Wednesday, January 14, 2009

4th Generation Tiller Pilot to Wind Vane Linkage

There are not a lot of autopilot options for a 15 ton vessel with an externally hung rudder controlled by a tiller. The simplicity of the steering setup is it's appeal. The largest tiller pilot is rated for a boat of about half this weight. Autopilots for larger vessels are intended for installation under the deck where they can integrate with steering cables or arms attached to the rudder post.

The solution most often chosen for this problem is an attachment of a tiller pilot to a wind vane. This takes advantage of the wind vane's natural mechanical advantage design. The solution, like the wind vane itself, is rather Rube Goldberg-esque.

My first couple of attempts involved mounting the standard ball-pin on a custom vane blade on the wind vane. This worked about 95% of the time. It would tend to fail when the pilot ram would bounce off of the ball in rough weather with the vane blade was at an extreme angle. This is caused by the rough weather and unbalanced sail trim. It is difficult for a singlehander to adjust sail trim at the same time the selfsteering fails. The distance of the wind vane from the tiller could also make it difficult to re-attach the linkage without letting go of control of the boat completely.

My next design, I did not implement. I have seen pictures of vanes controlled by Morse Cables. This allows the pilot ram to be mounted under the deck where it and any electrical connections can be protected from the elements. The pilot's ram and the vane blade require 8 to 10 inches of travel. Standard cables used for shifting and throttle control only allow 3 or 4 inches of travel. Added travel in these applications is provided by levers at both ends of the linkage. I felt that the complication of these levers would be difficult build reliably on a one-off basis. I believe that the correct way to implement this general solution would be to by a autopilot model that controls an rotary drive steering cable. One end of the cable could be adapted to attach to the vane. Having already purchased a tiller pilot. This would be expensive. If my current design does not work, that will be my next attempt.

My current design utilizes two Morse Cable ball joints connected by a 1/4-28 allthread rod. Dynamic Marine Machining built me a new aluminum end for my tiller pilot ram that anchors one of the ball joints. The other ball joint is attached to a new vane blade built from hardwood mounted on a 3 X 3 X 1/4 inch aluminum angle stock. The ball joints offer a a large range of movement while locking so that the linkage cannot come off. The joints have a spring-loaded quick connect mechanism that allows disconnection when we want to use the wind vane in it's normal wind-driven mode.

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