The stern light must be on whether steaming or sailing.

Masthead steaming light must be on when steaming or motor sailing. Can be off when sailing only.

When using a motor you need an allround white light which can be two separate lights, a stern light and a steaming light, and navigation lights. The steaming light fills in the remainder of the arc not covered by the stern light to provide the boat with an all round white light.

When sailing you need a stern light and navigation lights.

Therefore, the only difference when using the motor or sailing without the engine is that you need a steaming light when using the engine.

If the masthead light is a tricolor that is all you need for sailing. If you use the engine, or even have it running and available for use but not in gear, you also need the steaming light.

Thanks for that Morning Bird

Not sure your are right there Havefun. I have heard this before, including during my Yachtmaster Offshore theory, but have never been shown it in writing.

The following is direct from ColRegs. If there is something in the ColRegs somewhere else it would be good to know. In my reading a steaming and mast head stern light (in the tricolor) meets the all round white light requirement.

21. Definitions
"Masthead light" means a white light on the centreline of the vessel showing from right ahead to 22.5 degrees abaft the beam on either side of the vessel.
"Sidelights" means a green light on the starboard side and a red light on the port side each showing from right ahead to 22.5 degrees abaft the beam on its respective side. If the vessel is less than 20 metres (66 ft) long, the sidelights may be combined in one fixture carried on the centreline of the vessel.
"Sternlight" means a white light placed as nearly as practicable at the stern showing 67.5 degrees from right aft on each side of the vessel.
"All-round light" means a light visible from 360 degrees of the horizon.
"Sidelights" means a green light on the starboard side and a red light on the port side each showing from right ahead to 22.5 degrees abaft the beam on its respective side. If the vessel is less than 20 metres (66 ft) long, the sidelights may be combined in one fixture carried on the centreline of the vessel.

23. Lights displayed by power-driven vessels underway (interalia)
A power-driven vessel of less than 12 metres (39.4 ft) may display only an all-round white light and sidelights. However, in the case of a skiff a wooden clinker rowing boat which falls into this category only needs to be capable of showing a white light.

25. Lights for sailing and rowing vessels
Sailing vessels underway and vessels under oars:
(a) A sailing vessel underway shall exhibit: 1. sidelights;2. a sternlight.
(b) In a sailing vessel of less than 20 metres (66 ft) in length the lights prescribed in paragraph (a) of this Rule may be combined in one lantern carried at or near the top of the mast where it can best be seen.
(c) A sailing vessel underway may, in addition to the lights prescribed in paragraph (a) of this Rule, exhibit at or near the top of the mast, where they can best be seen, two all-round lights in a vertical line, the upper being red and the lower green, but these lights shall not be exhibited in conjunction with the combined lantern permitted by paragraph (b) of this Rule.
(d) 1. A sailing vessel of less than 7 metres (23.0 ft) in length shall, if practicable, exhibit the lights prescribed in paragraph (a) or (b) of this Rule, but if she does not, she shall have ready at hand an electric torch or lighted lantern showing a white light which shall be exhibited in sufficient time to prevent collision.
2. A vessel under oars may exhibit the lights prescribed in this Rule for sailing vessels, but if she does not, she shall have ready at hand an electric torch or lighted lantern showing a white light which shall be exhibited in sufficient time to prevent collision.
(e) A vessel proceeding under sail when also being propelled by machinery shall exhibit forward where it can best be seen a conical shape, apex downwards.

A sailing vessel under power needs an all round white light, This can be achieved with an all round white light or a stern light and a steaming light.

cisco said..

More correctly, any vessel under mechanical power in the hours of darkness must display port, starboard, stern and steaming lights separated by the correct distance as applicable to different sized vessels and defined in the I. R. F. P. O. C. A. S.

It matters not which state or country one's vessel is in. The rules are international.

Thanks Cisoc that now clear to me
Question when under sail at night would you have the stern light on plus the mast clear light + red and green?
Just trying to work out which way to wire the stern light in that is on a separate switch or combined with the all the Navigation lights

cisco said..

Port, starboard and stern lights on one switch (Nav Lights) and steaming light on it's own switch(Steaming Light).

I was thinking the mast tri color light would be my nav lights and the stern light the steaming light?

cisco said..
The steaming light must face forward and has a wider arc than a stern light.

Thanks Understood Cisco