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  • Writer's pictureBill Kinney

Rebuilding the Amel Bow Thruster.

Updated: Jul 1, 2023

The design of the main propeller drive system on the Amel has many advantages, but good maneuverability is NOT one of them. The prop is positioned at the back edge of the keel, a very long way forward from the rudder, so close quarters maneuvering would be charitably described as "sluggish". Enter the Amel bow thruster. The Amel bow thruster is an amazing piece of kit, and is really an important tool for maneuvering this boat.

The bow thrusters on Amel Super Maramus were far ahead of their time. They are uniquely powerful, generating over 10 horsepower from a large 24 Volt electric motor. The ability to extend the propeller far below the hull, means it does not suck air when it is used, then it retracts completely back into the hull leaving nothing exposed to add drag or get snagged.

In this design there are two moving systems, one has a linear actuator that serves to raise and lower the propeller, and the main drive motor, with its gearing to drive the propeller. For this discussion we are going to focus on the parts INSIDE the boat: The drive motor, and the linear actuator.

First a bit of history.

Like many things on Amel boats, the unique features are a combination of standard industrial parts, and some clever engineering by Amel. The linear actuator is in this category. It is a standard industrial part, with an add-on control system built by Amel.

Shortly after we bought Harmonie, we took both the main thruster motor and the little linear actuator motor into a motor shop for routine service. When they went to change the brushes on the little motor, the sheet metal brush holder broke. They could not source a replacement, so at the cost of many hours and dollars, had to fabricate one. In retrospect, this was NOT money well spent. The rebuilt brush holder cost us significantly more than a new complete linear actuator would have.

It has worked for the years since then... but... the motor in particular is showing showing its age. We have cleaned and repainted it several times, but a combination of aluminum and steel in the occasional presence of salt water drips meant we were hard pressed to keep ahead of corrosion, and this gave us no confidence in its long term reliability. A rating of IP54 indicated it was not really designed for places that might be exposed to significant water.

The original linear actuator installed on Harmonie by Amel. If my memory serves, the manufacturer is based in Switzerland.

Where we went with this...

The design of electric linear actuators like this have come a LONG way the the last 30 years. In more and more applications they are being used instead of hydraulics. Being used now on construction equipment and other weather exposed applications, means that waterproof and corrosion resistant construction is easily available in the market. Since we knew this one was on borrowed time, I took the original specs and ordered a replacement. Since this meant finding the time to redesign the control circuits as well, this sat on the shelf as a spare for some time.

The new one I specified has an IP67 housing, and is rated to pass a 500 hour salt water spray test. It is more compact, with no exposed aluminum or steel. It uses less power, and has its own internal limit switches for end of stroke. Operation is triggered not by switching power wires, but rather by low amperage signals. An original spec version was available from Amel the last we checked, but at significantly higher cost.

The original Magnetic and the 21st century alternative from Thomson.

The new one I installed has a few differences from the original Magnetic brand actuator Amel used.

  • Although it has the the same linear throw range (200mm) it is, as you can see in the photo, shorter than the original. That means the lifting cable will need to be a bit longer.

  • The fork ends have the same 10mm hole, but the fork is a little narrower. Check dimensions and have a plan if there is an issue there.

  • With this model, you do not switch the power to control the motor, instead low amperage 24V signals are used independent of the power supply. This made the job of selecting controlling limit switches much easier since they only needed to handle a few 10's of milliamps, not 6 Amps.

  • My design has the limit switches detecting the location of the drive motor, not the position of the actuator. The position of the drive motor is what we really care about. If for some mechanical reason, the actuator is not putting the motor in the right place, we need to know.

  • The original was rated for a load of 2000 N (~500 lbs, ~200 kg). That seemed WAY overkill for something I can pick up by hand. I specified a 1000 N model. I figure if the thruster will not come up with a 250 lb pull, something is very wrong and applying more force probably is not going to be a good thing.

The Thomson actuator has internal controls that shut down the motor if it is overloaded, avoiding blowing a fuse, or potentially causing mechanical damage. On our original installation, there was no such circuit and no fuse. If something jammed while lifting the thruster, the motor would burn up. In later models there was a fuse installed.

Control System Requirements

The original control circuitry for the linear actuator was integral with the assembly and was not easily transferred to a new unit. It was also not in great mechanical shape, so the decision was made to scrap it and start from scratch.

The new control system needed to do a few things.

  • Stop the actuator when the bow thruster was at the top or bottom of its travel.

  • Trigger the alert lights and buzzer when the actuator reached the end of its travel,

  • Prevent operation of the actuator if the locking pin was still in place

  • Prevent operation of the propeller unless the unit was in the proper "down" position.

Control System Design

The new linear actuator made most of these tasks simple. For example, there was no need for an external shutoff when the bowthruster was fully lowered. This could be handled by the internal limit switch.

There was one unexpected complication. My design used the same switch to supply the power to the bow thruster and to activate the signal voltage. The problem here is that if the power and signal are applied at exactly the same time, nothing happens. Power needs to be applied FIRST then signal. I had two choices. Run an extra wire from the helm station that powered on the linear actuator control system when the main thruster system switch was turned on, or add a time delay relay. I choose the lazy way out and added a time delay relay to pause the connection of the signals for a half second after the power was applied.

Otherwise, no relays were needed. Only four limit switches. I picked a switch design where all of them were identical, to simplify spare parts inventory. The four switches do the following things:

  1. Connect power to the light and buzzer if the thruster is down and the DOWN switch in the cockpit is ON.

  2. Disconnect the relays that run the thruster main motor until the thruster is in the down position.

  3. Break the signal circuit to the linear actuator if the locking pin is in place.

  4. When the thruster reaches the top of its travel, break the "UP" signal to the actuator, and connect power to the light and buzzer until the UP switch in the cockpit is turned OFF.

The "as-built" circuit diagram for the new control system.

Links to Parts

Fast forward to several weeks ago...

We were doing a routine bow thruster service as part of our maintenance haul out, and noticed that the base of the main thruster motor had begun to rust, as they do. I removed the motor to clean and paint it. In the process of removing it, the insulation on every one of the four power lead wires into the main thruster motor cracked and crumbled away, leaving bare wires behind. Age, heat, and ozone exposure from the arcing brushes over the decades had taken a toll.

What seemed a disaster at the time, was actually the best possible way to discover this problem. Having these wires short out during the operation of the motor could have been a very exciting event, and NOT at all in a good way. On a boat with an Amel OEM electrical system--without a fuse anywhere in the circuit--such a short could easily be a disaster.

I didn't get too far before I realized that replacing these lead-in wires was a job far beyond what I could do onboard. When Leroy-Somer was making these motors they cost about €1000. Unfortunately, Leroy-Somer no longer makes DC motors, so a replacement in kind was not an option. I know, given enough time, I could find a motor of similar specifications that, with the aid of a clever machinist, could be adapted to this application, but we did not have time for that project.

The motor was shipped out for a complete rebuild. New armature and field coil windings. Brushes, bearings, and seals. Commutator turned on a lathe. Basically putting everything back to the original specification as it came from Leroy-Somers 27 years ago. The motor is now not distinguishable from a new one, inside or out. The rebuild process was not cheap, or fast, but for such a critical piece of gear it was, to us, money well spent. Jeff Warfield Electric in Missouri did a great job with this project. They are one of the few places around who can handle rewinding a DC motor of this size. Depending on the level of rebuild needed, expect a bill between $500 and $3000.

There is not much on the interior of an electric motor that can not be rebuilt or replaced. With a bit of care, and the very occasional expensive trip to the motor spa, these machines can last almost forever. If repair ever becomes difficult or impossible, there are companies the do make custom motors on this size frame. There might be a lot of work needed to make the piece match up, but it is never hopeless.

Information about this drive motor

It's big. It's heavy. Ours draws about 485 Amps when we call on it to push the front of the boat around.

This is what is known as a "series wound motor." Unlike most of the other DC motors on board an Amel it does not have permanent magnets, but relies on current passing through field coils the generate a magnetic field.

It is "series wound" because the field coils are designed to run in series with the armature. Two of the four wires are connected to the armature coils, and two to the field coils. Current flows through from battery positive, through the armature, then through the field, and then back to battery negative. The direction of rotation is reversed by reversing polarity on EITHER the armature OR the field. If you reverse current on both the rotation does not change!

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