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Old 09-02-2018, 07:46   #1
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ETA/TTG

HI,
I have beeen recomeended to use OPEN CPN and I am very very pleased.

My subject is about ETA and TTG feature when I activate a route. I am working on a ship powered by engines (not a sail boat). I do complicated route planing along a broken coast. Whenever I activate a route, the data showing indicates values of ETA and TTG that seems related to VMG values. Is it possible to have this values in relation to SOG along a given route? I am asking because the value I get often doesn't make sense at times: for exemple, if I am steering away from destination (but following by plotted route) I don't get any logical readings for ETA and TTG. Is my explanation clear?

Many thanks
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Old 09-02-2018, 08:15   #2
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Re: ETA/TTG

I am working on an autopilot route plugin which will enhance this display.

For ETA to the next waypoint it should probably be based on VMG.. but could be filtered over a longer time.

For the total route, it could use a different algorithm. It could combine the ETA to the next waypoint, then compute the distance from there and use sog... Using vmg is not realistic because this value could change for the next leg significantly. It is also unrealistic to use sog, because the vmg is always slightly less than this... The autopilot doesn't perfectly follow the route, although maybe it can be nearly perfect.

What should we do?
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Old 09-02-2018, 12:27   #3
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Re: ETA/TTG

Quote:
Originally Posted by boat_alexandra View Post
I am working on an autopilot route plugin which will enhance this display.

For ETA to the next waypoint it should probably be based on VMG.. but could be filtered over a longer time.

For the total route, it could use a different algorithm. It could combine the ETA to the next waypoint, then compute the distance from there and use sog... Using vmg is not realistic because this value could change for the next leg significantly. It is also unrealistic to use sog, because the vmg is always slightly less than this... The autopilot doesn't perfectly follow the route, although maybe it can be nearly perfect.

What should we do?

Ia m not a specialist of programming software. But I am a professional sailor for 35 years. I do not want my views to be imposed. What I would like to bring to you is some features that are already used on ECDIS systems such as TRANSAS, FURUNU, JRC, etc.

These systems all include an ETA and TTG related to a given route and based on SOG. As you mentioned, this value can't be accurate 100%. Their could be small deviation in the route plan or a change of speed which will impact ETA. Nevertheless, this is a close enough value. Also there is an associtad option which allows to predict an ETA based on a different value than the current speed; in a box a different speed over ground value can be input. We use this option whenever we can predict adversed weather or ice condition that will significantly reduce our speed in the near future and consequently change our ETA. Nevertheless, the Distance to go should be along the planned route, not in direct line (which seems to be the case on the current Open CPN).

What I would suggest is 3 data boxes (they could be shown as roled down options) with any of the following feilds:
1. TTG, DTG and ETA based on SOG on a given route.
2. TTG, DTG and ETA based on a calculated speed (manually SOG input).
3. TTG, DTG and ETA based on VMG to suit the specific needs of sailboatd which do many tacks on a given route.

Again I am not a specialist but this is typically what I have seen on most ships that I have sailed on.

I hope this help.

Carry on the very good work!
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Old 10-02-2018, 02:07   #4
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Re: ETA/TTG

stephanguy...
Interesting suggestions and maybe some developers adapt it but it's up to there's time and joy.
Two comments though:
1.
Quote:
the Distance to go should be along the planned route, not in direct line (which seems to be the case on the current Open CPN).
This is not correct. If you make a curved route and activate it you'll see the total distance is calculated as the summary of all legs, not a straight line from first to last. (Click on the Route window to switch between Leg and Route view.)

2.
If you're heading towards next WP VMG and SOG are equal.

Håkan
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