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Sunday, April 06, 2008

Rudders, Steering, and Wide Angle Geometries

As mentioned in previous posts, we are planning to install transmission steering. One of the other advantages of transmission steering, (over pull-pull cable or hydraulic steering) is the mechanical advantage that can be achieved using wide angle geometry. Basically, the principle lies in the difference in lever arm lengths between the reduction gear box (right in the picture) and the rudder post tiller arm (left). The reduction gear tiller arm can swing through an arc of about 120 (60 degrees port and starboard) degrees. The typical 'useful' full swing of a rudder is about 72 (36 degrees port and starboard) degrees. By aligning the reduction gear, with the tiller, such that are linked in the "full swing" position, something mechanically interesting happens.
When the rudder is at 0 degrees, the mechanical advantage between the reduction gear and the tiller is about 1.5. As the tiller swings, the mechanical advantage progressively increases to nearly 3.0 (almost double) as the rudder approaches "hard over". This can be very useful if you are battling weather helm when underway.
The Jefa website has a more in depth discussion here.
What's more, if one wants the steering "constant", simply aligning the reduction gear, such that it's tiller arm is on center with the rudder post, will reduce the mechanical advantage to constant.
Or, aligning the gear somewhere in between will give you less of an advantage but more than constant at extreme rudder angles.
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