1. Vessels may enter or leave a harbour or a tributary waterway or enter or cross the waterway only after making certain that they can do so safely and without obliging other vessels to change their course or speed abruptly.
If a vessel proceeding downstream is obliged to turn upstream in order to enter a harbour or tributary waterway, it shall accord priority to any vessel proceeding upstream which also wishes to enter the harbour or tributary waterway.
In certain cases, waterways considered to be tributaries may be marked by one of the signs E.9 or E.10 (annex 7).
2. If the intended manoeuvre is likely or certain to oblige other vessels to change their course or speed, vessels except ferry-boats carrying out any of the manoeuvres referred to in paragraph 1 above shall announce the manoeuvre by sounding in good time:
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This provision shall also apply when the sign B.10 (annex 7) is placed on the main waterway near the exit from a harbour or tributary waterway.
4. If the sign B.9 (a) or B.9 (b) (annex 7) is placed near the exit from a harbour or tributary waterway, vessels leaving that harbour or waterway may enter or cross the main waterway only if their doing so will not oblige vessels proceeding on it to change their course or speed.
5. Vessels may not enter a harbour or tributary waterway when the sign A.1, together with the additional sign shown in annex 7, section II, paragraph 2 (b), is displayed on the main waterway.
Vessels may not leave a harbour or tributary waterway when the sign A.1, together with the additional sign shown in annex 7, section II, paragraph 2 (b), is near the exit.
6. If the sign E.1 together with the additional sign shown in annex 7, section II, paragraph 2 (a), is displayed on the main waterway, vessels may enter a harbour or tributary waterway even if their doing so might oblige vessels proceeding on the main waterway to change their course or speed. Vessels may leave if the sign E.1, together with the additional sign shown in annex 7, section II, paragraph 2 (a), is displayed near the exit; in this case, the sign B.10 (annex 7) shall be displayed on the main waterway.
7. The provisions of paragraphs 1 to 3 shall not apply to small craft in relation to other vessels, nor shall those of paragraph 4 apply to vessels which are not small craft in relation to small craft. The provisions of paragraph 2 shall not apply to small craft among themselves.