Zezo forum:

Post by Ursus Maritimus | 2018-01-02 | 00:10:44

Has anyone else seen this?
A sudden jump up in DTF, so almost +100nm hyperbeamed ...
No strange values in vC or delta_d though.

Time Rank DTL DTF Position HDG TWA TWS TWD aTWA aSail vR (kn) vC (kn) vT (kn) Δd (nm) Δt (s) Sail Gybe Tack
1-1-2018 21:12:28 UTC 340 - 18446.1 11�42'01"S 36�25'27"W 207.2 129.5 13.1 77.7 Yes - 21.25 21.25 21.25 (C0) 0.71 120 - - -
1-1-2018 21:10:28 UTC 340 - 18447.1 11�41'23"S 36�25'07"W 207.8 129.5 13 78.3 Yes - 21.15 21.15 21.15 (C0) 0.71 120 - - -
1-1-2018 21:08:28 UTC 339 - 17748.4 11�40'46"S 36�24'47"W 207.9 129.5 13 78.4 Yes - 21.15 21.15 21.15 (C0) 0.71 120 - - -
1-1-2018 21:06:28 UTC 339 - 17748.5 11�40'08"S 36�24'27"W 208 129.5 13 78.5 Yes - 21.14 21.14 21.14 (C0) 0.7 120 - - -

(any tips on how to include / upload images on this forum)

6 Comments

Post by Ursus Maritimus | 2018-01-02 | 00:13:09

delta_dtf is much bigger now: 1nm per update in stead of 0.1 nm

(HDG is 207 so moving east and south along the brazilian coastline)

Post by JohnT | 2018-01-02 | 09:39:55

Exactly the same happened to me, and at almost the same dtf: mine went from 17760.2 to 18466.1 @ ~11�33S 36�34W, which is hugging the Brazilian coast. My first thought was that the DTF algorithm had flipped to mearuing the other way around the world - but that doesn't make any sense in the Atlantic....

Post by zezo | 2018-01-02 | 11:45:46

This is the way it works.

On anything but a short straight course the DTF actually has two components:

Distance to next gate + supposed distance to sail from next gate to the finish.

Just most of the gates are invisible and used internally by the engine for that exact purpose.

Depending on gate definition and how exactly you clear it - like is it a point or two points, if a point you can clear it by sailing North of it or leaving it to port. If it's a line segment the distance may be calculated to the line or to one of the two points.

So hen clearing a gate two things can happen:

1) DTF decrease if you were far away from the gate and the distance decreases after crossing

2) DTF increase if your position is further away from the next gate than the supposed distance between the gates.

Post by Ursus Maritimus | 2018-01-02 | 13:10:55

ok, thanks for the explanation. I was thinking in that direction

so there are virtual gates in a virtual sailing game

Post by zezo | 2018-01-02 | 18:16:06

Not virtual, just invisible.

One more thing that could be visualized in the chart, but will raise more questions than it answers ;-)

Post by JohnT | 2018-12-03 | 19:46:09

My DTF started increasing again this morning while sailing towards it @ around 30knots :(
I assume the above logic explanation still applies, so thought I would re-surface this thread in case anyone comes across it for the first time.
For reference this time it happened around 55 S 58 E. So not obvious to me which point(s) on the next virtual gate the DTF calculation is using..