The Ship's Log of Danu

Every great shipwreck in Lake Michigan has a story, and almost no one gets to see them. They rest in cold, dark water — beautifully preserved, well beyond the reach of a weekend diver, and almost entirely unvisited.

We wanted to change that. Not with a crewed submarine or an expensive expedition, but with something we could build ourselves, at home, on a workbench: a small autonomous submarine that can swim out on her own, find a wreck, look it over, and return to tell us what she saw.

That is Danu.

She is being built from scratch — hull, thrusters, power, cameras, sonar, and a brain that can make its own decisions in the dark. No joystick, no tether: you point her at a mission and let her go, and when she surfaces she sends word home.

We are not a company or a laboratory. We are a father and a daughter, learning as we go — and writing all of it down: the clever ideas and the dead ends, the firsts and the floods. This log is that record, kept in the open so anyone curious can follow the reasoning, and perhaps build something of their own one day.

The lake first. The wrecks first. And someday, if she earns it, the open ocean.

*— logged at the workbench*

The main camera is a small, bare circuit-board camera with a very good low-light sensor (Sony's STARVIS 2). It sounds fragile for the deep — until you realize it never touches the water.

It lives dry, behind a clear dome at the nose. The dome is the waterproof part; the camera just looks through it. That means we can use a high-quality sensor for a fraction of what a fully-sealed underwater camera costs, because we're not paying for a pressure housing around every camera.

A counter-intuitive bit: the sealed "backup" camera actually costs more than the better main camera. You're paying for its machined metal housing and depth rating, not image quality.

One gotcha we wrote down for later: a dome is a lens. The camera has to be refocused after the dome is installed, or footage shot through it will be slightly soft.

Danu ends up with three cameras:

• Front, on a pan/tilt gimbal — the cinematic eye. It can aim without turning the whole vehicle.
• A sealed nav/backup camera — an independent eye in case the dome ever fogs or floods.
• A rear-facing camera — so we can see behind when backing away from a wreck or watching for snags.

We briefly considered making the rear camera a second high-end gimbaled one behind its own dome. We talked ourselves out of it. A dome on the tail fights everything: it blunts the streamlined shape we want for efficient cruising, it crowds the rear thrusters and fins, it shifts the balance backward, and it adds a second big leak path — all to point a premium camera at our own propeller wash.

The lesson that keeps repeating: put quality where it pays off (the nose), and keep the rest simple and rugged.

There are two body plans for an underwater robot. A hovering box (like a typical ROV) can hold still and spin in place, but it's draggy and slow to travel. A torpedo cruises efficiently over distance but can't really stop and pirouette.

For covering ground in a big lake, distance wins. So Danu is torpedo-shaped, with a layout biased toward efficient forward cruising plus enough side/vertical thrust to maneuver. The tail fins are fixed (no moving control surfaces) to keep things simple and reliable — we steer with thrusters.

We're deliberately not building it modular. Build it as one good shape first, then tame the drag with fairings, rather than over-engineering swappable pods up front. If we ever want true hover-in-place work, the cleaner answer is a small separate companion vehicle — not compromising this hull.

Most small ROVs hold their depth by constantly running vertical thrusters — like a helicopter that has to keep its rotor spinning just to hover. That burns battery fast, which is fatal for a vehicle meant to roam for hours.

Danu gets a buoyancy engine instead. By changing its volume a little, it can make itself slightly heavy (to sink) or slightly light (to rise) and then just... drift there, using almost no power. Submarines have done this for a century; we're building a small version.

This one decision unlocks a lot: long endurance, the ability to rest on the lakebed to collect samples, and — importantly for an untethered robot — the ability to bob to the surface to phone home and sink again without spending much energy. More on that when we get to communications.

We want Danu to pick up small objects — a sample, an artifact, something interesting on the bottom. A torpedo can't hover steadily enough to do delicate arm-work in mid-water.

So the plan is land-and-grab: settle gently onto the lakebed (or brace against structure), *then* work a gripper. Wide feet keep it from sinking into silt or kicking up a cloud that blinds the camera. The buoyancy engine makes the set-down soft and controlled.

It's a good example of letting the vehicle's strengths dictate the method, instead of forcing it to do something it's bad at. We don't need a fancy hovering manipulator — we need a stable place to stand.

A natural question: should we add infrared / night-vision to see in dark water? The answer is no, and the reason is pure physics.

Water absorbs infrared light within centimeters. The night-vision trick that works on land (invisible IR floodlights) is useless underwater — the light is gone before it reaches anything. That's also why deep water looks blue: the warm colors get absorbed first.

So "seeing in the dark" underwater isn't about a special camera. It's two things: - Bright white light plus a genuinely good low-light sensor (which we have), for close-up detail. - Sonar for everything beyond the reach of light. Sound travels beautifully in water where light can't. An imaging sonar paints a picture of the surroundings in total darkness or murk.

That sonar turns out to be the key to autonomy — it's how the vehicle will spot things worth swimming over to look at.

Here's the hard truth that shapes the whole mission: radio doesn't go through water. Wi-Fi, cell, GPS, satellite — all dead within centimeters of the surface. While Danu is submerged, it is on its own and silent.

So we can't watch a live video feed mid-dive. Instead: - Everything is recorded onboard. - When the vehicle surfaces, it sends a quick update — a satellite text-message anywhere on Earth (tiny, but global), or a real data upload over cell signal when it's near shore. - The full footage comes off the vehicle when we recover it.

We considered a clever trick — reeling a little floating antenna up to the surface while the body stays down. But that's just a thinner tether, with all the snag and complexity we're trying to avoid. The buoyancy engine already lets the whole vehicle pop up to report and sink again cheaply, so that's the plan: dive, explore, surface to check in, repeat.

And Starlink? Great — but on the support boat, not the sub. A satellite dish is far too big and power-hungry for a small submarine bobbing in waves.

This is the part that makes Danu more than a fancy RC toy. The onboard computer runs a loop:

1. Search a planned area, scanning with sonar. 2. Notice something that stands out — a hard, geometric, or unusual shape against the natural bottom. 3. Go look: steer over to it, switch from sonar to camera and lights, and record. 4. Judge: decide if it's actually interesting (man-made? worth a closer pass?). 5. Report: if it found something good, surface and ping us — "found something here," with a thumbnail when there's signal — and optionally wait for a yes/no before continuing.

Two pieces of hardware make this real, and we just moved both into the main build: the imaging sonar (the eyes in the dark) and a navigation sensor that tracks motion against the seabed so the vehicle knows where it is and can return to a spot precisely. Without that, "go back to that object" is just guessing.

The onboard AI doesn't get to invent wild behavior — it chooses among options we've pre-approved (investigate, skip, mark, come home). Autonomy with guardrails.

We just got our hands on a proper navigation sensor — a DVL (Doppler Velocity Log), which tracks the vehicle's motion by bouncing sound off the lakebed. It's the difference between "I think I'm somewhere over there" and "go back to that exact spot."

But a single sensor is a single point of failure. So we're also building a second, independent way to know how we're moving — in software, for basically free.

It's called visual-inertial odometry. The downward camera watches the bottom slide past; we measure how far the texture moves between frames, scale it using the sonar altimeter (how high we are off the bottom), and correct for tilt using the motion sensors. Out comes a velocity estimate — a "fake DVL" that costs nothing but code.

It's not as precise as the real thing, and it needs a lit, textured bottom to work. But running both at once means if one drops out, the other keeps us oriented. That theme — never trust one sensor — runs through the whole vehicle.

Here's the nightmare: months of work swims off into Lake Michigan and never comes back. Preventing that is a top design rule, and the answer is layers — no single failure should be able to keep the vehicle down.

Make it come back up Normally a buoyancy engine surfaces it gently. But if everything fails — flooded, dead battery, frozen computer — a last-resort drop weight releases, making the vehicle lighter than water so it floats up on physics alone. A separate watchdog brain triggers this even if the main computer is silent.

Find it once it's up When it surfaces it reports its GPS position. We also want a recovery beacon that runs on its own separate battery and brain, so it can still call home even if the main systems are dead. Add a bright strobe for night recovery, and an underwater "pinger" to home in on it with a microphone if it's stuck on the bottom.

Don't fail in the first place Backups everywhere: two depth sensors, two battery banks, multiple cameras, two independent ways to navigate, a watchdog that forces the vehicle to surface if the main computer goes quiet.

And early on, we cheat The first dives happen on a safety line, so a rookie mistake can't cost the whole vehicle. Trust is earned before the leash comes off.

When people picture losing a submarine, they imagine it getting crushed by the deep. In a Great Lake, the likelier killer is far more mundane: getting tangled.

Lakebeds — especially around the shipwrecks we want to visit — are draped in lost fishing line, old nets, ropes, rigging, and weed. The thrusters are the weak point: a single strand of monofilament can wrap a propeller and stop the vehicle cold. And the worst instinct is to power harder to break free — which just winds it tighter.

So we design against it in layers: - Guards over the thruster intakes so props can't grab line — the biggest single win. - A smooth, faired body with no open hooks or loops for a net to catch. - Snag detection in software: if a thruster strains while barely turning, the vehicle *stops* instead of fighting, tries a gentle wiggle to shake loose, and if that fails, drops its weight and floats up. - Stay out of trouble: keep a respectful distance from wrecks and never go inside one — penetrating a wreck is how machines get trapped forever.

And the honest part: monofilament line is nearly invisible to sonar, so you can't detect it ahead. You can't beat it completely — only stack the odds and carry a way to be found and recovered if the lake wins a round.

The dream is a vehicle that lives in the lake for months and looks after itself. That turns out to be less about a big battery and more about two things: harvesting energy and barely spending it.

Harvest: a solar fin A flat strip of cells on a round hull catches almost nothing. So we're adding a dorsal solar fin — a printed spine that rises from the round hull to a flat top deck, so head-on the vehicle looks like a T. It carries far more upward-facing panel, doubles as the mast for the antennas, and — with a buoyant top and a heavy keel — makes the vehicle naturally float panel-up to charge. We shape the spine like a teardrop (an airplane-wing cross-section) so it doesn't drag too badly underwater.

Spend almost nothing The real trick is consumption. Most of the time the vehicle should be asleep, drifting and charging, woken only by a timer. Instead of running motors to hold depth, it uses its buoyancy engine. Missions happen in short bursts, then it goes back to sleep. Done right, it sips a few watts — little enough that a sunny day refills more than a quiet day drains.

The catch nobody mentions The thing most likely to end a months-long mission isn't the battery — it's biology. In the Great Lakes, mussels and algae will grow over the hull, the solar, and the camera lenses in a matter of weeks. We can fight it with coatings and wipers, but realistically we'll still have to pull the vehicle out now and then to clean it. Nature is patient.

We got excited about a submarine that lives in the lake for months. Then we did the honest math and the honest risk assessment, and dialed it back to days — and the project got better for it.

Why the change: - Biology wins the long game. Mussels and algae would foul the hull and solar within weeks, forcing us to pull it out anyway. "Months unattended" was partly a fantasy. - You can't lose what you can reach. Leaving it far out for weeks is the easiest way to lose a year of work. Days-long trips that come home keep it recoverable. - It matches the actual goal: transit out to a further wreck, explore for a few days, come back.

So the new target is multi-day missions. That means a bit more battery — but the real trick isn't a bigger battery, it's using less: cruise slowly (power grows with the cube of speed), rest on the bottom between legs using the buoyancy engine, and sleep the computer when there's nothing to do.

And we finally have somewhere to start small: a swimming pool for first wet tests, then a reservoir for autonomy, then Lake Michigan. Crawl, walk, run.

Before building anything bigger, we did the energy math — how far can it really go, and for how long?

Slow is the superpower The power to push through water grows with the cube of speed. Double the speed and you need eight times the power. So the whole game is cruising slow: at a gentle walking pace the vehicle sips only tens of watts to move, which is what makes multi-day trips possible at all.

How long A realistic day — some transit, a bit of filming, lots of resting on the bottom — works out to roughly a few hundred watt-hours per day. The current battery is good for about 3 days. Want a week? Add battery (a longer tube, same diameter) — but we'll measure the real draw in a reservoir first instead of guessing.

How far With energy held back for the trip home, the practical reach is on the order of tens of kilometers out and back — enough for plenty of Lake Michigan wrecks near and moderately offshore. Crossing the whole lake in one go is out of reach for this pack. Currents and weather will trim those numbers, so we treat them as optimistic until real-world tests refine them.

The ocean is a different animal The dream is oceans someday. Honest answer: a battery-powered thruster sub can't cross an ocean — it'd need ~20× the battery. The machines that cross oceans don't really use motors; they glide on buoyancy or harvest waves and wind. So the ocean version is a *different vehicle* down the road. This one is how we learn to build it.