Seafarer Mini-Seacourse

I have been asked a few questions about this venerable product, being the proud owner of one that came from a boat jumble a few years back and so I thought that I would create a webpage about it. This is the beginning...

Basically, if you have one and it works, use it . Dont throw it out until you have tried it.
It will outlast many other more flashy units which get seawater in and die.
If it nearly works, you should be able to get it fixed fairly cheaply by the 'radio ham' type of character :-)


The weak point of this product is the plastic end of the actuator. Intended to break before the rest breaks but no longer replaceable. I had to get one turned out of brass bar by a friend to replace the one that was broken off when I bought it.

Internally the actuating part is via a nice lead screw attached to a geared motor . The motor is a really good quality one and it makes  'robot' noises when it is moving. No belts or channels full of ball bearings here . The down side is that if it hits the end stop it tends to jam slightly and you often have to wait for it to unstick, by adjusting the course knob to encourage it to move the actuator the other way.

Excess water drains out through holes in the bottom. The electronics board is clamped to the motor in mid -air  so there is less possibility of water damage. The control knob protects the hole in the top of the case.


These units are basically a textbook example of how to make an autopilot with the least possible electronic components.
It features overload protection by measuring the motor current .  It ticks if the motor is stalled. Each tick is an attempt to restart the motor.
All of the electronic components apart from the sensor are still available and can be replaced with equivalent parts. No microprocessor here.

The compass unit is a magnetic compass in an  alcohol-filled bulb with a yin and yang shaped shutter that is attached to the needle. The yin and yang obstruct beams of light from LEDs to photocells.
The idea is that the compass unit is rotated until it points north at which point the actuator will stop, with yin and yang equally obstructing the light beam. Turn one way or the other and one of the light beams gets more obstructed and the other gets less obstructed. Electronics detects this and decides the direction of movement of the actuator.

For fans of control systems this is a bang-bang system with a deadzone. Integral feedback is achieved by turning the compass unit with a bit of string attached to the shaft driving the actuator as the actuator moves.

The weakness here is the string slips and the electrical contact from the compass gets dirty.

String can be replaced and the contacts cleaned.


As far as I can tell the best installation is with the unit level with the boat at rest and attached at right angles to the tiller, 50 cm from the pivot point of the tiller on the starboard side.of the boat.

The pin on the base is slightly larger than normal Autohelm - I had to drill out a socket. The socket will engage with the Autohelm bracket on the tiller.

Because of the non-gimballed compass do not expect the unit to work above about 20 degrees of heel. At that point the compass jams and you start going in circles. The GK24 never really gets too heavy on the helm when heeled sailing to windward , so overloading the motor is not an issue. This domain of sailing is really a job for a windvane or a human.

Getting inside the box

The bit that went wrong on mine was some dirty electrical contacts under the compass bit - to get inside pry off the little cap on the top of the black compass knob and then unscrew the screw. Undo the screws underneath the unit, remembering which one went where as they are different lengths.

The works will then be revealed.

If you want to get at the contacts under the compass , you need to note how the string is fitted around the black compass unit and remove one spring and detach the string. Undo 4 screws and lift up the compass unit. Under there are some brass contact wires . If they look tarnished use brasso or fine emery paper.

Rea-assemble in the reverse order. The long screws are extremely difficult to line up properly and extremely easy to cross the threads. Dont force them in .


It would be easy to drive this from a PIC microcontroller and provide more sophisticated control from e.g an NMEA input. Provide a switch to disconnect it and then you have the best of both worlds. An autopilot that fills up with seawater and dies but can steer to GPS and another that still works.

Inside views of electronics board


I took this picture on the 28th January when I pressure washed the boat on the scrubbing piles, and had some spare time. This shows the controller board. The big resistor is 0.33 ohms.

The idea is that the C106A  thyristors are arranged in a bridge arrangement with the TIP41 acting as an on/off switch.
To turn the motor ,  the TIP41 is turned on and two of the four thyristors are triggered. They remain on until the TIP41 turns off, either because the autopilot wants to or the voltage across the 0.33 ohm resistor becomes sufficient to turn on one of the other transistors, because the motor is stalled.
About 2 amps would seem to be right. If that happens the TIP41 is turned off for a while. A bit later on it is turned on again, and off immeidately if there is an overload still present (the voltage across the resistor is still large enough due to the current). If the overload has gone then the motor will draw less than 2 amps (or does RV1 set the current limit to something higher) and so the autopilot continues to work. If you completely short the motor the protection is probably not fast enough to prevent things blowing up.

Equivalent parts : I checked Cricklewood Electronics website via Google and they either have the parts in stock or still list them on the catalogue
MC14001= CMOS 4001
SFC2458=MC1458 op-amps
BFR79=BC557 : different pin out : need to refer to datasheet.
C106A= thyristor with a sensitive gate pin : any 50V 5A device I suppose

According to the date codes on the devices here it was built using parts made between 1979 and 1983internal view from a different angle

String Layout

I have been asked a couple of times to provide detail of the string layout inside the unit as there is a great tendency for 'someone' to have a fiddle and mess it up.

Compass End

Compass end view mini seacourse string
Here we can see the ram at near full extension. The string is attached to the end of the spring which is nearly fully extended (this is actually a bit tight). It then wraps clockwise around the compass unit for 1.5 turns and then goes to the little gearing wheel at the other end of the

Motor end

gearing wheel
What is critical here is that at this point there is still at least a couple of turns of string on the larger bottom spool of the gearing wheel (it reduces the movement of the ram to a smaller movement of the compass)
gearing wheel top view
This is the same thing from the top. String to left goes round compass. String to right goes to the end of the ram.

More Documentation

I have subsequently recieved some more JPEG files giving the circuit schematic and the theory of operation
I include these here




Page © Mike James 13th April 2008
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