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Old 01-08-2019, 23:08   #1
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Equations for calculating true wind with attitude?

Cheers,

We have B&G instruments on our new Hanse, but unfortunately not the ones (H5000 series) that account for pitch and roll when calculating wind angle and speed.

I'm running my own Raspberry system, though, and can make the calculations myself, if I only knew how

From the NMEA2000 network I get AWA, AWS (and also B&G's pitch+roll). From the Raspberry I get pitch and roll (in degrees or radians). Can anyone point me towards the math I need to a) adjust AWA/AWS and b) calculate TWA/TWS?

Kindest regards and thank yous
Mikael
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Old 02-08-2019, 00:47   #2
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Re: Equations for calculating true wind with attitude?

It's not the angle of pitch/roll that you need, it's the rate of change.
You need to divide the change from one reading to the next by the sampling frequency to get degrees per second and then multiply that by the height of your mast to get the speed of movement in both pitch and roll. Then simple vector trigonometry will give you a speed and direction of movement of the wind instrument relative to the hull. You can then use that data to correct AWS and AWA with another simple trig calculation.
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Old 02-08-2019, 00:52   #3
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Re: Equations for calculating true wind with attitude?

To answer the second part. TWS/TWA are again simple vector trigonometry once you know AWS, AWA and boat speed.

For both problems, drawing the vectors on a piece of paper and working out where you need sines, cosines to determine the missing data points is the best way to understand it.
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Old 02-08-2019, 01:38   #4
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Re: Equations for calculating true wind with attitude?

Thank you!

I found this post as well: https://kingtidesailing.blogspot.com...or-vessel.html

But returning to your comments: the rate of change, ok!, but won't the wind sensor return a different wind angle just being steadily tilted to one side? Due to the effect of gravity, if nothing else?
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Old 02-08-2019, 04:07   #5
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Re: Equations for calculating true wind with attitude?

Quote:
Originally Posted by mglonnro View Post
Thank you!

I found this post as well: https://kingtidesailing.blogspot.com...or-vessel.html

But returning to your comments: the rate of change, ok!, but won't the wind sensor return a different wind angle just being steadily tilted to one side? Due to the effect of gravity, if nothing else?

Iterations 1 and 3 are covered in my earlier posts.


I am doubtful about Iteration 2.
"If you take a normal anemometer and tilt it 90 degrees to the side, it will stop working because there is no more wind that can turn it" That depends on which way you tilt it If you tilt it 90° to the side in a headwind or tailwind, it will be unaffected. If you tilt it 90° fore or aft in the same situation, it will come to a stop.


"But a wind vane tilted X degrees to the wind will register wind depending on the angle.", At any reasonable degree of heel, say up to 20-30°, I would expect the tail of the wind vane to still trail away from the wind.
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Old 02-08-2019, 05:05   #6
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Re: Equations for calculating true wind with attitude?

An interesting technical exercise, but will it ultimately solve a pragmatic problem? For example, will it yield data which can be used to sail more effectively than just using a dampening factor?
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Old 02-08-2019, 05:14   #7
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Re: Equations for calculating true wind with attitude?

Just after we left New Zealand our wind instruments broke...somehow we managed to get to Japan and we never missed them!
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Old 02-08-2019, 06:17   #8
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Re: Equations for calculating true wind with attitude?

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Originally Posted by s/v Moondancer View Post
Just after we left New Zealand our wind instruments broke...somehow we managed to get to Japan and we never missed them!
Certainly not necessary, but useful and fun. My anemometer has been out of commission for several seasons now. I plan to fix it for this season...mostly so I can play around with instrumentation like Open Chart Plotter.

Sometimes when I teach sailing I cover the instruments so students have to learn to sense the wind. My Hobie 33, which I original acquired for the sailing school, intentionally has no instruments (I ripped the old ones out and glassed in the holes).
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Old 02-08-2019, 09:28   #9
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Re: Equations for calculating true wind with attitude?

Quote:
Originally Posted by belizesailor View Post
An interesting technical exercise, but will it ultimately solve a pragmatic problem? For example, will it yield data which can be used to sail more effectively than just using a dampening factor?
I want to create polars to compare with the published ones. Eventually trim, record data, see what I can do to improve sailing speed (and/or comfort).

I've (a bit superficially) went through all the calibration thingies according to the manuals but when tacking a) true wind jumps around way too much (like 10-15 degrees) and b) close hauled apparent wind angles are likewise quite different (like 40 deg vs 29 deg).

I want to solve this so that I can get reliable recordable data

When reading the B&G manuals I saw that the more expensive sailing computers also correct for pitch and roll, and have a couple of additional correction tables as well. I don't know how big these corrections are, but just trying to figure out everything bit by bit.

Standing rigging trim is on the todo list also.
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Old 02-08-2019, 09:34   #10
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Re: Equations for calculating true wind with attitude?

Here's the project: https://community.nakedsailor.blog/b...C2X9X2LbkzMhFm (can take a while to load)

It's "recording everything" and it would be nice if what's recorded is at least somewhat accurate
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Old 02-08-2019, 10:02   #11
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Re: Equations for calculating true wind with attitude?

Quote:
Originally Posted by mglonnro View Post
I want to create polars to compare with the published ones. Eventually trim, record data, see what I can do to improve sailing speed (and/or comfort).

I've (a bit superficially) went through all the calibration thingies according to the manuals but when tacking a) true wind jumps around way too much (like 10-15 degrees) and b) close hauled apparent wind angles are likewise quite different (like 40 deg vs 29 deg).

I want to solve this so that I can get reliable recordable data

When reading the B&G manuals I saw that the more expensive sailing computers also correct for pitch and roll, and have a couple of additional correction tables as well. I don't know how big these corrections are, but just trying to figure out everything bit by bit.

Standing rigging trim is on the todo list also.
Potentially fun and useful !

So, your hypothesis is that these variations are caused by mast movement, right?

Matreon, and others, solve this problem by putting sensors in their transducer. Putting an accelerometer at the masthead would be one way to solve this problem with direct readings rather than a secondary reading (sensor in an offset location). Any sensor not at the masthead will be offset both vertically and horizontally...making the calculations more complicated (unless maybe you can mount it precisely under the mast). Seems to me if you want very accurate readings then putting at the masthead would simplify matters.

https://www.maretron.com/products/wso100.php

As I recall there is a thread here discussing the accuracy of the Matreon product and it wasnt very favorable. Might be worth finding that thread. I think "Dockhead" started it.
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Old 02-08-2019, 10:24   #12
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Re: Equations for calculating true wind with attitude?

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Originally Posted by s/v Moondancer View Post
Just after we left New Zealand our wind instruments broke...somehow we managed to get to Japan and we never missed them!
Exactly, turning the wind speed meter off is my best practice when sailing. I can readily trim sails and adjust course without any electronic input. I especially hate it when someone is monitoring the wind speed meter and saying the indicated speed with each passing gust. And my neck greatly appreciates not looking up at even a simple windex. A glance at the tell tales on the sails and on the shrouds provides the guidance I utilize and of course just the feel of the speed and direction, especially at night when I don't desire to have my night vision impaired by any glow of instruments. Call me simple.
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Old 02-08-2019, 10:26   #13
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Re: Equations for calculating true wind with attitude?

Other than mental exercise, WHY?
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Old 02-08-2019, 10:28   #14
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Re: Equations for calculating true wind with attitude?

Quote:
Originally Posted by belizesailor View Post
Potentially fun and useful !

So, your hypothesis is that these variations are caused by mast movement, right?
I think my base calibration is off as well, but mast movement certainly affects it to some degree. I'd be happy to get readings that are +/- a couple of degrees from reality.

Quote:
Matreon, and others, solve this problem by putting sensors in their transducer. Putting an accelerometer at the masthead would be one way to solve this problem with direct readings rather than a secondary reading (sensor in an offset location). Any sensor not at the masthead will be offset both vertically and horizontally...making the calculations more complicated (unless maybe you can mount it precisely under the mast). Seems to me if you want very accurate readings then putting at the masthead would simplify matters.

https://www.maretron.com/products/wso100.php

As I recall there is a thread here discussing the accuracy of the Matreon product and it wasnt very favorable. Might be worth finding that thread. I think "Dockhead" started it.
I will look for it!

B&G with their Hercules CPU (I think) use the standard gyro mounted somewhere down in the hull. I wonder what degree of accuracy they give? As I wrote above, getting to zero point something isn't what I'm looking for but +/- 10 degrees isn't very fun
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Old 02-08-2019, 10:33   #15
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Re: Equations for calculating true wind with attitude?

Quote:
Originally Posted by Montanan View Post
Exactly, turning the wind speed meter off is my best practice when sailing. I can readily trim sails and adjust course without any electronic input. I especially hate it when someone is monitoring the wind speed meter and saying the indicated speed with each passing gust. And my neck greatly appreciates not looking up at even a simple windex. A glance at the tell tales on the sails and on the shrouds provides the guidance I utilize and of course just the feel of the speed and direction, especially at night when I don't desire to have my night vision impaired by any glow of instruments. Call me simple.
I could do it that way as well, but for that I'd need two boats
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