Upgraded Autopilot

For cruising long distance we are really dependent on our autopilot to steer the boat hour after hour, day after day. The Amel was designed as an “electric boat” with power produced by the diesel generator, stored in the batteries, and used to do everything on board. Electric autopilots unfortunately do not have a great reputation for reliability.  So what to do if you really need an electric autopilot to steer the boat?  Well, you have two of them!  In fact, two completely independent systems.

After 20 years, the electronic brains of Harmonie‘s autopilots are getting more than a bit out of date.  One of them, a vintage Autohelm ST7000 is especially lacking in modern features and capabilities, so it was slated for replacement with a modern Navico system that fully integrates with the new B&G instruments we have been installing.

Now, while the electronics parts of the Autohelm are obsolete, the electro-mechanical parts that are used to actually do the steering are robust and reliable. In fact they are still in current production.  So I wanted to keep those parts. The old mechanical parts would be easy to connect to the new electronics…  I thought.

One of the great thing about the new generation of navigation electronics is that the communication between the separate pieces is so much more straightforward and efficient than the “old days.” Many fewer wires are required and yet all the pieces talk back and forth to each other easily and simply.  In removing the old system we replaced ten wires by 4. It was nice to complete a project that ended up with many fewer wires running through the boats’s wire chases!

But there was ONE catch. Harmonie‘s main electrical system is 24 Volts DC, not the 12 Volts which is more common.  This isn’t an issue in and of itself because the new autopilot handles either system in stride.  The issue came up with the drive unit that actually turns the rudder–the part from the old system we wanted to keep. These units have a main drive motor and a clutch.  The main drive motor is 24 Volts, but the clutch runs on 12 Volts. Why would Autohelm (or Raytheon or Raymarine or Flir, the product line was spun off and sold several times in recent years) design a system where the motor runs on 24V and the clutch on 12V?  Who knows.  I suspect it was done to avoid someone doing just what I wanted to do: Mix and match manufacturers in single system. The new autopilot electronics assume that if the motor is 24V, so is the clutch.  Simply connecting the two would burn out the clutch coil in no time when twice as much power charged through it as it was designed for. Fortunately Ohm’s law came to the rescue.  Adding a power resister to the circuit dropped the voltage available to the clutch to 12 V and all is happy. Kind of a brute force fix, and a waste of 7 Watts of power, but it works!

Since some rewiring was needed, now was the time to be fix the installation so that I could have either computer run either drive, and make sure that there was no way both computers and drive could try to steer the boat at once.

I got two of these 4-Pole/Double-Throw switches:

The switches were mounted to a bracket inside the cabinet over the sink. Images below show the mount and the schematic of the installation.

The new system is so much smarter and more capable than the old one.  The old one did one thing.  It steered the boat to the commanded compass course.  That was it.  The new one will steer to a compass course, and with a new and improved compass and rate of turn gyroscope it steers much more accurately. Or it can steer the boat at a constant wind angle.  Or you can use it as power steering.  If you want to steer to a close hauled wind angle, it will optimize your course to maximize your speed made good relative to the wind.  You can navigate to a route you specify on the chart plotter.  Smart auto tack and auto gybe.  Etc, Etc.  All is good and happy.