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Old 02-08-2019, 11:03   #16
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Re: Equations for calculating true wind with attitude?

And here I thought you only wanted to figure wind speed when you were pissed off...
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Old 02-08-2019, 11:44   #17
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Re: Equations for calculating true wind with attitude?

You mentioned the correction tables. I think they might be in the end a more exact and easier way of making the corrections to the measured wind speed and direction than trying to base these corrections on pure math and trigonometry.

I'm not sure how easy it is to collect such data. Maybe you can start by rocking the boat (at anchor or mooring) and checking how much that changes the results.

Maybe you would need a second set of devices and measurements from a nearby island if you want to find out the impact of large waves and related yaw and roll. Or maybe it is enough to make the measurements first behind a flat island, and later at nearby open sea (with constant speed).

How about boat heading (compass?) vs direction (GPS?). Will you measure those too?

In my boat I'd also need corrections for the log (because of different results on different tacks). Tides and currents are not a big deal in Finland, but it would be nice to be able to see the (accurate) difference of speed in the water vs GPS speed.

People asked if this is useful. Although I do most of practical sailing based on looking at the sails and telltales, having accurate information would help e.g. in telling me if my settings were good or if some other settings would have been better. Having data from before and after making some adjustments (one minute of both) would tell me right away if those adjustments were good or bad. After few such exercises I'd be much more confident in making adjustments in the sails (without any such modern tools), and I could concentrate again on just sailing and enjoying the day . If you plan to race, then these devices and measurements could be really valuable.
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Old 02-08-2019, 12:30   #18
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Re: Equations for calculating true wind with attitude?

There're two factors that don't seem to have been mentioned and which influence the computations: The extent to which the wind instrument averages its "instantaneous" measurements for delivery to the user and the location of the wind instrument relative to the center of mass of the boat.

For example, if the sampling rate of the instrument is a millisecond, then the data rate from the instrument would about a thousand samples per second... far too often to be of use for most sailing purposes. If the instrument uses internal averaging and delivers, say, a new apparent wind speed every few seconds, all accurate information about faster 3D motions of the boat are lost.

Also, the instantaneous velocity (speed and direction) of the instrument differs from the velocity of the center of mass (COM) of the boat. This is important since it's the motion of the COM that's important for positional and heading estimates relative to the water as well as the speed along the desired geographic course. Good luck with sorting all of this out!

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Old 02-08-2019, 13:22   #19
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Re: Equations for calculating true wind with attitude?

Or you can just integrate over several roll pitch cycles which is far simpler mathematically. In the end the mast top will travel the same distance as the boat centre of gravity unless there is something seriously amiss.
Real time instantaneous calculations to be accurate do require that each component used has the same latency and integration time constant, which in most cases are unknown and difficult to establish.
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Old 02-08-2019, 13:23   #20
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Re: Equations for calculating true wind with attitude?

Quote:
Originally Posted by Montanan View Post
... turning the wind speed meter off is my best practice when sailing.
Best practice? No sorry, do not do this.

As a real sailor, learning since when I was a child, and still to this day being hyper sensitive to any change in my environment, even on land in an apartment, sorry but I can still not agree with this in any way.

Why deprive yourself of useful information? No, the best sailor never relies solely on one source of information and instead uses all possible data to assess the situation.

Switching off data, no, sorry, for the average sailor that is extreme foolishness.

Why do people recommend such advice to the general community?

No, this is not correct.

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Old 02-08-2019, 14:09   #21
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Re: Equations for calculating true wind with attitude?

It is very challenging to measure wind in a moving sailboat in choppy seas. Not only must the sensor be adjusted for the offset error, but you should also adjust the wind measurement för heel, leeway and upwash. And most of the basic wind systems on the market have no such capabilities. Normally you can just make the offset adjustment and then they perform the basic wind triangle calculation. Note that also the speedlog should be carefully calibrated and preferably also adjusted for the effect of heeling.

So unless you have a system that enables all these adjustments and (on top of all) also put the great effort to thoroughly calibrate your measurements you can never expect any accuracy from your wind system. And without accurate wind info it is impossible to pick trustworthy TBS values from your polar table.

One final comment. Although it would seem useful to adjust the wind measurement also for the (dynamic) roll and pitch motions the integration time of most wind sensors is simply too long for a meaningful compensation. So in reality this will unfortunately just add extra noise to the wind measurement.
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Old 02-08-2019, 16:27   #22
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Re: Equations for calculating true wind with attitude?

sailing since I was 13 and never understood why knowing true wind is helpful. I seem to be able to know all I need to know from windex, heel, weather helm, tell tails etc. and yes, I too have wind instruments that were installed by a previous owner and I suppose it's a nice gee wiz thing to see how the wind blows at the dock so I can know which direction I can throw the ball for my dog the farthest to get his exercise but under sail? don't need it and have the trophies to prove it.
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Old 02-08-2019, 16:34   #23
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Re: Equations for calculating true wind with attitude?

Quote:
Originally Posted by jmh2002 View Post
Best practice? No sorry, do not do this.

As a real sailor, learning since when I was a child, and still to this day being hyper sensitive to any change in my environment, even on land in an apartment, sorry but I can still not agree with this in any way.

Why deprive yourself of useful information? No, the best sailor never relies solely on one source of information and instead uses all possible data to assess the situation.

Switching off data, no, sorry, for the average sailor that is extreme foolishness.

Why do people recommend such advice to the general community?

No, this is not correct.

actually it is correct. sailing since I was thirteen and whining trophies since I was 15 and never relied on wind instruments. at best they are lousy and there is much m
ore information available just by being aware of the boat's performance and parameters available from the windex, angle of heel, tell tails and helm pressure.
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Old 02-08-2019, 17:27   #24
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Re: Equations for calculating true wind with attitude?

Not mentioned so far is the great performance to windward under auto pilot when a properly compensated wind instrument is fitted, to say nothing of sailing deep angles with a kite up. The solo racers have such devices and can sleep while the boat blasts along at high speeds. This isn't too important to most day sailors, but for short handed cruisers it would be useful. I know that i'd like it... but not at the commercial price point. The OP's efforts could lead him in that direction if he was interested.

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Old 02-08-2019, 18:12   #25
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Re: Equations for calculating true wind with attitude?

Quote:
Originally Posted by jrbogie View Post
sailing since I was 13 and never understood why knowing true wind is helpful.

Allows you to predict apparent wind following a coming heading change?
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Old 02-08-2019, 18:17   #26
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Re: Equations for calculating true wind with attitude?

Quote:
Originally Posted by jrbogie View Post
actually it is correct. sailing since I was thirteen and whining trophies since I was 15 and never relied on wind instruments. at best they are lousy and there is much m
ore information available just by being aware of the boat's performance and parameters available from the windex, angle of heel, tell tails and helm pressure.

Funny how often we see a poster bragging about how good they are and how many races they won advocating the opposite of what the vast majority of very experienced sailors, who don't feel the need to brag about their exploits, practice.
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Old 02-08-2019, 23:04   #27
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Re: Equations for calculating true wind with attitude?

Quote:
Originally Posted by Juho View Post
You mentioned the correction tables. I think they might be in the end a more exact and easier way of making the corrections to the measured wind speed and direction than trying to base these corrections on pure math and trigonometry.

I'm not sure how easy it is to collect such data. Maybe you can start by rocking the boat (at anchor or mooring) and checking how much that changes the results.
Yes, might be! B&G uses both tables and trigonometry, themselves, I believe, but there certainly are big question marks there, starting from getting good source data to do the calculations.

Quote:
Maybe you would need a second set of devices and measurements from a nearby island if you want to find out the impact of large waves and related yaw and roll. Or maybe it is enough to make the measurements first behind a flat island, and later at nearby open sea (with constant speed).

How about boat heading (compass?) vs direction (GPS?). Will you measure those too?

In my boat I'd also need corrections for the log (because of different results on different tacks). Tides and currents are not a big deal in Finland, but it would be nice to be able to see the (accurate) difference of speed in the water vs GPS speed.
Yes, I'm measuring those as well. And trying to calibrate them . Actually I have gigabytes and gigabytes of raw data already and it would be fun to try to implement some sort of "continuous calibration" system.

Quote:
People asked if this is useful. Although I do most of practical sailing based on looking at the sails and telltales, having accurate information would help e.g. in telling me if my settings were good or if some other settings would have been better. Having data from before and after making some adjustments (one minute of both) would tell me right away if those adjustments were good or bad. After few such exercises I'd be much more confident in making adjustments in the sails (without any such modern tools), and I could concentrate again on just sailing and enjoying the day . If you plan to race, then these devices and measurements could be really valuable.
Agree!
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Old 02-08-2019, 23:09   #28
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Re: Equations for calculating true wind with attitude?

Quote:
Originally Posted by ChrisseH View Post
It is very challenging to measure wind in a moving sailboat in choppy seas. Not only must the sensor be adjusted for the offset error, but you should also adjust the wind measurement för heel, leeway and upwash. And most of the basic wind systems on the market have no such capabilities. Normally you can just make the offset adjustment and then they perform the basic wind triangle calculation. Note that also the speedlog should be carefully calibrated and preferably also adjusted for the effect of heeling.

So unless you have a system that enables all these adjustments and (on top of all) also put the great effort to thoroughly calibrate your measurements you can never expect any accuracy from your wind system. And without accurate wind info it is impossible to pick trustworthy TBS values from your polar table.

One final comment. Although it would seem useful to adjust the wind measurement also for the (dynamic) roll and pitch motions the integration time of most wind sensors is simply too long for a meaningful compensation. So in reality this will unfortunately just add extra noise to the wind measurement.
I think it should be possible to do what I'm trying to do because I think B&G does it themselves, with the H5000 Hercules CPU and the same sensors I have. I'll have to check whether they are using the same gyro/3D motion sensor, but if so, then I should be able to do the same transformations to the raw data as they are. (I think.)

EDIT: No, in fact, it looks my (default) pitch and roll data is originating from the Precision9 compass and the H5000 series has its own sensor including rate of pitch/roll: https://www.bandg.com/bg/type/instru...-sensors/list/.
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Old 03-08-2019, 00:18   #29
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Re: Equations for calculating true wind with attitude?

It would be absolutely wonderful to implement something like this, table based or next gen cooler than everything AI self adjusting, as part of the Signal K server. The server has a plugin system that allows simple installation and provides configuration management out of the box.

So instead of everybody doing their own bespoke systems from scratch people could collaborate on the algorithms and make more progress.

There is already a Derived Data plugin for straightforward stuff like true wind, true heading etc, but for advanced stuff like this a separate plugin would work very well.

I can help out with getting up to speed, let me know if you are interested.

https://github.com/SignalK/signalk-server-node/
https://github.com/SignalK/signalk-derived-data

Disclosure: I am one of the primary authors of Signal K spec and the server
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Old 03-08-2019, 00:47   #30
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Re: Equations for calculating true wind with attitude?

Quote:
Originally Posted by mglonnro View Post
Actually I have gigabytes and gigabytes of raw data already and it would be fun to try to implement some sort of "continuous calibration" system.
I wonder if "continuous calibration" would mean real-time modifications to the parameters while sailing, or if post processing after the sail (to improve the model and make permanent changes to it) would be enough. At least the latter case could be doable.

If you have gigabytes of data, and you have marked some segments of it as "under constant conditions", then you could automatically (or manually) adjust the parameters until your model gives you constant TWS and TWA for all points of sail, and for all other changes in your trim (for each data segment under constant conditions). You can also check the results against the (reliable and accurate) GPS measurements.

If you have multiple alternative models/algorithms for calculating the corrections, you could also check which one of the models gives you the best / most stable results.

I note that real-time calibration could be needed for the magnetic compass, since magnetic fields may change from place to place.

If you want to make a perfect system, maybe you need to install some additional wind measurement devices, e.g. closer to sea level, maybe bow and aft to always get clean winds. A gimballed wind measurement system could also help, at least in the testing phase.
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