Setting Up Rudder Servos and Linkages on a Model Boat

What a good rudder setup should do

The rudder servo turns your steering stick into rudder movement. A good installation:

• Has the rudder straight when the servo is centred, with little or no trim needed.
• Moves freely through its full travel, with no binding or tight spots.
• Has little slop, so the rudder does not wander or flutter.
• Gives enough rudder angle to turn the boat, without driving the rudder or servo against its stops.
• Keeps the servo and rudder tube dry.

Most of this comes down to where the servo sits, the type of linkage you use, and the geometry of the arms. Steering is one part of the wider installation covered in our radio control setup guide for model boats.

Choosing a rudder servo

For most scale models and small to medium sport boats, a standard-size servo is the usual choice. Small plastic kit conversions may need a mini or micro servo to fit. Things to consider:

• Torque. Larger rudders, faster boats and boats with big props directly ahead of the rudder put more load on the servo. A heavier hull turning hard at speed needs more torque than a slow tug. If in doubt, choose a servo with some margin above what you think you need.
• Water resistance. The rudder servo often sits right at the stern, where water collects. Waterproof or water-resistant servos are worth considering there. Labels vary, so treat “water-resistant” as splash-resistant at best.
• Gears. Metal-geared servos cope better with knocks transmitted back through the rudder, for example when the boat hits the bank or debris. Plastic gears are fine for many lightly loaded scale models.
• Voltage. Check the servo is rated for the voltage your ESC’s BEC or receiver battery supplies.

Sailing yachts are a slightly different case: the rudder servo is usually small, but the sail winch is a separate, more powerful unit. See our guide to RC model sailing yachts.

Mounting the servo

Mount the servo on a servo tray or bearer — a small plywood or plastic plate glued or screwed into the hull — rather than directly to a thin hull skin. Points to keep in mind:

• Position it to give a straight linkage. The pushrod or cables should run as directly as possible from the servo arm to the rudder tiller. Long, curved runs add friction and slop.
• Keep it as high as practical, above the bilge water that tends to gather at the stern.
• Use the rubber grommets supplied with the servo, and tighten the screws until snug, not crushed. The grommets help isolate vibration.
• Make it removable. Servos fail eventually. Screws or a removable tray make replacement much easier than a servo glued in place.

Double-sided servo tape or hook-and-loop strips are sometimes used in small models. If you do this, clean and degrease both surfaces first, because tape on a damp or oily surface can let go under load.

Linkage options

Pushrod and clevis

The most common arrangement is a stiff wire or rod pushrod between the servo arm and a tiller arm on the rudder post. Typically:

• One end has a Z-bend or a clevis at the servo arm.
• The other end has an adjustable clevis or ball link at the tiller, threaded onto the rod so you can fine-tune length.
• A short length of fuel tube or heat-shrink slid over a clevis stops it springing open.

Use rod stiff enough not to bow when it pushes. On long runs, a pushrod in an outer guide tube (sometimes called a snake) keeps it from flexing. Plain steel wire can rust in a damp hull; stainless rod or carbon with threaded ends is an alternative.

Pull-pull cables

In a pull-pull setup, two cables run from each side of a double-ended servo arm to each side of a double-ended tiller. The servo always pulls one cable rather than pushing a rod, so there is no risk of the rod bowing. This suits long runs and installations where a straight pushrod path is hard to find. The cables need to be tensioned evenly, enough to remove slack without loading the servo, and the two arms should be the same width so one cable does not go slack as the rudder moves.

Other arrangements

• Direct tiller. In a small hull where the servo can sit right beside the rudder post, a very short link or even a slot-and-pin connection can work well.
• Quadrant or tiller with drag link. Some scale models replicate the real ship’s steering gear for appearance. Make sure the scale parts are strong enough for the load or hide a working linkage underneath.
• Flexible cable in a sleeve. Useful where the path must bend, though it adds friction and some slop.

Getting the geometry right

Centring mechanically first

With the transmitter and receiver on, all trims and sub-trims set to zero, and the servo connected to the steering channel:

1. Fit the servo arm so it is as close to square to the pushrod as the spline allows. Try rotating the arm a spline or two, or turning it over, to get closer.
2. Fit the tiller arm on the rudder post at right angles to the rudder blade (or as near as the design allows), and lock it in place.
3. Adjust the pushrod length with the clevis until the rudder is straight.

Only then use sub-trim for any small remaining correction. Sub-trim works best for small adjustments; using it to correct a badly centred arm can make travel uneven between the two sides.

Arm lengths and rudder throw

The amount the rudder moves depends on the ratio between the servo arm and the tiller arm:

If the servo arm and tiller arm are the same effective length, and both are square to the pushrod at centre, the rudder moves through roughly the same angle as the servo. For an even response each side of centre, keep the two arms parallel at centre.

As a starting point, aim to get roughly the throw you want mechanically, then fine-tune with the transmitter’s end points. A linkage set up for far too much throw and then cut back heavily in the transmitter uses only part of the servo’s travel, which loses resolution and gives the servo less leverage than it could have.

How much rudder movement you need

Many modellers set the rudder to swing somewhere around 30 to 35 degrees each side of centre. Beyond that, a rudder tends to stall and act more as a brake than a steering surface, so extra angle often does not tighten the turn. Fast boats usually need less movement than slow scale models; very slow displacement hulls and boats steering behind a big prop sometimes benefit from a bit more. Try it on the water and adjust.

Twin and multiple rudders

Many twin-screw ships have two rudders, one behind each prop, including all-weather lifeboats such as the Severn class. The usual way to drive them is with one servo and a tie bar linking the two tiller arms:

• Fit identical tiller arms to both rudder posts, at the same angle and the same distance from the post.
• Link them with a rigid bar that is the same length as the distance between the rudder posts, so the tiller arms and bar form a parallelogram. Both rudders then stay parallel as they turn.
• Drive the bar, or one tiller arm, from the servo.
• Set each rudder straight by adjusting the tie bar ends before connecting the servo.

Models with three rudders use the same idea with a longer tie bar. Some builders use a separate servo for each rudder and mix them in the transmitter; this works but adds parts and is harder to set up evenly.

If the model has twin motors, differential throttle mixing can tighten turns at low speed; this is covered in our RC setup guide.

Transmitter settings for steering

• Servo reverse — if the rudder turns the wrong way, reverse the steering channel rather than rebuilding the linkage.
• End points (EPA / travel adjust) — limit the rudder so it stops just short of any mechanical stop. If the servo buzzes or strains at full stick, reduce the end point on that side until it is quiet.
• Sub-trim — small final centring only.
• Expo — softens the steering around centre, which can help fast boats that feel twitchy.
• Failsafe — decide what you want the rudder to do if the signal is lost. Many modellers set it to centre or hold; the throttle channel’s failsafe (motor stop) matters most.

Sealing the rudder tube

The rudder post runs through a tube in the hull, and it can leak just like a stern tube:

• Where possible, have the top of the rudder tube above the waterline.
• Bond the tube into the hull with epoxy and fillet the joint inside.
• Pack the tube with waterproof grease, which reduces water creeping up the post and makes the rudder move smoothly.
• Fit a collar above the tube to stop the rudder dropping out.

The same methods used for the prop shaft apply here; see our guide to fitting and sealing a stern tube for bonding, greasing and leak testing.

Testing and troubleshooting

Before the first run, operate the steering on the bench for a minute or two. Listen for buzzing at the ends of travel and watch for any tight spots. With the transmitter off, move the rudder by hand: it should feel firm but move smoothly when the servo is not powered.

On the water, start slowly and check the boat turns equally both ways and runs straight with the stick centred. A little trim to offset prop torque is normal on single-screw models. Note the final pushrod length and arm holes once you are happy, which makes replacing a servo later much quicker.