Field Notes: Refit Day — Taking Drifter 7 Apart to Find Its Faults

Drifter 7 came into the shop on a Tuesday with 4,300 survey kilometres on its log and the slightly haunted look of a vessel that has spent two seasons working reefs nobody else wanted to survey. Refit days are my favourite days in the calendar. Not because things are broken — though some things are — but because a mid-life teardown is the most honest design review a piece of engineering ever gets. The workshop doesn't care what the CAD said. The workshop shows you what the ocean did.

The Vessel in Question

Seven is one of our original Block 2 Drifter hulls — solar-electric catamaran, six metres, sonar and camera survey payload plus the Holdfast line-laying rig — and it has worked harder than any other boat in the fleet. Two Tasmanian seasons of seeding, one Mendocino deployment by road freight, and the small matter of the afternoon it came home on its own initiative after a link dropout in weather. If any hull in the fleet has opinions, it's this one.

Reading the Wear

A refit starts before a single bolt turns: you walk the hull slowly and read it. Wear patterns are data. Here's what two seasons wrote on Seven:

The skegs and running gear. Prop blade edges nibbled — that's kelp. Not fragments of rope or plastic: kelp stipe, chopped by the hundred. The bite pattern includes telltale twin-radius cuts from bull kelp blades, and the shaft seals showed the fine scoring that comes from line fibres wrapping and turning into rope-yarn sandpaper. Conclusion for the design log: the weedless prop guards from Block 3 get retrofitted, no debate.

The sonar face. Micro-pitting across the transducer window — not impact damage, biofilm history. Seven spent weeks loitering in warm, rich water on survey holds, and anything that loiters grows a beard. A fouled transducer reads soft bottoms wrong, which matters enormously when the survey map is what decides where Holdfast lines go down. Refit adds a wiper ring and revises the ops manual: transducer faces get inspected and wiped every seventy-two loiter hours, not every week.

The mast base. Hairline gelcoat cracks radiating from the folding-mast foot — the same stress riser we documented when the mast sightline was scoring on earlier hulls. Block 3 answered it with a redesigned folding mast; Block 2 boats like Seven get a reinforced collar and a torque schedule on the hinge bolts. Old hulls get the lessons new hulls taught us. That's how a fleet actually improves.

The seed drum. Salt crystallisation in the tension-sensor housing — a known weakness, and the reason salt-fog testing got added to the trials checklist for Hull Nine. Meanwhile the drum bearings, which everyone worried about, came out looking nearly new. You learn which worries were real.

The Electronics Autopsy

Then the hatches come off. Two seasons of coastal humidity write their own report inside an electronics bay. Seven's was mostly good news — desiccant discipline works — with three finds:

1. A corroded earth point hidden behind the comms tray, caught early, twelve months from being a midnight instrumentation ghost.
2. A battery module running measurably hotter than its siblings under load. Swapped, sent for internal inspection, flagged in the fleet app. Thermal drift is how packs age into problems, and catching it at teardown is exactly what refit is for.
3. A nest. An actual nest — spiders, in the cable void behind the payload rack, who had moved in during a road freight leg and prospered at sea in defiance of all logic. Relocated, respectfully, to the woodpile. The log entry reads: unauthorised crew, no sea time credited.

What Goes Back Together

The refit sheet for Seven: new props and guards, wiper ring on the sonar face, mast collar, drum sensor housing replaced with the Block 3 sealed unit, one battery module, all through-hull fittings pulled and re-bedded, every connector opened, cleaned, re-pinned where tarnished and re-potted. Software: latest control firmware with the station-keeping gains we tuned on Nine's trials, and Seven's original survey-calibration constants preserved — calibration is a boat's memory, and you don't wipe two seasons of known-good behaviour on a whim.

Total: nine workshop days, one argument, zero parts invented on the spot. The one argument was about the paint scheme for the mast collar, and it was resolved the traditional way — the person doing the fibreglass work chose.

Why Refits Are the Real R&D

People imagine engineering progress happens on screens. At the small end of ocean robotics it mostly happens on a bench, with a vessel that's been scraped by reefs, nibbled by kelp, fouled by warm water and colonised by one enterprising spider. Every reef Seven surveyed fed data into Canopy Ledger maps; every fault it logged fed a design change into the next hull. That loop — work, wear, teardown, redesign — is the whole craft, and it's the part of ocean-tech work that never makes the recruitment video.

Seven goes back in the water next week, bound for the winter survey blocks off the Tasman Peninsula. It'll come back next refit with new handwriting on it. I'll be there with a notebook, on a Tuesday, trusting the machine to tell the truth one more time.

Key Takeaways

• A teardown is a design review: wear patterns on props, transducers and mast fittings are field data no simulation provides.
• Kelp chips props and fibres score seals — working hulls need weedless gear, not careful skippers.
• Biofilm is a data-quality issue: a fouled sonar face quietly degrades the survey maps everything else depends on.
• Fleet learning must flow backwards: fixes discovered on new hulls should be retrofitted to old ones.
• Preserve what's proven — calibration constants and known-good settings survive refits untouched.

FAQ

How often does a Drifter get refitted?

Major teardown annually, minor inspections monthly in season, and an unscheduled haul-out after any grounding, line entanglement or collision event. Working boats in kelp country live under a harsher maintenance rhythm than recreational craft.

What's the single most common refit find?

Connector corrosion. Salt air finds every imperfect seal eventually. The fix is unglamorous: better potting, dielectric discipline, and opening every connector on schedule rather than when symptoms appear.

Do you reuse parts between hulls?

Structural and cosmetic parts, yes, after inspection. Anything fatigued, salt-damaged or load-critical is retired permanently — and retired parts go into the teaching collection that trains new crew on what failure actually looks like.

Why preserve old calibration constants instead of recalibrating fresh?

Stability beats novelty. A sensor suite with two seasons of validated behaviour against known reefs is an asset; recalibration happens when hardware changes, not as routine. When we did refit the sonar face with a wiper ring, the calibration was re-verified against a reference target before sign-off.

Who does the refit work?

A permanent workshop team of marine technicians and mechatronics engineers, with the vessel's regular operators embedded for the teardown week. Nobody understands a boat's quirks like the people who've been deploying it, and refit week is when that knowledge gets written down.