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Thread: True Wind

  1. #41
    Join Date
    Dec 2003
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    3,880

    Default Re: True Wind

    Quote Originally Posted by noelex View Post
    I am no expert, but I think modern autopilots when steering via true wind or even apparant wind don't just use wind data like say a windvane would. They are integrating heading and rate of turn information into the mix.
    Indeed, and what I alluded to in post #20. The trick is to combine wind sensor and STW data with the vessel motion data output by the rate sensor. The so-called 9-axis systems basically monitor the x,y,z spacial vectors to orientate the compass, also the first derivative of this (i..e rate of change in any direction) and the second derivative (i.e. the acceleration in any direction) The benefits of this are numerous:

    1. A much more accurate determination of TWS and TWA

    2: The AP can potentially learn 'a sea' if the software is up to it (big strides being made here)

    3. The AP knows not only that it has gone off track (in relation to the AWA, TWA, or Compass Heading it is directed to follow); it knows how fast it is going off track in each of the three axes and knows whether the vessel is accelerating further off track in any of these dimensions, or decelerating down to a steady rate of turn. Finally, the AP knows the plane the rudder will work in at all times.

    Put it all together: a boat sailing downhill on STB tack is lifted by a big wave, the boat heels to port, steers to port and the STW increases sharply.

    Tthe AP 'thinks': ..."Crikey that was a big-un and I'm turning fast to port! Okay rudder hard to STB, no even harder, but not too much, while the boat is still moving off track the rate of turn is slowing, ...okay coming back on track and I've learnt that the boat is going to swerve to STB as it heels to windward, so I'll straighten-up a bit and monitor............"

    Nobody is quite yet there but strides are being made all the time. Crack this nut and a rock stable steer-to-TWA downhill capability without risk of gybing/broaching becomes possible . From a racing perspective it also helps to identify shifts more quickly and more accurately.

    Not an expert on the extent civilian GPS could fill part of this space -- is 50Hz now available? -- but I've heard that noise goes up as the refresh rate increases and that numerous hard barriers exist in practice. Happy to be corrected on this.
    Last edited by dom; 18-07-17 at 13:57.

  2. #42
    Join Date
    Apr 2017
    Posts
    26

    Default Re: True Wind

    Quote Originally Posted by lw395 View Post
    I don't think gps update rate is the only issue.
    To steer to true wind, you need AWA, AWS and STW.
    To steer to ground wind, you need AWA, AWS, SOG, COG and compass heading.
    All those quantities are subject to delays and errors.

    How much that matters will depend on what demands you are making on the autopilot.
    The greater the delays and errors in the loop, the harder it has to work to hold a course.

    Could you explain why compass heading is needed above for if using COG rather than STW ? I can't get my head to understand ! Thanks... ps does that mean that without compass heading GWA (ie angle of ground wind to bow) is inaccurate ?

  3. #43
    Join Date
    May 2007
    Posts
    26,798

    Default Re: True Wind

    Quote Originally Posted by Yealm View Post
    Could you explain why compass heading is needed above for if using COG rather than STW ? I can't get my head to understand ! Thanks... ps does that mean that without compass heading GWA (ie angle of ground wind to bow) is inaccurate ?
    To keep the boat pointing relative to the ground wind, you need to know where the boat is pointing.
    That's different from COG due to tide and leeway.

  4. #44
    Join Date
    Apr 2017
    Posts
    26

    Default Re: True Wind

    But my boat doesn't have a compass heading. Tackticks merely substitutes COG for STW. Does this means that the displayed (ground) wind speed and angle relative to the boat bow are not correct ?

  5. #45
    Join Date
    Dec 2003
    Posts
    3,880

    Default Re: True Wind

    Quote Originally Posted by Yealm View Post
    But my boat doesn't have a compass heading. Tackticks merely substitutes COG for STW. Does this means that the displayed (ground) wind speed and angle relative to the boat bow are not correct ?
    Okay, I don't know the Tacktick instruments, but let's go with that. Now imagine such a system sailing in a light/medium wind, on either a heavy rolling, or pitchy/surfy sea. The rate sensor will be located as close to the boat's fulcrum of motion as poss, but waves are 3-D animals, so just imagine the error terms set up within such a system if it can't distinguish between COG and HDG, between STW and SOG, and only a very short time-averaging (i.e. high sensitivity) is employed. Moreover, the problem will not be solved - poss made worse - by increasing to a say a powerful 20Hz GPS system.

    The sea off Cherbourg tomorrow is forecast to have a height of c.2m and period of 5-8s (0.2-0.12Hz); the wind is forecast to be c.15kts. Now consider how a system could accurately resolve that little lot without knowing either STW, or HDG.
    Last edited by dom; 19-07-17 at 06:55.

  6. #46
    Join Date
    May 2007
    Posts
    26,798

    Default Re: True Wind

    Without a heading sensor, your gps does not know whether your boat is going nicely forwards or going sidewise in the tide, or being carried sidewise down a wave.

    This may not be an issue for people using low power autopilots in benign conditions.
    But is it any better than the old-school autohelm steering a compass course?

    Alex Thomson has different needs from your cruising yachtsman who just wants to put the kettle on.

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