Nothing Grows on a Crewed Hull for Long

Week 39 | September 2026

Ocius will tell you its Bluebottle can stay out almost indefinitely. The little Australian USV runs on solar and wave power, so it does not drain a fuel tank and turn for home the way a crewed patrol boat does. Read the company's own material closely though, and the caveat sits right next to the boast. Endurance limited only by biofouling.

Back in March, the Australian Defence Force signed on for 40 more Bluebottles, at a cost of A$176 million over five years, expanding the Royal Australian Navy's fleet from 15 to 55. As Defence Minister Pat Conroy stated at the announcement, Bluebottles already at sea average 75 days continuously, with some deployments running past six months. Whatever the 75 days assumes, a hull left in the water that long will grow something. The only question is how much, and what it costs the mission.

Start with what an endurance figure actually measures. Maritime Robotics rates its Mariner X at 25 days, and that figure is bounded by the engine and the fuel aboard, with a maintenance interval every 250 hours. Hull condition appears nowhere in the spec.

Now the physics pulling against it. The IMO's GloFouling programme found that a slime layer half a millimetre thick across half a hull lifts fuel burn and emissions by 20 to 25%. Let a light crust of barnacles form and a mid-size container ship can watch that climb towards 55%. Michael Schultz's work on naval hulls put a harder number on it again. Once barnacle coverage passes 10% of the wetted surface, a ship needs 36% more shaft power to hold the same speed.

Those numbers come from powered ships, the kind that can answer drag by burning more. A Bluebottle cannot. It harvests what it uses, so on a boat like that the drag has only one place to land. It lands on speed, and eventually on the station-keeping the whole mission depends on. A clean-hull assumption sits quietly inside every endurance line on every spec sheet, and almost nobody says it out loud. And that 75-day figure is already a March 2026 snapshot. Ocius's own updates have mission duration climbing all year, past 100 days by some accounts, which means the fouling clock is running faster than the conversation about it.

Drag is the half of it you can calculate. The other half grows on the payload.

Saildrone learned this on a survey that ran close to 300 days across the Caribbean. Growth kept forming on the sound-velocity sensor, so the team built new ways to catch fouling early and clear it remotely before the data went soft. That is the part that should worry a procurement officer. A fouled hull is slower. A fouled sensor goes blind, and blindness is the one state a surveillance or survey USV cannot afford, because sensing is the entire reason it is out there.

Here is where the materials side of this walks in, and the material that matters is a rulebook.

The international regime governing what you may put on a hull to stop growth is the IMO Anti-fouling Systems Convention. Tributyltin was out years ago. Cybutryne, one of the booster biocides, has been prohibited since January 2023. Copper still does most of the work and is under growing regional pressure from Europe and parts of the United States. Every line of that framework was drafted for commercial ships that run to dry-dock cycles and repaint schedules measured in years.

Sitting on top of it is the biosecurity layer, and this is where Australia gives the story its sharpest local edge. Since June 2022, any vessel arriving in Australian waters has had to declare how it manages biofouling through the pre-arrival report, with a short set of accepted routes: an implemented management plan, cleaning within 30 days of arrival, or a pre-approved treatment. The Australian Department of Agriculture, Fisheries and Forestry (DAFF) can and does inspect submerged hulls and niche areas to check. New Zealand runs a stricter version again under its Craft Risk Management Standard. Arrive with a clean hull, and clean in-water only at approved sites. For anything over 120 metres there is no haul-out option at all.

Read those rules with an uncrewed hull in mind and the seams show. Every path assumes a vessel that arrives, submits to inspection, and carries someone who can be told to act. Who does a biosecurity officer direct when the boat has no master aboard and no crew to send over the side with a scraper? These rules predate the uncrewed hull entirely. They were built around a crew that could be inspected and instructed, and that assumption runs through every clause.

So what do the people buying these platforms say about fouling? Almost nothing.

Go through the public procurement material for uncrewed vessels and the silence is consistent. The US Navy's own MASC materials cite range and speed but no maintenance-free duration at all. The oft-cited '60 days without maintenance' figure for their medium/large USV family traces to 2019 reporting on an unreleased internal requirements document, not to any published Navy spec. The one number the Navy has put on the record itself, a December 2024 NAVSEA engine-test milestone, is 30 days, and that's a single component under test conditions, not a fielded hull. Either way, none of it says anything about the growth that decides whether the number is real.  The Mariner X spec, the MUSV and LUSV requirements that have reached the public record, every one carries an endurance figure and none carries a fouling assumption to sit beside it. The requirement gets written down. The thing the requirement quietly leans on does not.

I acknowledge care is required here, because a documented absence is not proof nobody has thought about this. Some of the detailed requirement documents are not public, and fouling may well be buried in an annex or document I cannot read. But the specs open to read are silent, and that silence is the finding. It leaves the operator holding the question the paperwork skipped. Across a 25-day mission, or a 75-day one, who cleans the hull, and who pays for the recovery when they do?

The obvious question is whether someone has solved this with a coating or a gadget. The honest answer is that the fixes exist, but none of them was built for this problem.

Presently, the most mature answer is a robot that cleans the hull gently and often, before growth takes hold. Jotun's HullSkater and Greensea IQ's EverClean are the two names that matter, and Greensea in particular carries real defence traction, with US Navy and Marine Corps work behind it. Both assume a crewed ship returning to a home port on a schedule, and the cleaning happens there. Neither has been fielded to chase down a lone USV parked on station for two months.

The current coatings do not fit the duty cycle either. Foul-release silicones, the biocide-free kind, work by being too slippery for growth to grip, but they need the vessel moving at speed to shed whatever does settle, which a slow station-keeper never manages. Ultraviolet systems that bathe the hull in light remain pre-commercial. Ultrasonic units protect sea chests and internal pipework well enough without covering a whole hull. And the bio-based coatings, the genuinely regenerative-materials answer you would hope closes the loop, still sit in the lab. Promising chemistry, and panel tests that have not left it. No production scale, and not one defence or offshore-energy qualification among them.

That is the gap in a sentence. The hardware to keep a hull clean already exists. So far as we have found, nobody has qualified it, or bought it, for a boat with no crew aboard to run it.

The endurance is real. The reef that grows on the hull is real too, and it grows faster than the doctrine is moving. The regime that decides who scrapes it off was drafted for ships with crews and dry-dock dates, and the spec that bought the endurance never mentioned the reef at all. Fifty-five uncrewed hulls is a lot of unmanaged surface area on which to learn that lesson in the water rather than on paper.

Next week is a Deep Dive, looking at Saronic building a factory and Kraken building a partnership network. Two bets on how you get from tens of uncrewed hulls to hundreds, with the US Navy buying from both and backing neither. Deep Dives are a piece in the OTI paid tier. If you are not on it yet, upgrade above to read the full piece when it lands Monday.

Since you have been, thanks for reading.

Cheers,

Mick

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