A vane gear for Weaverbird

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  • 15 Sep 2017 18:37
    Reply # 5262583 on 4048415

    There's another significant difference between Weaverbird's system and the Hebridean that may be related. As far as I can see from your picture you have no way of tilting the vane away from the wind. The Hebridean has a clamp specifically for this purpose. You might be able to make it out in this picture https://flic.kr/p/vMdBWr. The Hebridean's vane is never vertical.

    Of course this doesn't alter the axis of the vane so it may be irrelevant. But I'd like to understand your decision.

  • 13 Sep 2017 23:37
    Reply # 5160683 on 5153480
    David Tyler wrote:
    The answer is simple. Build in a little bit of inclination of the vane axis. It doesn't have to be as much as on conventional servo pendulum gears, but it really ought to be there.
    A bit like mast rake. I can easily as some with a few washers on my Hebridean, but I don't seem to observe a problem so I'm not sure I'll see an effect.

    In any case I have a second reason to visit you now.




  • 13 Sep 2017 22:18
    Reply # 5153480 on 5151281
    Richard Brooksby wrote:

    No, but the tilt effect still applies somewhat on upwind courses. John Fleming mentions this in his plans. And that it's less satisfactory downwind.

    And there we have it. Yes, it still applies, though the trigonometry gets complicated - so long as there is weather helm and so long as the boat doesn't roll to windward. If there is lee helm, it doesn't apply. I've rolled a little to windward, in light airs and a bit of a chop, and the vane has immediately gone unstable. Yes, it's less satisfactory downwind, because the Hebridean's tilt of the whole servo and vane doesn't have so much effect on the vane's true angle of inclination as it does on other points of sail.

    The answer is simple. Build in a little bit of inclination of the vane axis. It doesn't have to be as much as on conventional servo pendulum gears, but it really ought to be there.

  • 13 Sep 2017 22:00
    Reply # 5151281 on 5147351
    David Tyler wrote:

    Richard, I think you have misunderstood.

    Very likely and thank you for taking the time to explain yourself.

    All those photos appear to show the vane axis pointing directly fore and aft.

    But you don't sail directly into the wind, do you?

    No, but the tilt effect still applies somewhat on upwind courses. John Fleming mentions this in his plans. And that it's less satisfactory downwind.

    This, in the end, is why vanes with a horizontal axis are unsatisfactory.  They will only work when they have an appreciable load to work against, so that they do not deflect too far during a roll to windward.
    Is that not provided by the pendulum and rudder damping in the water? When I crossed the North Sea with my Hebridean there was significant roll to my 20' boat from waves on the quarter, but the Hebridean kept an excellent course somehow. You can see it in my video “Trials of my Hebridean” (can't readily link while sailing sorry).

    In fact the drag of the pendulum in the water when the boat was rolled was beneficial, but that's probably another discussion.

    The Hebridean principle has one great  advantage over other servo pendulum gear geometries: that it greatly simplifies the primary linkage between vane and servo, and greatly simplifies the construction of the whole vane gear. It's true that the inclination of the power axis adds more negative feedback as the servo swings over, but all other pendulum gears do this anyway, by the way that the primary linkage is arranged relative to the power axis.

    Often with a right-angle gear or somesuch, if I understand correctly.


  • 13 Sep 2017 21:45
    Reply # 5149948 on 5128623
    David Tyler wrote:I do know that whenever my vane gets its axis "low end forward", it gets unstable and goes to its fullest travel in either direction.

    John Fleming probably advocates a neutral helm because the angle through which the servo will swing is much smaller than for a horizontal axis pendulum, and so the ability to apply large quantities of weather helm is not there.

    Now I'm quite puzzled. The Hebridean pendulum feathers to the direction of the boat as it swings precisely because it is angled and the pivot is angled. The vane also feathers and tilts because of the pivot angle. In fact that's what I assumed you meant by “the Hebridean principle” because it's so fundamental. What stops your vertical pendulum always swinging right out? The Hebridean will always stop at an angle corresponding to the angle of the vane to the frame because the angles provide negative feedback, and so it applies a tiller angle corresponding to the vane angle (after a useful damping delay).

    I will stare harder at your pictures :)



  • 13 Sep 2017 21:20
    Reply # 5147351 on 4048415

    Richard, I think you have misunderstood. All those photos appear to show the vane axis pointing directly fore and aft.

    But you don't sail directly into the wind, do you?

    Consider the least difficult scenario to understand. You are on a beam reach, and the vane axis is pointing directly athwartships. Now a wave rolls underneath the boat. On the face of the wave, she rolls to leeward, and as it passes, she rolls to windward. And so the angle of roll needs to be either added or subtracted from the angle of inclination when the boat is level. In one sense, they reinforce each other and apply more negative feedback. In the other sense, they tend to cancel each other. If the angle of inclination is zero to start with, then a roll to windward must result in an overall negative angle of inclination - and the vane becomes unstable. If there is some positive angle of incidence on the the vane axis, then the vane will only become unstable when this angle is surpassed by the angle of roll to windward. This is the simplest case. On other points of sail, the same principle applies, but we would need to get tangled up in some trigonometry to work out the actual angles.

    This, in the end, is why vanes with a horizontal axis are unsatisfactory.  They will only work when they have an appreciable load to work against, so that they do not deflect too far during a roll to windward. And this is one of the reasons why they are totally unsatisfactory for driving a well balanced control surface. The Hebridean vane gear is perhaps more prone to this instability, but it will show up in all cases where there is a horizontal vane axis, to a greater or lesser extent. 

    The Hebridean principle has one great  advantage over other servo pendulum gear geometries: that it greatly simplifies the primary linkage between vane and servo, and greatly simplifies the construction of the whole vane gear. It's true that the inclination of the power axis adds more negative feedback as the servo swings over, but all other pendulum gears do this anyway, by the way that the primary linkage is arranged relative to the power axis.

  • 13 Sep 2017 20:00
    Reply # 5139060 on 5125689
    Richard Brooksby wrote:
    David Tyler wrote:

    I've learned something more over the past few days: that a vane gear using the Hebridean principle must always be working so as to counteract weather helm - or must be fooled into thinking that it is.

    If there is weather helm, the servo swings out to weather, and so the the vane turret tilts to leeward, thus putting some positive inclination onto the vane axis.

    If there is lee helm, the servo swings to leeward and the vane turret tilts to windward, thus putting some negative inclination onto the vane axis, resulting in unstable steering.

    Are you sure this is true of the original Hebridean design? I've not noticed it on mine, but I do usually have sine weather helm. John Fleming claims the Hebridean wants a neutral helm.

    Right, here are some pictures of the vane turret on my Hebridean in roughly central position and pulled over. https://www.flickr.com/photos/rptb1/shares/0g6BHX. Note the turret tilts forward when the trunk is rotated in either direction.

    I may have misunderstood you completely!

    In haste — setting sail!

  • 13 Sep 2017 18:24
    Reply # 5128623 on 4048415

    In reply, I can only quote the late lamented Professor C E M Joad:

    "It all depends..."

    ... on the amount of balance on the servo blade

    ... on the amount of balance on the rudder

    ... whether there is a skeg

    And doubtless some other variables. I don't suppose there's a simple answer to this. I do know that whenever my vane gets its axis "low end forward", it gets unstable and goes to its fullest travel in either direction.

    John Fleming probably advocates a neutral helm because the angle through which the servo will swing is much smaller than for a horizontal axis pendulum, and so the ability to apply large quantities of weather helm is not there.

    Last modified: 13 Sep 2017 18:27 | David
  • 13 Sep 2017 18:05
    Reply # 5127013 on 4878186
    Arne Kverneland wrote:

    David,

    So, in other words, the windvane gets positive feedback with a lee helm? I start to wonder if it would be better to separate the vane turret from the servo blade. With the vane turret fitted in the traditional way, fixed to the boat, you could still use the same servo blade with the 45° power axis. This axis will always produce negative feedback, and if you in addition tilt the vane axis a bit aft (preferably with an adjustable tilt angle), you should have a more stable system. That tilted power axis on the servo blade still appears to be a very good idea to me.

    Arne

    I have wondered the same thing, and yet John Fleming says the connection is important to the negative feedback. I suspect the angle of the “extension” (that holds the vane) to the pivot attachment is quite critical to this. I'll take a closer look when in aboard.

    I do like the look of David's design. I've spent many hours thinking about how to simplify the Hebridean (especially the linkage) and I think the ideas are great. I was thinking about low friction bicycle brake cables, but I'm not sure how long these would work at sea. The drum-and-cable approach is very neat.

     
  • 13 Sep 2017 17:52
    Reply # 5125689 on 4876186
    David Tyler wrote:

    I've learned something more over the past few days: that a vane gear using the Hebridean principle must always be working so as to counteract weather helm - or must be fooled into thinking that it is.

    If there is weather helm, the servo swings out to weather, and so the the vane turret tilts to leeward, thus putting some positive inclination onto the vane axis.

    If there is lee helm, the servo swings to leeward and the vane turret tilts to windward, thus putting some negative inclination onto the vane axis, resulting in unstable steering.

    Are you sure this is true of the original Hebridean design? I've not noticed it on mine, but I do usually have sine weather helm. John Fleming claims the Hebridean wants a neutral helm.

    Weaverbird's helm is neutral, when sailing to windward in light conditions. Sometimes there is lee helm, and the steering goes haywire if I have the steering lines to the tiller at equal length. But if I shorten the lee side line, this induces a kind of artificial weather helm, and the steering becomes stable again.

    I have a chain across my tiller that sits on a pin, allowing me to compensate for the helm while keeping the vane central. The pin often ends up at the mirror image position on the chain after a tack. See https://flic.kr/p/xR1Tuj

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