- The boat has approached the mark using TWA
- The rounding has not happened according to the server
- The track of the boat shows that the boats has rounded the mark on the correct side
- Usually also the sailor says that the TWA predictor line showed that the mark should be rounded correctly
There are actually two problems (or perhaps better term would be features) affecting the issue.
1. The boat tracks in the client are not really accurate when the boat has been on TWA
I haven't been in SOL that long but I have been told that originally there was no TWA command, only CC. The tracks were then straight lines between the points where new commands were issued.
Later the TWA command was introduced which brought up the problem that the track between the points where course was altered by commands was not straight any more. To handle this issue the server sets up track points to the boats when they have altered course enough from the course they had when the previous track point was set.
It would be impractical to have too many track points for the boats because they would take too much space in the DB and more importantly, dataset of the boat tracks would also grow pretty big and would be difficult to load to clients having slow internet connection.
So, while reality the boat has traveled a curved line according to the wind direction changes, the track seen on the client is series of straight lines between the places where the server has decided to add a track point for the boat. This means that you can't really accurately determine from a track has a boat rounded a mark or not if the rounding has been very tight.
2. The predictor line in the client is not 100% accurately the path the boat will sail
The TWA predictor that you see is calculated in your client (that is, in your browser) according to the weather information received from the server, your current heading and time. Although it has proven to be pretty accurate, it's still not doing the calculation with the same precision as the server does. This can lead into small differences, which can be meaningful if trying to set the course really accurately using TWA predictor.
I personally have been caught by trying to make a really tight mark rounding using TWA and ending up missing the mark. I believe most of us who have been in SOL for a longer time have. Nowadays I always try to leave some margin (how much, it's up to you and how much you want to risk) if I use TWA or then switch to CC just before the rounding.
In my mind, this issue is bit similar to the fact that in real sailing, you always leave some room to the mark to allow waves moving it a bit. If you don't do it you might still make a clear rounding with luck but you are not guaranteed to do so.
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