Foto: Werner Karrasch; © Vikingeskibsmuseet.

Tacking abilities

In this exercise you will investigate the ability of The Sea Stallion to tack against the wind. Download, unzip and open the navigation data from the voyage. You need to use data from stages when the ship sails as close as possible to the direction of the wind – under sail, of course! Use the map on the website to find suitable stages by switching on show positions. Find these stages in the downloaded data set. (Alternative solution: Open the downloaded data set in a GIS programme where you can see the route mapped at an even greater level of detail.)

Click on the points in order to see the course and wind direction. A zigzag course could be what you are looking for. Try answering these questions:

  • How close to the wind can The Sea Stallion sail?
  • Can it sail equally close on both tacks – i.e. with the wind coming from both sides?
  • Can it sail equally close at all wind speeds? And what if the sail is reefed (the area of sail is reduced)?
  • What is the relationship between the distance travelled when tacking (in zigzag) and the travelled distance as the crow flies (DMG: distance made good) when sailing directly into the wind?
  • What is The Sea Stallion’s speed forwards against the wind (the so-called VMG: velocity made good)? Is it worthwhile, if not a great distance, rowing directly into the wind rather than tacking using the sail? (Find stages with rowing and compare.)

(Tip: You will need to calculate the angle between the ship’s steered course and the wind direction.)

A series of exercises has been produced which aim to analyse The Sea Stallion’s handling abilities under sail and oar power respectively. In other words, the pupil is able to carry out him- or herself much of the research which the Viking Ship Museum is to embark upon after the voyage to Dublin. The exercises are best tackled in the order shown, and it is a good idea to present the results together in a final report.

The data set to be processed comprises the ship’s geographic co-ordinates (recorded with the aid of a GPS) as well as a series of other navigational and meteorological measurements. All of this is assembled in a zipped, semicolon separated text file Datalogger which can be downloaded from Tools in the right hand column of the exercise. This table can be used as the basis for a large number of exercises involving data validation, source criticism, searching and sorting, calculations and column updates and much more. The exercises are best executed with the aid of a spreadsheet and/or GIS and can be incorporated in a wider teaching context dealing with, for example, the GPS system, statistics and cartography.

In Tacking abilities a calculation has to be made of the difference between the wind direction and the steered course (Geography: also possible to query the relationship between the steered course, BoatHdg, and course made good, COG). If this exercise is solved using a spreadsheet, it is necessary to create a formula which accommodates positive and negative angles greater than 180º. Subsequently, a simple sorting exercise is involved in finding the minimum value for the measurements when the ship is under sail. It may be a good idea to focus on a tacking stage or tacking attempt: for example the 13th and 20th July and the 2nd August.

Subjects: Maths, physics, geography, general study.

Level: Sixth form/A level.