Day skipper theory got us started on the basics of chart work. This was actually fairly straightforward given that I’ve spent a lot of time with maps before.
The key points are being able to accurately plot lat/long pairs, measuring the same for a defined mark, measuring distances and finding bearings from one point to another. I guess the idea here is to get used to the tools – I now have my own portland plotter and dividers, and I know how to use them.
This section also introduced the idea that there are 3 types of bearings; true, magnetic and compass. True points to the geographical North pole and is what you use on a chart. Magnetic is the bearing from where you are to the magnetic pole, which moves over time (it’s currently somewhere in Canada). Compass refers to the direction your boat compass is currently pointing.
To convert between true bearing and magnetic bearings, you need to account for variation. Variation is the measure of how much magnetic north differs from true north in the local area. This depends on where you are in the world – around Gibraltar it’s roughly 1 degree west, but in Northern Canada above the magnetic north pole it could be 180 degrees, i.e. the North end of your compass would be pointing due south.
Deviation is the final weirdness. This depends on the boat and what’s on it. There are a variety of things on board that have or can generate a magnetic field – magnets in speakers, large lumps of metal (like an engine, for example), or electronics. Any of these can affect the ships compass, and how it affects it depends on the boat’s heading. If, say, the compass is at the stern and the engine is midships ahead of it, when the boat is heading North or South the engine’s magnetism lines up with the Earth’s magnetic field and so has no effect on the compass’s direction. When heading East or West, the engine pulls the ship’s compass to one side or the other.
To take account for that, the boat has a deviation chart which shows how much error the compass has along any heading. In fact, we have 2 of them, one for each of the compasses we have by the 2 wheels.
There are 2 things to worry about with tides.
Firstly: Am I going to have enough water under the boat to keep afloat? For this we’ve been learning about tide tables, where high and low water times and heights are predicted for many ports. Each of these ports has an associated tidal curve, which shows how the height of tide varies between one low tide and the next. Once you know when high tide is on the day you’ll be in the area, you can use the tidal curve to calculate the tidal depth at the time you’ll be there.
Together with the chart work above, you start to see how you can plan a journey taking into account how far you have to go, how long it’s going to take, and whether there’ll be enough water under your keel to clear any obstacles when you get there.
Secondly: Which way is all that water going, and how fast? When a tide comes in or goes out, the water has to flow somewhere and that flow is called the tidal stream. In some places, this doesn’t make much difference, but if you’re crossing the straits of Gibraltar, for example, all the while you’re pointing the boat at your destination, the water flowing into the Mediterranean is (most of the time) pulling you eastwards. On a boat travelling 5 knots, a 2 knot tidal stream will take you 2 miles sideways for every 5 you go forward. In order to account for that, you need to know the state of the tide throughout your journey so you can adjust your course upstream. So for this, we’ve learnt to use either a tidal atlas, which shows the strength and direction of tidal streams in an area for 6 hours either side of high water, and tidal diamonds, which do something similar in tabular form and are usually written on the chart itself.
Combining chart bearings and tidal streams, you’re able to calculate a course to steer that will take you where you want to go in the shortest possible time. It feels weird pointing the boat 20 degrees away from the marina you’re aiming for, but it works.