Expedition Navigation Software 
1 of 1- nav
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Offline - Registered: 12/20/2015
- Posts: 91
Rotation and acceleration
Hello,
I have a question on how the rotation in Expedition is calculated:
Is there any reduction in velocity included during the turn?
Lets take the following example on a 100'+ boat: the boat goes 10knts in the pre start and has to do a 120° turn to approach the starting line. As far as I understand the time burn is now calculated using the turning rate for the current boat speed and then the time from the position of the end of the turn to the line at polar speed for the corresponding wind angle.
What I think is not included (correct me if I'm wrong), is that the boat slows down 3 knots during this turn, so it comes out of the turn at 7knts. After the turn the acceleration is used to calculate the new time burn and this is all of a sudden much lower than it was before the turn. So calling a turn at 0 seconds time burn will let you end up maybe 10seconds late, depending on how far to go to the line after the turn. Of course one could just set the turn rate lower, but that is not very accurate as the program thinks you come out at a different position after the turn as the turn radius becomes much bigger.
It might be useful to add an optional function like velocity loss in knots per degree of turn, so the programm can determine, at which speed you exit the turn and then include the acceleration from there.
Last edited by nav (6/13/2016 11:22 pm)
- Nick
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- Registered: 9/16/2015
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Re: Rotation and acceleration
That is the brake calibration. There is a description of it in the help.
- nav
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- Registered: 12/20/2015
- Posts: 91
Re: Rotation and acceleration
Ah, I see. Thanks, I guess that explains everything...
Last edited by nav (6/14/2016 3:15 am)
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Thread Starter- nico_sanfe
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- From: Italy
- Registered: 5/20/2019
- Posts: 15
Re: Rotation and acceleration
Hi. I use this thread since I have a question about rate of turn calibration, used for the prestart fase.
Rate of turn units are degrees per second and are a function of boat speed (according to the manual).
The boat I sailed is a Swan50.
I have analyzed different tacks performed during prestart, with different boat speeds at the beginning of the turn. I have computed the rot in °/s as: the angle we span divided by the time between start & end time of the turn; for 7kn BS at the beginning, I have obtained a rot of 6/7 °/s.
If I use this value, the predicted turn is way to big. I have used the playback mode, to compare prediction vs reality.
A good rot is 16, in order to have reality and prediction similar.
Am I missing something in my approach ?
Is there a "mathematical/statistical" approach to compute rot for the start turn ?
I hope to have explained my question.
Thanks,
Nicolò
- Nick
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Re: Rotation and acceleration
Hello,
One thing is that it is always a model and you might turn faster or slower etc.
Yes, ROT is a function of bsp - the faster you are going, the faster you can turn. Acceleration is modeled as a linear function. Dfw used a log curve, but in all the AC etc testing we did, it really is more linear.
One idea I used to use was to select ROT to make it roughly match the track circles. Whether that is a valid or good idea ...
Generally, people have decided the Brake cal isn't necessary. I supposed the less complications the better.
- nico_sanfe
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- From: Italy
- Registered: 5/20/2019
- Posts: 15
Re: Rotation and acceleration
Hi Nick, thanks for the explanation.
I know reality is different than a model prediction.
I used your same approach (prediction vs track) on average typical turn to compare with my computations (btw, I have analyzed 25/30 racing & training port to starboard tacks in prestart, in order to have an idea of average tack).
Still have not study in deep the brake function. Tomorrow I'll do.
Living in Italy, now in quarantine due to covid19, we have lot of free time .
Ciao, Nicolò