A vane gear for Weaverbird

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  • 03 Jun 2017 15:07
    Reply # 4878328 on 4048415

    Arne,

    Then I would be back to almost exactly the vane gears that I designed for Fantail and Tystie. Both were OK, though Bryan has reported problems with Fantail's gear that I cannot diagnose from a distance. The 45° power axis brings with it a greatly reduced arc of operation, which the vane mounting on this new gear has improved again, so I wouldn't want to take a step backwards.

    No, this gear has the potential to be better than either, but like all new pieces of machinery, there are things to be learned as I go along, and things to optimise. 

    To alter the gear as you suggest would mean a complete redesign and rebuild. Before doing that, a simple modification of the vane turret is the obvious first step and could be all that's needed.

    As I say, this is an issue on a boat with a light and neutral helm, and almost certainly does not occur on most boats.

    Last modified: 03 Jun 2017 15:11 | David
  • 03 Jun 2017 11:42
    Reply # 4878186 on 4048415
    Anonymous member (Administrator)

    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


  • 02 Jun 2017 10:01
    Reply # 4876186 on 4048415

    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.

    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.

    Is there a better solution? I'm beginning to think that I should rework the vane turret to add some positive inclination to the vane axis under all points of sail, as servo pendulum gears usually have.

    That said, when the gear is working well, it is certainly giving me the best vane steering that I've ever experienced, on a boat type that has always been the most difficult to steer with a vane gear - that is, with a partially balanced spade rudder.

  • 26 May 2017 16:14
    Reply # 4854352 on 4048415

    Ah.  That actually makes perfect sense. I was completely leaving effects of acceleration out of the equation.  On the ethereal frictionless plane, the gravitational acceleration and inclination would exactly cancel each other out, I believe, and the vane would not move one inch. The ethereal apparent wind would change, though - another factor I completely didn't think about.  Once all those dynamic effects come into play, it's pretty clear the solution is to become engineers and not physicists:  do what David's doing and make one. Trial it, tweak it.

    And after studying the photos in your album, like Arne, I finally get how the thing works.  That was not an intuitive process.  I had to make a few small models out of crudely folded paper.  I have tough time picturing movement in three directions: the z-axis effects on the other two overwhelms my bio-neurolical video graphics card's capabilities.

    Last modified: 26 May 2017 16:14 | Anonymous member
  • 25 May 2017 22:02
    Reply # 4853178 on 4853072
    Scott Dufour wrote:

    David,

    In heavy following seas, surely the boat inclines more than 4 degrees fore and aft.  Is this not the inclination you're concerned about?

    This is an exceedingly complex dynamic  situation. As a wave passes, there are at least pitch, roll, yaw, heave and acceleration affecting the apparent wind that the vane sees. So there's not going to be one correct angle of inclination of the vane axis. We can say that when on the face of a wave, the inclination will be increased, but acceleration will have decreased the apparent wind speed, and who knows what the roll is doing? And, of course, on the back of a wave, the reverse is true - inclination probably going negative and deceleration probably increasing the apparent wind. This, travelling fast in a seaway, is the most taxing situation for a vane gear. This is where the servo pendulum scores, as it has inbuilt yaw damping which tends to ignore the vague and variable signals the the vane is sending, in the light apparent wind that is constantly changing due to yaw and roll. The strongly trailing action of the Hebridean is the most effective of all in this. So what we're asking from the vane is that it shouldn't send strong, false signals to the pendulum, tending to override the yaw damping action. And the best way to do this is with somewhat more inclination of the vane axis than is necessary on other points of sail. Does that make any sense?
  • 25 May 2017 20:19
    Reply # 4853072 on 4842113
    David Tyler wrote:

    One thing I've been finding is that the vane axis must remain either horizontal, or inclined with its forward end higher at all times, or it becomes unstable. This is fine, with the vane axis horizontal and parallel to the base on which the vane turret is mounted, so long as that base remains truly horizontal. When sailing to windward or on a beam reach, the angle of heel will supply a little positive inclination. On a run, it doesn't. If the whole gear droops down aft, even if only by a little, the vane axis inclination goes negative and the stability of the steering is adversely affected. So my current thinking is to build in a little insurance by inclining the vane axis at say 2°, and then also raking the whole gear forward by 2°. This would give 4° of inclination when running, reducing to 0° when head to wind.

    David,

    In heavy following seas, surely the boat inclines more than 4 degrees fore and aft.  Is this not the inclination you're concerned about?

  • 20 May 2017 15:54
    Reply # 4843479 on 4048415
    Anonymous member (Administrator)

    David,

    I think it is to ask for a lot from a self-steering gear to control a boat which is not course stable in the first place. My limited experience with windvanes was mostly on my 23’ Malena. She had a big skeg in front of her rudder and was definitely course stable. As a result, my Belcher vane, Otto steered her straight as an arrow right downwind with mainsail and jib to either side. Another boat I sailed was a Maxi 95 with a (Wind Pilot?) windvane driving its own aux. rudder. With the main rudder locked, this worked very well even downwind in high seas.

    In case a boat is directionally unstable, I would first correct this, instead of trying to build God into the windvane. On Weaverbird, with that powerful wind vane system already operating, I would simply add an auxiliary rudder on the transom. This rudder would normally be locked on the CL, or offset a bit to trim out any weather helm. It would thus act as a steering feather or skeg, making your bird want to fly straight.

    As a bonus you get a spare rudder in hot standby in case the deeper main rudder gets knocked off. Many mayday calls could have been avoided if yachts were fitted with an easy to use spare rudder.

    Arne



    Malena with the Belcher type OGT Mk II windvane ("Otto"). No feedback remedies needed on that course-stable boat.

    Last modified: 21 May 2017 08:34 | Anonymous member (Administrator)
  • 20 May 2017 11:47
    Reply # 4843300 on 4048415

    Only just directionally stable. She won't hold a course for more than a few seconds when sailing, and flicks into a turn under power, if I let go the tiller.

  • 20 May 2017 11:42
    Reply # 4843298 on 4048415
    Anonymous member (Administrator)

    Just one more question before heading on to the harbour:

    Is the Dueta directionally stable in the first place, or will she flick into a sharp turn if one lets the tiller go?

    Arne

    PS: I just found the string of photos of the vane in your album, which lifted much of my mental fog....

    Last modified: 20 May 2017 11:44 | Anonymous member (Administrator)
  • 20 May 2017 11:10
    Reply # 4843273 on 4843249
    David Tyler wrote:

    Arne,

    3) Right. The blade is antifouled and remains in the water. I have seen no Ill effects in previous gears. A bolt-on or clamp-on blade could be made, at the expense of more complication in the construction.

    I should say that I arranged the inclined set of pintles and gudgeons such that the gear is captive. Equally, they could be arranged such the the gear is shipped and unshipped like a dinghy rudder, with a split pin through a pintle for security.
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