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WL is waterline with vessel on even keel. XZY is parallel to WL passing through the point P on the uppermost continuous deck, vertically below A. A is the foremast light; D is the mainmast light. It is required that ZD is at least 4.5 metres more than AP.

 

1. To find the Angle C (with the aid of the Ships drawing)

Horrizontal distance CL + LH + HF = CF

Vertical heights DE + EF = DF

Tan C =

2. To find BH (The height where the dotted line crosses the
formast

Tan C =

^BH = CH x Tan C

3. To find height of B above A (with the aid of the Ship’s drawing)

AB = BH - AH

WL is waterline with maximum trim by the stern.
C is 1000 metres ahead of the vessel’s stem.
A is the foremast light; D is the mainmast light.
CD produced cuts the foremast at B; for the foremast light to be seen from C distinctly separate from and below the mainmast light, it must be placed a minimum distance of about 0.5m below B at position A.