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Thursday, February 21, 2008

Steering Design Updated

One of our stated goals for the Westsail this year is to get the steering mechanism fitted. We are going with Jefa's transmission steering system. Different than typical cable pull-pull and hydraulic systems, this uses a series of torque transmission tubes and bevel/reduction gears to transmit force from the wheel to the rudder. Advantages: easy to install, compact, low maintenance, sensitive and accurate rudder feedback. Disadvantages: not cheap.
One of the design problems presented with this type of steering is arriving at a good angle geometry. Ideally the torque tubes and the input shafts they connect to on the gear boxes would be along the same line in the same planes. But of course this is ideal. The joints at the end of each torque tube allow the tube to be "off" up to a certain angle. Beyond that, the joints will exhibit a sort of "cam flex" action and put a bit of stress on the hardware which you might feel at the wheel. Since we arent talking high speed rotations here, its probably not that big of a deal. But, if it can be avoided, all the better.
With the Westsail, the challenge comes with the skeg hung rudder that is canted 18 degrees aft. With the torque tube running directly under the floor, paralell, the final 18 degrees must be dispersed over the aft bevel gear, the bevel box, and the drag link.
The solution is to rotate the bevel box nine degrees (half of 18) and mount it at a height such that the angles are evenly split between the drag link and the final torque tube. I suspect this will allow for smooth steering rotation.
Also, note that the side view drawing suggests a tube running directly down under the pedestal. This is not the case. The top view, not shown, has the tube angling off to starboard. This is necessary to clear the engine. When fitted, we may have to move the forward bevel box aft against the primary bulkhead. One cannot tell till we actually dryfit the system.
Still left to design is choice of reduction gears and the emergency tiller fittings.

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