The Electric Ghost of the ST6000+: How a Crushed Cable Kept Us at Bay Mid-Voyage

1. The Sleepless Night

Sailing under canvas, in the early hours before dawn. The sea is calm, the moon provides enough light, and the ST6000+ autopilot steers the course without a hitch. Everything seems peaceful aboard the Moody 40 "Azul."

Suddenly, the cockpit pilot display shows a message you never want to see in the middle of the night: "SeaTalk Error."

The pilot stops responding to the ST6000+ keypad. I can't change direction, I can't adjust the course. Yet the wheel stays put and the boat holds its heading. It's as if the system froze, locked into the last command it received.

The temporary fix: physically power down the instruments and steer by hand. The error appeared on different passages, always under similar conditions: sailing at night, in the early hours, battery sufficiently charged but with the alternator not charging.

It wasn't the first time this had happened. I'd been seeing this sporadic error for a while, but I'd always put it down to some one-off issue with the SeaTalk system. Until one night, after it happened for the third time on different passages, I started the engine by chance and saw that the error disappeared instantly.

That's when the lightbulb went on. It wasn't a SeaTalk communication problem. It was an electrical problem.

2. The Revealing Symptom

When the engine was running, the error vanished and the pilot worked normally. I started connecting the dots:

• Battery at 80% (12.4 V), but the alternator pushes the voltage above 13 V.
• The Neco actuator draws between 3 and 10 A when steering.
• The PC100 electronics are sensitive to voltage drops.

My gut feeling was: the high current draw of the Neco is causing voltage drops in the PC100's power supply, and under load the voltage falls enough to partially reset the PC100 or corrupt the SeaTalk communication. But I still didn't know why.

It was time to open the panel and have a look.

3. The Inspection: The Previous Owner's "Bodge Job"

When I opened the electrical panel and examined the PC100 power terminal block, I found the first clue:

Two 6 mm² cables crammed into the same terminal on the PC100 power strip.

It's physically impossible to fit two cables of that gauge into the terminal's space. The previous owner's "solution": strip both cables down to reduce their cross-section so they would fit. This left the cables damaged, with copper partially exposed and a poor connection. It was obvious that this terminal block was never designed to power the external motor. In fact, if the PC100 were intended for that, it would have its own power output for the actuator.

I also found a Finder relay already installed in the system. This sent me down a rabbit hole: what is it, how does it work, and what's its purpose? I discovered it's an intermediate relay that allows the PC100 to switch power to the Neco using low-current signals. It was probably installed when the instruments were upgraded. The Neco appears to be the boat's original actuator — a true "workhorse" from the 70s or 80s — but the instruments were replaced in the early 2000s in Lisbon. The original instruments must have been much older, and the Neco is a remnant of that era.

4. The First Fix: Separating the Power Supplies

I decided to intervene. The principle was simple: physically separate the control (low-current) circuits from the power (high-current) circuits.

The plan:

1. Power the PC100 from a dedicated breaker with its own fuse.
2. Power the Neco actuator from an independent breaker, also with its own fuse.
3. Keep the Finder relay so the PC100 can switch power to the Neco.

The wiring diagram ended up as follows:

The initial result was promising: for 2 days, the system ran without errors. The pilot steered correctly, no error messages, everything seemed fixed.

But the calm didn't last long.

5. The New Problem: The Fuse That Couldn't Take It

On the third day, the F5AL250V fuse on the PC100 blew. I replaced it, and when I powered up, it blew again immediately.

I observed the pattern:

• First blown fuse: with the Neco OFF (only instruments active).
• Second blown fuse: with the Neco ON beforehand.

This was new. Since I've owned the boat, this fuse had never blown. The first time was two days after changing the power supply.

Something didn't add up. Before the modification, the fuse held. Now, with a stable power supply, it blew immediately.

Then it hit me: improving the power supply had revealed a problem that was previously hidden. The voltage drops had been masking an intermittent short that, now with stable voltage, the fuse detected and couldn't handle.

6. The Diagnosis: Scientific Method Onboard

Time to apply the scientific method. The goal: isolate the problem.

In the cockpit I have 4 displays connected to the SeaTalk network: the ST6000+ autopilot, the wind instrument, the tridata (GPS, depth, speed log), and the bidata. There's also another display in the cabin.

I started disconnecting equipment one by one, thinking one of them might be causing the fault. It was a 15-day process, because I also discovered that the fuse only protects the SeaTalk network, not the entire PC100 as I initially thought.

I disconnected:

1. The wind instrument → the fuse still blew.
2. The tridata → the fuse still blew.
3. The ST6000+ pilot → the fuse still blew.
4. The bidata (the last one in the cockpit) → the fuse still blew.

Now only one cable was left: the SeaTalk cable coming out of the PC100 going nowhere, since all the instruments were disconnected.

The problem wasn't in the equipment. It was in the wiring.

I grabbed the multimeter and set it to insulation resistance scale (2 MΩ). I measured between the red wire (+12 V) and the black (ground):

• At rest: 0.2 MΩ (200,000 ohms). This was already an abnormally low leakage.
• When moving the cable: the resistance climbed to 0.8–0.9 MΩ.

Diagnosis: variable-resistance leakage, indicative of mechanical damage or intermittent contact. The cable had a physical problem.

7. The Physical Discovery: The Crushed Cable

I visually inspected the entire cable run. I reached a bulkhead pass-through where the cable bundle passes through the wood.

What I saw was revealing:

• The cable bundle was crushed against the wooden structure.
• The black wire (ground): was completely blackened, charred. I discovered it came from an old installation without individual insulation, with exposed copper.
• The red wire (+12 V): had a burn mark (blister) on the insulation right where it touched the black wire. The insulation was almost pierced, about to make contact with the conductor.

The root cause: the previous owner's "solution" of stuffing two 6 mm² cables into the same terminal block. Over time, friction and vibration at the bulkhead pass-through finished off the insulation.

My hypothesis: the previous owner had much smaller, poorly installed batteries than mine. He probably never sailed with the autopilot, because his battery wouldn't last long enough. By improving the electrical installation, the problem he left hidden came to light.

8. The Permanent Fix

With the cause identified, it was time to repair:

1. Cut out all damaged sections: remove the charred and burned section until finding clean copper and good insulation.
2. Wire-to-wire splice: use adhesive-lined heat shrink tubing (marine standard) to ensure a watertight, durable joint.
3. Mechanical protection: install a cable gland in the bulkhead to prevent the new wiring from being crushed against the wood again.

The repair was simple but meticulous. It wasn't just about splicing wires — it was about leaving the installation better than it was.

9. The Acid Test: 6 Days at Sea

We did the entire repair in Ceuta, in port. It took us nearly 15 days between figuring out the fuse problem, understanding what it actually protected, and confirming the system was stable. In port I had verified that the system worked, but I still didn't know if it would hold up under full load. And I hadn't been able to confirm whether the "SeaTalk Error" was truly gone.

But we had a scheduled passage from Ceuta to La Graciosa (Canary Islands) — six days of sailing. That would be the real acid test.

The result: 148 continuous hours without a single "SeaTalk Error."

The system performed perfectly throughout the entire passage. The pilot steered for virtually the whole journey, with brief breaks to hand-steer for pure pleasure. The repair had been a success.

10. Technical Appendix: Modified Wiring Diagram

For readers who want to replicate this modification on their own boat, here is how the installation ended up:

Modification summary:

Applied principle:

• Control circuit (low current): powered from the instrument breaker.
• Power circuit (high current): powered from a dedicated Neco breaker.
• Complete physical separation between both circuits.

The F5AL250V fuse protects only the SeaTalk network, not the entire PC100.

11. Conclusion and Advice for Fellow Sailors

This experience has taught me several lessons I want to share:

1. Intermittent electrical symptoms usually hide physical problems. A crushed cable, damaged insulation, or a poor connection can cause faults that seem like electrical "ghosts." The "SeaTalk Error" wasn't a communication problem — it was an intermittent short that manifested as a communication error.

2. If improving the power supply makes a symptom worse, it doesn't mean you did something wrong — you've unmasked the real problem. Electricity doesn't forgive bodge jobs. When you give the system more stability, hidden weaknesses come to light. In my case, the fuse had never blown until I separated the power supplies.

3. The fuse is an indicator, not the problem itself. If a fuse keeps blowing, don't just replace the fuse — find the cause. And make sure you know what that fuse actually protects (in my case, only the SeaTalk network, not the whole PC100).

4. Check your bulkhead pass-throughs. Cables crushed against wood are a very common cause of electrical failures on boats. A simple cable gland can save you a lot of trouble.

5. When you buy a second-hand boat, inspect the previous owner's "bodge jobs." Change batteries if needed, check the wiring, and don't take anything for granted. Just because "it's always worked like that" doesn't mean it's done right. If the previous owner had a poor electrical installation, they may never have exposed the problem you'll encounter when you improve the system.

6. Invest in adhesive-lined heat shrink and cable glands. They're inexpensive materials that can prevent serious breakdowns down the line.

7. Don't underestimate the value of good intuition. My first suspicion was a voltage drop, and that intuition led me down the right path. But intuition alone isn't enough — you have to confirm it with method, patience, and a good multimeter.

Have you had a similar problem on your boat? Has improving one part of the system ever revealed a hidden issue? Tell me about it in the comments. Together we can learn from these experiences.

"Electricity on a boat doesn't forgive bodge jobs. When you improve one part of the system, hidden weaknesses come to light. Don't ignore them — they're the clue to finding the real problem."