Bryan Tuffnell wrote:Thanks David and Annie. Unfortunately I've had to return to Christchurch to deal with post-earthquake issues, but I did number of tests before returning:
- The oar has stabiltiy; it trails centrally with the vane removed. If disturbed from its stable, central position it will gently but positively return to centre.
As well as removing the vane, did you also untie the primary linkage, so that the lead weight is not centralising the servo blade?
- The boat can be readily steered with very light pressures by rotating the vane hub by hand with the vane removed.
- The vane is also stable. With the the lines to the oar removed it also returns to an upright position when disturbed.
- There is some motion in the blocks on the vane-to-oar lines, but it's minimal.
- Wanda-the-windvane only wanders when the wind is aft of beam. She does this with the lines as tight as seems reasonable. The degree of wandering depends on conditions, but say <10 degrees on a beam reach to perhaps 50 degrees on a run.
- When running, once the vane begins to tumble the tiller begins to move and the boat begins to turn... the vane tumbles further, the tiller moves further, the boat turns further... the vane reaches its maximum tilt and begins to return to upright; the tiller reaches maximum throw and begins to return; the boat begins to straighten... the vane, tiller and boat pass through their 'centres' with little if any stability, and the process repeats on the opposite tack.
- The system is beautifully stable when going to windward.
It occurs to me that if the boat yaws, the oar may sense this and may respond by moving. If any yaw-induced motion is detected by the self-steering it will be in a co-operative sense as far as corrections are concerned if the wind is forward of the beam and contrary when the wind is aft. If that makes any sense?
Most boats and their steering form stable systems when going to windward, as witnessed by the fact that just lashing the helm is enough to hold a course - until the wind changes in strength. What you're describing when off the wind, is classic rhythmic oversteer, because the boat and steering are unstable, just like a weight suspended on a spring oscillating, and continuing to oscillate, because there is little damping supplied by the air it's in. If you've eliminated friction, and backlash/lost motion, then the further factors to consider are inertia (we can't do much to improve things there, the vane is already pretty light), getting the period of oscillation of the boat out of sync with the period of oscillation of the vane gear, and avoiding over-rotating the balanced spade rudder.
If you're certain that the servo blade is not overbalanced (I'm not - see my note above), the next thing you could try is to move the steering ropes forward along the tiller, so that you get less helm angle as the pendulum swings. This will address the last two items, and is what we had to do on Blondie's Pilmer to get the Hasler SP pendulum gear to function without oversteering - balanced spade rudder again. You could add more balance weight to the vane, but this would make the gear dull in light airs, and would be a bad "fix".
The geometry of pendulum servo that you have, with its power axis at 45 degrees, is about as good as you can get for coping with this kind of rudder. I'm surprised that you're getting oversteer, and and although my memory is hazy, I don't think the vane gear did it initially - something's changed. We just have to establish what it is.