A little technical piece

15 10 2011

You might have thought from my previous blog that we are just trundling along out here contemplating the stars and our navels. Far from the case. There is one thing that we are obsessed with and occupies our conscience every mintue of every day: Polar Percentage. What? you might ask?

For the uninitiated heres a little technical piece…

For every wind speed and wind angle relative to the boat, there is a certain speed that the boat should go. These are called polars, a matrix of numbers covering 180 degress of wind angles (the boat is assumed to be symmetrical) and wind speeds from say 2 knots to 40. After 40 you are not caring about how fast, but how safe. These speeds initally come from a designer, and you develop them as you sail the boat.

So for every wind angle and wind speed, a little number is spat out saying you should go this fast if you are sailing to your best performance. Problem: You would think that when sailing in the middle of the ocean, everything is nice and stable and the wind is consistent. Far from the truth – right now as I sit at the nav table the wind is fluctuating every few minutes by about 15 degrees, and from 11.5 to 14.5 kts, and of course we cannot steer a perfect straight line. So, you can imagine that the theoretical speed we should be going is fluctuating quite rapidly.

This is where polar percentage comes into it. By taking our actual speed and comparing it to the polar speed as percentage, we have a number which is insensitive to fluctuations in wind angle and wind speed. We have a clever little piece of software called Expedition Navigation Software (see our suppliers page for more info) that does all the calulations for this and we have a dedicated readout on deck (supplied by Stowe – again see our suppliers page) and we keep an  eye on this number at all times. I have even moved the window of this text editor so I can see the polar percentage on the screen as i write this below deck and the pilot is on. It is doing quite a good job I have to say, averaging 107% so I need to adjust our polars…

Polar percentage tells us a lot – if the polar percentage is below 100%, there is something wrong. Sail trim? Boat stacked incorrectly? Something on the keel? Helmsman tired? Wrong sail up? A few percent makes a huge difference over hours and days. Right now 5% is 1.5 miles every 3 hours to H&M ahead of us.

We monitor how well the pilot steers by polar percentage – if it cannot do 100% then we hand steer. If we are hand steering and getting tired and struggling to maintain 100% then engaging the pilot and monitoring its percentage can give us a rest. if the pilot cannot maintain a % better than hand steering then we are back to steering.

There were masses of weed through the doldrums. Hard to see the keel all the way to the top – the most telling way to know you have caught some is in polar percentages – a few tenths of a knot of speed is hard to see in boatspeed, but easy when you cannot get better than 95%. It is an honest number too, no point in being all pleased with yourself for doing 11 knots when you are only doing 97% – there is another 0.4 of a knot somewhere you need to find, and that equals almost 10 miles in one day…..

So, not all boring and groundhog day really. We are obsessed by boatspeed and this is the best and most efficient way that we can make sure we are sailing to our optimum, and keeps us extremely busy trying to maintain 100% or better.


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