Navigation lights- The horizontal cut-offs

Navsregs>COLREGS>Horizontal cut-offs

A quick guide to some of the details in COLREG Annex 1

Sidelights on MV Balmoral

Following the theme of Navigation Light horizontal arcs this post now descended into the details of Annex 1.

Colreg Annex 1

What part of the Rules defines the horizontal cut-offs of Navigation Light?

Annex 1 section 9

What is the horizontal cut off for sidelights in the forward direction?

In the forward direction they should show the minimum required intensities.

The intensities shall decrease to reach practical cut-off between 1 degree and 3 degrees outside the prescribed sectors.

What is the cut-off requirements for the other sectors?

For sternlights and masthead lights and at 22.5 degrees abaft the beam for Masthead lights MV Balmoralsidelights, the minimum required intensities shall be maintained over the arc of the horizon up to 5 degrees  of the prescribed limits. 

From 5 degrees within the sectors the intensity may decrease by 50 percent up to the prescribed limits; it shall decrease steadily to reach practical cut-off at not more than 5 degrees outside the prescribed sectors.

How much can an all-round light be obscured?

Not more than 6 degrees, except anchor lights which need not be placed at an impracticable height above the hull.NUC lights

If it is impracticable to comply with these requirements by exhibiting only one all-round light, two all-round lights shall be used suitably positioned or screened so that they appear, as far as practicable, as one light at a distance of one mile.

Sidelight screens

The sidelights of vessels of 20 metres or more in length shall be fitted with inboard screens painted matt black, and meeting the horizontal sector  requirements of  Annex 1.

On vessels of less than 20 metres in length the sidelights, if necessary to meet the horizontal sector  requirements of the  Annex, shall be fitted with inboard matt black screens.

With a combined lantern with a very narrow division between the green and red sections, external screens need not be fitted.

There is another level of detail below COLREG Annex 1, and that is contained within an IMO Resolution.


Perfomance standards for Navigation lights

Within MSC 253 there are paragraphs giving more details on cut offs and intensity.


In the horizontal directions where decrease of luminous intensity to ìpractical cut-offî is required by section 9 of Annex I to COLREGs, the luminous intensity should be no more than 10% of the average luminous intensity within the prescribed sector for vessels not less than 12 m in length.

Uniformity of intensity

Within the prescribed sector in which the minimum luminous intensity is required  the horizontal intensity distribution of the light should be uniform so that the measured minimum and maximum luminous intensity values do not differ by more than a factor of 1.5 for vessels not less than 12 m in length.

This is to avoid luminous intensity changes which may result in the appearance of a flashing light

Click here for IMO MSC 253>

Click to view this Handy Revision Guide for Kindle on AmazonA Really Handy Guide to learn the Collision Regulations

This was the first in the series of Kindle revision guides covering the COLREGS.

Click here to view on Amazon>


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