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Old 23-08-2016, 12:49   #466
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Re: Plugin: DashBoard

Rick..
True, it can for sure be made more sophisticated but for me those "simple" calculations gave me a "good enough" true wind indication. My use is for example when running by motor and you want to know if there's a reason to switch it off and hoist the sails.
Håkan
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Old 23-08-2016, 12:56   #467
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Re: Plugin: DashBoard

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Originally Posted by fgd3 View Post
That's not correct. While a purist (or a pedant) might argue that because you are sailing on the water true wind should be measured at the water's surface rather than the ground it's just a semantic quibble.
I don't agree that it is just semantics. One use of true wind data is to use a polar diagram to estimate what sail configuration and trim should be used to maximize a course made good. Polars are based on wind speed relative to a boat in water not on ground. It may not seem obvious but they are not the same. If the boat is making progress through the water but over ground it is moving differently the sails don't care about the ground. They care about the wind and the keel which is driven by water.

Another use of true wind data is to compare with Grib predictions. These are relative to ground and so using ground speed/direction is appropriate for that comparison. But the more useful data when sailing is wind relative to water.

Polars take into account leeway but perhaps not drift (but apparent wind takes into account leeway and drift). There is no perfect true wind formula that satisfies every requirement.
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Old 23-08-2016, 13:35   #468
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Re: Plugin: DashBoard

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Originally Posted by Hakan View Post
Hasse..
I'm aware of the discussion above but I use SOG, speed over ground, because I want to know the true wind over ground
I can see a point in using SOG since it is difficult to get good precision in STW depending on mounting, heeling and fouling.

To get the correct wind over ground don't you have to take in account the fact that your wind transducer is not pointing in the direction you are moving according to COG (leeway + current). Am I correct ?

The angle difference between COG and heading would off course be easy to calculate if you have heading info and make use of the WMM plugin.

This would give true wind over ground.

/Hans
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Old 24-08-2016, 05:33   #469
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Re: Plugin: DashBoard

I agree that if the speed through water sensor is inaccurate then speed over ground would at least help compute true wind over ground. And true wind over water can't be computed anyway so there is no need to bother trying.

I suppose there is no reason not to have both types of true wind available. I'll think about it a bit and see if Dashboard can be coerced into providing computed true wind of both types. What would we name them to make the meaning clear?
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Old 24-08-2016, 12:36   #470
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Re: Plugin: DashBoard

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Originally Posted by hasse_A View Post
I had a look at true wind angle and wind speed.
As I understand it you have to input this info via NMEA sentence to have the meters show them.

A lot of us don't have this info accessible.
Would it not be possible to calculate it from apparent wind speed, wind angle and boat speed ?

These are much more frequently available.

If true angle and speed are not present the dashboard could calculate the values, otherwise use the supplied NMEA data.

/Hans
Hans,

give tactics_pi a try, it can do this calculation. It's based on dashboard_pi, they look the same and you can run both in parallel ...
Thomas
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Old 24-08-2016, 20:34   #471
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Re: Plugin: DashBoard

Thanks Tom, those are very good links and graphics.
Sort of responding to TDan and Hasse

To record a good polar diagram, the effect of drift and set, leeway need to be removed, For example trying recording your track while tacking up or downwind going against or with current! There is a huge difference.

Additionally, use of the polars, when you've got meaningful polars, they will provide more effective guidance (Targets) if the software can also include the effect of current and a meaningful estimation of leeway.

What I am trying to say, is I think it really does make a difference how you record your boat's polars and what values you use HDG/STW or COG/SOG and how you account for Drift/Set and leeway.
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