23-Aug-2004 -- We are proceeding with our bananas towards S.-Petersburg, and today we
passed once again the Østerrenden-Bridge over the Great Belt.
Shortly south there is 55°N 11°E.
Looking to North, although 37 km away, we still clearly see the Østerrenden-Bridge, linking the Danish Islands Fyn and Sjælland.
To the East we see the tiny island of Vejrø and to the SSE the Island of Lolland. On Lolland there are a lot of wind rotors, and only a glance on the radar screen shows us
that many of them are not standing ashore, but built into the sea. They are
orientated towards NW, which is the predominant wind direction in this area.
To the West we finally see the coast of the island of Langeland. A final look on the radar screen shows us the whole area
around the confluence.
Seeing the date display on the GPS I realized, - but better late than
never - that today is my 45th birthday.
This visit compared with that of 26 August is a very good example for the importance of the height of eye above the sea level when doing offshore confluences. The visitors of visit #2 were on a yacht and had probably a height of eye of not more than 2-3 metres, whilst I have one of more than 30 metres!
The yachtsmen see not much from the confluence, although the closest land
(Langleand Island) is only seven km away. We on the cargo ship, instead,
under good visibility conditions can comfortably see land which is even more
than 100 km off (provided the elevation of the coast is sufficient.)
This gives me the opportunity to talk about the so-called "DISTANCE OF THE
The distance of the sea horizon in nautical miles (1 nm = 1.852 km) is
2.095 x square root "h"
(where h = the height of eye in metres). Thus, for example, when h = 30 m, the distance of the sea horizon is 11.5 nautical miles (21.3 km).
How do we calculate when the shore has a certain elevation?
Thus, the question is: At what distance (in good visibility of course)
should an observer whose height of eye is e.g. 30 metres be able to sight
land of a height of e.g. 170 metres?
Distance of horizon for the height of eye 30 m: 11.5 nautical miles
Distance of horizon for the height of shore 170 m: 27.3 nautical miles
Sum 38.8 nautical miles
Hence, the object should be visible at a distance of 38.8 nautical miles
(71.9 km). Now let's compare the yacht and the cargo ship on this particular confluence (Langeland has an elevation of roughly 30 metres):
For the YACHT (height of eye 2 m)distance of horizon: 3.0 nautical miles. Distance of horizon for the heigt of shore (30 m) 11.5 nautical miles
SUM 14.5 nautical miles
Hence, the yacht should be able to see Langeland at a distance of about 27
km, and I wonder why on the pictures no land at all is visible. Probably the
atmospheric conditions close above the sea level were not good, it is
very common that the air just above the sea is "thicker" and hazier than
several metres above it.
For the CARGO SHIP (height of eye 30 m) distance to horizon: 11.5 nm. Distance of horizon for the heigt of shore (30 m) 11.5 nautical miles
SUM 22.5 nautical miles
Hence, the cargo ship should be able to see Langeland at a distance of about
41.7 km. On the other hand, a big disadvantage of a cargo ship when doing offshore confluences is her draft restriction. For me any waters with a depth of less then 10 metres are absolutely "off limits", whilst a yacht, not having mach
draft, may comfortably enter shallow waters without any risk of grounding.
So the "justitia aequatrix" is working as well in offshore confluencing.