I'm inclined to think that the scalloping downwind is due to a combination of Fantail's behaviour and the design of the servo. Once Fantail begins yawing, the servo blade swings to the wrong side to correct this. I'm guessing that the reason is because the angle of attack of the servo blade, which is set by the combination of the vane angle and feedback from the rudder, overpowers the servo blade's tendency to swing due to the yawing action of the hull. My guess is that the motion of the tiller feeds back through servo, which is resisted by the vane, resulting in the pitch of the servo blade being set in the wrong direction. All this happening in concert with the correct action of the system makes it hard to analyse.

Annie, has the vane ever operated successfully on a run? To my untrained eye it looks as if the servo tiller needs to be above the rotating axis of the servo in order to prevent the tiller feeding the servo blade pitch incorrectly when running. I'd like to be wrong about this as it would be a major redesign to correct. 

I'd really like to hear from David and anyone else who has ideas...

As for the cloth being taut - well temperature affects it a bit, but being nylon and about three years old, I fear that if Bryan so much as took a firm grip on it, his fingers would go right through!

David Tyler wrote:

The point of the 45 degree power axis is that the self-centring action of the servo is absolutely yuge, to the point where, if something is managing to override its strong urge to stay central, that something must also be yuge. 

Try this diagnostic test. Disconnect the primary linkage lines that connect the vane to the servo, and connect the servo to the tiller. Sail downwind. The boat should very slowly alter course one way or the other, as the servo is sensing a yaw and trying to damp it, but the vane is not there to apply a correction. If this is satisfactory, steer the boat using the pressure of one finger on the little tiller within the servo. It should be possible to push that tiller a little, resulting in a stable course correction, and then when it is released, it should slowly, but stably, return to a central position. 

You say that there is a rhythmic oscillation. First, are you sure that there's not just a wildly fluctuating apparent wind due to sailing downwind fast? Second, are you sure that all moving parts are exhibiting a mildly self-centring action? That is, that the vane is not top-heavy (even when wet); the servo blade is not over-balanced; the centre of gravity of the servo blade and its tiller are just forward of the axis around which it rotates (the tiller should have a lead weight on it); the vertical servo shaft is moving very freely (grease it); Fantail's main rudder is not stiff or over-balanced; and anything else of this sort. Any of those factors might be to blame.

It's very difficult to try to diagnose this problem without being on the spot. Would you be willing to buy me a first class air ticket to NZ? Failing that, could you take a photo of the gear from side on, in its current state, and take a video of the gear when this kind of action is actually in progress?

Right, tests done. Yugeness follows:

With the vane to servo lines disconnected, the servo can sort-of lock the tiller in place. In the somewhat fluky offshore wind just south of Cape Brett, it's a bit much for the servo to keep a small sway under control, but it will more or less tame Fantail's need for weather helm once she is not dead downwind. Starting from a pure run though, Fantail will soon round up. If the servo is set to provide a small amount of helm, it will hold a less-than -pure downwind course for a while. Fantail can be steered by the oar tiller... the pressure is 'moderate', and considerably more than is required to turn the oar tiller with the rudder tiller disconnected.

I'd love to blame Fantail's antifoul-blistering speed downwind, but sadly no... not today. There's a little slop in the vane bearings... she deserves better blocks... but I can't help feeling that feedback is the issue.

I've uploaded a video of what's going on - link to follow. I can't afford your air ticket but I could scoot over in my wee two seater and pick you up...

https://www.youtube.com/watch?v=n_Fw_BRWPo4

Side on photo to come...

(Note that the miserable state of lines and poor adjustment in the video is because I'd just retied the lines by leaning over the transom after the the previous test. The behaviour is just as bad if everything is tweaked to perfection.)

David Tyler wrote:

Bryan Tuffnell wrote:

I wonder if there's some instability caused by the turning of the boat driving the servo blade - have you seen this happening? If the boat is yawing, the servo blade - being outboard will 'feel' that yaw and might rotate in response, further driving the helm. ?

The servo blade, feeling a yaw, tries to work against it and damp it. That's the very big plus that the servo pendulum has over all other types of control surface. Ironically, though, if the servo blade is overbalanced, this effect will be increased. The problem is that it will over-correct. An overbalanced servo blade is trying to throw itself right out to the end of its stroke, and then as the vane finally gets enough incidence on, and manages to turn it, right out to the other end of its stroke.

What I'm wondering is that while the oar itself feels yaw and responds in the correct sense to damp it, the pitch axis of the oar is fed in the incorrect sense by the rudder during yaw for a constant vane angle. It's a real challenge to figure out which dominates - a bit like trying to decide which rider on a tandem bicycle is pedaling harder.

If the above is the case, the solution would appear to be relocating the oar pitch tiller above the oar's axis of rotation. 

I may be missing the point of the 45 degree angle of the oar rotation axis. The plane of the course setting axis, which sets this angle exactly, means that as the oar swings away from vertical, it arcs forward somewhat (to the extent that it can clout the transom).

Is AskDavid open for business? 

Earlier in this thread I have expressed my preference for windvanes over sheet to tiller self steering. Besides, for my modest sort of sailing, I have made do with a simple tiller lock.

However, after having read John Letcher’s book (..thanks, Annie...), in particular chapter 3, on ways of hooking the sheet up to the tiller, I am about to be converted  -  or at least very curious. On my Ingeborg, the sheet and tiller layout happens to be very well suited for sheet to tiller control. It will not cost much in work or money to try it.

I still regard the windvanes  as the best for long distance cruising, since they can hold a course even straight downwind. Still, if the windvane breaks down, a sheet to tiller arrangement could be a useful backup.

Let’s hope the winter will be short  -  I want to try this!

Cheers, Arne

PS: This topic has now found its place among my favourites...