Why does the predicted route significantly change when changing the number of days?
Post by iconPost by pgfiore | 2011-11-18 | 10:29:53

Ciao folks, I'm wonder why if I switch from the one day prediction to two the days (or more) one the calculated route is very different.
I mean the starting part of the new track! Should be always the same, shouldn't?

Yesterday evening the SS even changed from a broad reach computed by the 1day prediction to gybeing and a beam reach computed by the 2-7days one...

Thank in advance for any explanation. rgds. pf

commenticon 6 Comments
Post by iconPost by zezo | 2011-11-18 | 11:42:41
Predictions with different duration just see different things.

I'll use the current situation in the South Atlantic as example:

Thu current strategy suggests catching a low pressure system somewhere around 35S/15W. This is expected to happen in 4-5 days. But the prediction does not see that system coming if it's shorter than 5 days. It will keep more or less the direct route, because it does not know that it will lead to the wrong end of that system with strong headwinds in just few days.

The same thing goes for expected weather more than 7 days ahead. If you have checked the forecast at the gate few days ago with destination set at Capetown, it would suggest going the great circle route even if there was weather system forming around Cape Horn. Here is where human knowledge comes.

Real sailor would know that crossing the South Atlantic involves getting in the roaring 40's sooner or later, and would seek route South. That could go close the Brazilian coast, or as was our case - much further West. So you could set a destination around 45S/10W and see how to get there faster.

Shorter forecasts, on the other hand, have advantage in more unstable and unpredictable conditions. A 1-week forecast may suggest chasing a weather system 1 week ahead which is very unlikely to appear at exactly the predicted place and time, so it will lead in somehow wrong direction. What's worse is that it will lead in another slightly wrong direction the next day when the forecast changes. You end up making a zigzag track and losing time. So it's better to decide on a long term strategy and stick with it if possible.

Hope this helps

Cvetan
Post by iconPost by pgfiore | 2011-11-18 | 15:15:43
Many thank for yr prompt and clear reply Cvetan. Lot of stuff to weight!
I have a light skill in sailing, but with no idea about ocean sea and long-duration races.
For example I'm never sure if to point higher and getting the best from light spi or to bear away for making less miles...

Cvetan, does your SS considers other sails too?
It is suggesting default medium spi while, onto the game, it seems slightly better the light spi.

Ciao
Post by iconPost by zezo | 2011-11-18 | 18:08:36
The simulator model uses one (two, with/without pro options) envelope sail
, consisting of the best possible sail for every TWA. Then it just displays you the appropriate sail to use. You can choose standard/pro sails with right click.

I have real sailing experience only in Black and North Aegean seas, but have crossed the Atlantic quite a few times virtually, and that teaches you something about meteorology ;)
Post by iconPost by pgfiore | 2011-11-19 | 10:58:12
Scenario
Now at 10°48'N 27°53'W - Fernando is in a 3dd sailing.

SS suggests: 188° for the 3dd forecast and 185° for the 5dd one.

Since my own concern is about "how much water to pay for getting speed", I'd like to discover firstly why the SS is making that difference (the race field and forecast are the same for both) and then more generally which could be the best choice.
It's just half a knot (and 3° on the other side), but for the next 200nm!

Sorry for bothering... ciao
Post by iconPost by pgfiore | 2011-11-19 | 13:51:36
... About 3 hours later (12:50gtm+1) and 40-odd nm south
10°02'N 28°W

A new 3-dd run of SS now shows 169°, much more "larger" from the target (Fernando)... The forecast is (should?) be the same and I'm getting lost

... Luckely the only trip in which I crossed the Gibilterra (to Canaria) I was not the tactician! :D:D:D
Post by iconPost by zezo | 2011-11-19 | 14:50:43
It sees that you're going to sail against the wind in two days and goes a bit to the West so you can get better TWA without tacking.
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