be deemed to be an overtaking vessel, and no subsequent alteration of the bearing between the two vessels shall make the overtaking vessel a crossing vessel within the meaning of these rules or relieve her of the duty of keeping clear of the overtaken vessel until she is finally past and clear.

As by day the overtaking vessel can not always know with certainty whether she is forward of or abaft this direction from the other vessel, she should if in doubt assume that she is an overtaking vessel and keep out of the way.

  ART. 25.  In narrow channels every steam vessel shall, when it is safe
  and practicable, keep to that side of the fairway or mid-channel which
  lies on the starboard side of such vessel.