Steve Edwards, crew Coronation Gulf, 03.00 UTC 8 Sept
As a young boy scout I was told to always trust my compass when navigating. That advice is good as long as you are in the UK, however in the North West Passage we are close to the magnetic North Pole and this has two effects. Firstly the North Pole magnetically is much closer than the real or “true” North Pole, so we are going past it quite quickly and therefore the direction of apparent North is changing quite quickly. Secondly the earth’s magnetic field is usually about horizontal and compasses are built assuming that this is the case. However when you are so close to the pole the magnetic field is mostly vertical as the field disappears into the ground to join up with the South Pole field in the centre of the earth. This means that not only are magnetic compasses quite unreliable in the NW Passage but that the errors can vary enormously from compass to compass depending on the details of how they are built.
This picture shows the chart plotter where the boat is following a course of 70 degrees towards out next waypoint. Just looking out it is clear that we are travelling in the direction that the boat is pointing, so the boat must be pointing at about 70 degrees. However if you look closely at the chart plotter you can see that the boat symbol is pointing quite along way to the right of the course, about 105 degrees I would guess. This is because the navigation system is getting inaccurate information from its electronic compass which believes the boat is pointing at 105 degrees. The navigation system uses its GPS system to get both position and our real direction of travel so it is forced to assume that we have a very strong current pushing us sideways and is happy to accept pointing the boat is a very different direction from the course we are successfully following. The overall effect is that we follow the correct course as set out by the GPS but the boat symbol is pointing in a funny direction – all is well.
The bottom of the picture shows the traditional ship’s compass which is built in a different way from the electronic compass and is showing about 002 degrees, not the 105 degrees of the electronic compass! If we were trying to navigate by map and traditional compass we would be in serious trouble.