Woorabinda Lake - Stirling South Australia

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Rudder and Servo Setup - by Ben Morris

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Pre-built yachts will already have the rudder and servo set up ready to sail.  If you are building your own yacht or fitting out a new hull or replacing a servo or rudder then maybe a glance through these ideas may help.

A servo has a typical swing of about 110°.  This means it swings 55° either side of a neutral position.  The rudder does not need to swing through such a great angle probably only about 35° or so.  This is because the rudder will stall if it makes any angle of attack to the water flow of greater than about 12°.  See 'Getting the most from your rudder' for more detail.  But it is important to use the full 55° of movement of the servo as any only then will the maximum resolution be obtained.  To achieve these angles the ratio of the arms (really the ratio of the distance to the hole being used on the arm) on the servo and rudder should be in the ratio 55:35 or 11:7.  This means making the arms something like 22 mm and 14 mm respectively.  In order to make the rudder move equal angles either side of neutral then the connecting rod MUST be at right angles to the arms on the servo and rudder stock.  The simplest situation would be to make the rudder arm at right angles to the rudder and the servo at right angles to the servo at its centre position.  Notice that the centre of the servo then is offset 8mm from the centre line of the rudder!  If this simple arrangement is not possible then above everything else the rudder arm, the connecting rod and the servo arm MUST be at right angles to each other when the rudder is at the centre or neutral position. 

To set this up you need to do things in this order

  1. Decide on a servo arm and rudder arm length in the ratio 7:11 (in my example 14mm and 22mm).  Use holes or drill them at appropriate distances from the centre.  It is a good idea to have at least a spare hole inside and outside of your designated hole to make adjustments to the ratio later.

  2. Connect the rudder arm to the rudder and secure the rudder so that it is in line with the hull.  I will be setting up another page to discus options for connectors later.  In general the must have little or no slop and must be secure to the arm.

  3. Secure the rudder servo in the hull such that the connecting rod is at right angles to the rudder arm when it passes over the correct hole in the servo arm

  4. You need to turn on the radio and set the trim on the rudder to centre then adjust the servo arm to its correct angle by resetting it on the splined stock.  You can bet your life one spine will be too far one way and the next too far the other.  Chose the closest one.  If you have a programmable servo then adjust the arm to the correct position using the servo program.  If you have a computer radio then use the programmable sub trim to adjust the arm.  If you have none of these then leave it for the moment ie do not use your manual trim on the transmitter - leave it centred. 

  5. Place fittings on the rod to attach to the two arms and adjust its length using the screw adjustment on the rod until the rudder is back in its centre position

  6. View the rudder while moving the stick from side to side.  It should move equal angles to port and starboard.  DO MAKE SURE the movement is in the correct direction to turn the boat the correct way.  If wrong use the channel reversing switch or programming option to reverse its motion.  You may need to restart the process from point 4 again as many radios are not quite symmetrical in their motions when the channel is reversed.

  7. Test the boat in the water with no wind or sails loose by giving the boat a firm straight push and ensure it goes straight.  Adjust any slight movement to left or right with the programmable sub trim if you have a computer radio or with the programmable servo if you have one or by adjusting the length of the rod if you have neither.  You should aim to have a yacht that steers straight with the rudder trim centred.  That way you always know where it should be for straight steering.  If you have to move it a click or two to deal with weather helm etc on one tack and go to the other tack centre it first then add any minor trim needed.  This way you always have a reference point to return to.  Most radios these days give an audio beep when the trim is centred so its easy to find it.

  8. Should you find the rudder is turning too far or not far enough then the extra holes on your servo and rudder arms can compensate.  Move further out on the servo arm or closer in on the rudder arm to increase the movement. and vice versa to decrease movement.  In order to decrease the effect of any slop in the end connectors and to reduce the effect of any dead band in the servo it is better to use a longer arm on the rudder wherever possible.