As I write this the Eurodam is halfway across the North Atlantic Ocean on her way to Europe. After a very successful Caribbean season we are now getting ready to take on Europe again. The Eurodam will spend one month in the Mediterranean before heading up to the more familiar waters of the Baltic.
The repositioning cruise is a 16-day voyage from Fort Lauderdale, ending in Rome. Instead of calling at the Azores our itinerary has us make our way directly and non-stop from the U.S. to Lisbon, Portugal in 8 days. In the world of cruise ships where we rarely spend more then two or three days at sea this presented me with the unique opportunity to choose and follow a so called “great circle” track across the North Atlantic Ocean.
A Great Circle route is the line of intersection of the earth and a plane through it’s centre. In this way a Great Circle is the shortest distance between any two points on earth and thus, in theory the most economical route.
When we sail shorter distances we always use Rhumb Line tracks (otherwise known as a loxodrome) which have the advantage of a constant heading. On shorter routes the savings on distance sailing Great Circle versus Rhumb Line are negligible, particularly if they have a large north/south component in them.
In our case as we are crossing west/east the distance savings are quite considerable, almost 100 Nautical Miles. Our Great Circle started at the northern end of the Straits of Florida and ends at a point just to the west of the Tagus River that leads to Lisbon .
The potential disadvantages of choosing a Great Circle track are that this track does not have a constant heading and is therefore slightly more challenging to plan and to navigate. For example we started on a heading of 065 degrees and by the time we reach Lisbon our track will be 098 degrees. To avoid having to change heading degree by degree we have planned the route using the so called chord method, basically making a Great Circle out of many short Rhumb Lines!
The Great Circle which is always turned towards the pole with its curve also takes the ship further north versus the Rhumb Line, in our case by as much as 470 Nautical miles. For example our Great Circle track takes us north of Bermuda and the Azores, the Rhumb Line would have taken us south of these.
After careful consideration and monitoring of the long-term weather forecast and sea conditions, I elected to put the Eurodam on the great circle track. We have enjoyed excellent weather and a very smooth ride thus far.
Normally we use nautical charts with Mercator projection (Parallels and Meridians in a grid pattern) for navigation where the great circle would actually be a circle. In this case we are using a chart with Gnomonic projection which has the Meridians converge towards the pole and which shows the Great Circle as a straight line…
On the below snapshot from our weather forecast system one can see our Great Circle route in blue. This is also gnomonic so it shows as a straight line, to the north of Bermuda and the Azores. By comparison, the Rhumb Line (red) is curved and leads south of Bermuda and the Azores.
Captain Jeroen van Donselaar is Eurodam’s master.