You may recall that over chocolate biscuits and sailor’s tea, Ahab introduced the notion of true and magnetic north to his willing sailor in training, Ernest. [link to blog post]
Ernest had discovered there are two Norths – true north which is the north pole which is arbitrary and magnetic north which is real but also, confusingly moves.
“But not by a great amount,” Ahab answered Ernest when he’d asked how far magnetic north was known to wonder, “but it is enough for us to take into account when we plot our course. And that’s why you need to know about true virtue makes for dull companions, add whisky.”
“What on earth is that? I’m not keen on whisky…”
“It’s a mnemonic, a phrase that we chant to remember an important principle, a bit like Richard Of York Gave Battle In Vain to remember the colours of the rainbow.
We have to account for the difference between true and magnetic north because the numbers on the chart won’t… what?”
“Line up with a compass”, replied Ernest.
“We’ll make a sailor of you yet Earnest! You’re right. And so, we use the mnemonic true virtue makes for dull companions, add whisky to help us. What it means is this, “continued Ahab. “True, variation, magnetic (for) deviation add west. In other words, this is the order of the calculation we need to work through to compensate for the true north and the moving, magnetic north”
“How do I do that then?” asked Earnest, although to be honest, his head was swimming a bit with all this information.
“You’ll need to do some more reading, most of us sailors have our head in a book but seeing as you have that new-fandangle smart phone with you, take a look at some sailing websites, something like School of Sailing that has more information on how to plot a course taking in account true and magnetic north”.
“But before you do that, let’s have a quick-fire quiz. Ready?” Ahab didn’t wait for an answer.
“What do you think a leeway is? It has something to do with what we’ve been discussing, setting a course and all that…”
“Get a notebook out and make a note of these things because you will need them. Leeway is the effect of the tide and/or win that pushes your boat off the course you are steering”.
Ernest frantically scribbled in his notebook, “You will need to look at what you do about that too, Ernest. Right, second question, what is dead reckoning?”
“Use that smart phone thing and find me a definition”.
After much tapping, Ernest had the answer, “dead reckoning is the process of calculating the one’s position, usually at sea, from estimating direction and distance travelled rather than by using landmarks or the stars.”
“Excellent, Ernest, excellent. We’ll do more about that in the future, probably as we bob along, the wind in our sails. But we’ve covered a lot about charts, courses to steer, three-point fixes, the true and magnetic norths and more. Take another look at everything we’ve covered because we are going to move on to tides”.