Wing Power System: Build and Mounting Guide v9.6

Companion to:

• Build Guide v9.6 sec 6 (the free-rotating wing and the self-contained pod) and sec 3.3 (the rig mount, fixed can, and slewing ring the charging transformer references),
• the Wing Construction Guide v9.6 (the wing build this power system installs into),
• the Wing Trim Actuator Installation Guide v9.6 (the actuator, its cable, and the compliance link that makes feather independent of power).

Scope. This document covers only the wing's own power system: how the wing feeds itself, how much power it makes and stores, how it steps down when short, and, in detail, how every part is built into or onto the wing. It is the design authority for the contactless charging across the bearing that Build Guide sec 6.6 points to. Changelog, version history, and decision notes for this guide live in the central Changelog. Terms used here.

Term	Plain meaning
Feather	The wing turned edge-on to the wind, making no force. Its safe resting state
Trim tab	The small flap on the tail. The only powered moving surface in the rig
Pod	The sealed electronics box at the wing root, on the turning side
Split transformer	Two facing coils that pass power by magnetism across an air gap, with nothing touching
Slip ring	The usual way to get power across a turning joint, by sliding contacts. Deliberately not used here
Ideal diode	A one-way electronic valve, so a dead battery drops off the bus instead of dragging it down
Awareness rail	The 3.3 V supply feeding only the computer, sensors, and radio
BMS	Battery management system. The little board that protects a battery pack
Compliance link	A spring-loaded section in the tab pushrod. A wind-load fuse: it lets strong wind push the tab straight whatever the actuator is doing, so feather never depends on power (Trim Actuator Guide v9.6 sec 6)
MPPT	Maximum power point tracking. A small charge controller that pulls the most it can from a solar cell as the light changes
RS485	A rugged two-wire way of sending serial data. Immune to electrical noise
CAN	A two-wire data bus with error checking built in. What the Thomson actuator speaks

1. The idea in plain terms

The wing turns on a bearing to follow the wind. Nothing electrical crosses that turning joint by wire, a deliberate choice, so there is no slip ring to wear out. Instead the wing is its own small power island: it makes power, stores it, and uses it, all on the turning side. The only things that cross the joint are magnetism from a split transformer that can charge the wing from the hull without touching, and a short-range radio for commands and readings.

The wing needs very little power

Its only real load is the small actuator that drives the tail's trim tab, plus a wind sensor, an angle sensor, a small computer, and the radio. The tab is what keeps that load small. The tail flies itself, and the actuator only positions a small flap against a light spring, so holding a trim setting draws essentially nothing. On a good day a small solar panel covers all of it.

The one guarantee

The wing never loses the ability to trim itself for lack of power. And if it ever does, it simply feathers, turns edge-on to the wind, the safe resting state, and drifts, costing nothing but a little speed. Three independent ways to keep it powered, so no single failure takes away wing control:

1. Solar cells built into the wing skin.
2. A no-contact split transformer that charges from the hull across the bearing.
3. Two batteries on the wing, either of which alone can run everything, plus a tiny backup cell that keeps the sensors and radio alive no matter what.

One thing to keep clear: safety never depends on any of this. Feathering needs no power: the compliance link in the tab pushrod is a wind-load fuse that lets strong wind push the tab straight whatever the actuator is doing, and the light hinge spring centers it (Trim Actuator Guide v9.6 sec 6). The power system only buys performance, meaning the ability to keep actively working the wing, and awareness.

2. The electrical backbone (every voltage, in one place)

The whole wing runs on two rails, and everything in this document hangs off one of them.

Rail	What it is	What lives on it
24 V bus	Both batteries (25.6 V nominal LiFePO4) joined through one-way ideal diodes, so either feeds the bus and a dead one drops out	Tab actuator, battery chargers, the receiving coupler's output
3.3 V awareness rail	Stepped down from the 24 V bus. Falls back automatically to the backup cell when the bus is dead	Computer, wind sensor, angle sensor, radio

Charging sources all land on the 24 V side. The solar cells feed it through two small MPPT charge controllers, one per face. The contactless coupler, a small 24 V inductive coil pair, feeds the wing charge controller like a panel (sec 4.2). The backup cell is a 3.6 volt lithium primary. It feeds only the 3.3 volt awareness rail, through its own regulator and diode, switching in by itself when both batteries are gone. Nothing about the backup cell involves the actuator. It cannot move the tab and never needs to, because feather is the compliance link's job, not power's.

3. What lives on the wing (parts overview)

Part	What it does	Roughly
Solar cells, both faces	Make power in daylight	4 sq ft total, about 2 lb
Battery 1 and Battery 2	Store power. Either one runs everything	24 V (25.6 V nominal) 6 Ah LiFePO4, about 150 Wh and 2.9 lb each, as bought
Backup cell	Keeps sensors and radio alive if both batteries are flat	OmniCel ER34615 lithium D-cell with leads, about 0.25 lb
Receiving coupler (turning half)	Receives charge from the hull, with no contact	Receiving coil of a 24 V inductive coil pair bought on Amazon 2026-09-15; part number and rating to be logged at the next mail audit. Light; weight to be measured
Pod electronics	Computer, radio, angle sensor, charge controllers, watchdog	Existing pod
Tab actuator	Drives the tail's trim tab. Lives in the sealed boom root fairing, 47 in up, fed by one wiring conduit in the trailing edge. The only real power user	Thomson Electrak MD, about 2.4 lb. Trim Actuator Guide v9.6 sec 2
Wind sensor	Reads the wind at the wing tip. The cross-check for the primary unit on the instrument post	Calypso CMI1017 ultrasonic (RS485), on a short arm ahead of the nose at the tip

Total added weight at the wing root is about 9 to 10 lb, all of it low. Both battery packs ride in the external nose locker on the root leading edge (Wing Construction Guide v9.6 Stage 10), where the same weight does balance work ahead of the pivot. Batteries the wing carries anyway replace lead. The actuator rides at the boom root and is carried in the same moment budget: about 2.4 lb for the Thomson MD.

4. The three power sources

4.1 Solar cells in the wing skin

Two square feet on each face, both faces, for 4 sq ft in total. Why both faces. The wing is a tall vertical shape that swings to point into the wind, so a panel is never aimed straight at the sun. Cells go on both faces and the system simply draws from whichever side is brighter at the moment. They are built flush into the skin (sec 8.2) so there is nothing sticking up to peel off. Rough daily output:

Where	Output
Tropics	About 45 Wh per day
Mid-latitudes	About 35 Wh per day
Southern Ocean, worst and cloudy	About 10 Wh per day

4.2 Charging across the bearing (the split transformer)

This is a bought part, not a project. The part: a 24 V inductive coil pair bought on Amazon 2026-09-15, part number and rating to be logged at the next mail audit. About $45. One coil is powered on the fixed side, the other receives on the turning side, and power passes by magnetism across a small air gap with nothing touching. The rating is not known yet, so the check is written down here. The wing itself uses about 20 Wh a day fully active (sec 5), which any coupler of this class refills in a few hours whenever hull power is healthy. The load that decides the part is the heater pads: the pod needs about 20 W if both pads run from the coupler, and a few watts otherwise. So either the coupler carries the pad load, or the pads run from the pod packs (sec 8.3). Measure it at the sec 10 coupler test. The receiving coil feeds the wing charge controller as if it were a solar panel. History, one line: the Phoenix Contact NearFi 200 pair (50 W, about $650) was the specified coupler through v9.6 revision 1. It, its M12 cordsets and its G10 standoffs are deleted, 60x oversized for this wing. Do not re-propose it. This lets the hull charge the wing without a slip ring. It only works when the hull's main power is alive; when the hull is dead the wing runs on its own solar and batteries instead. Mounting is in sec 8.5.

4.3 The two batteries and the backup cell

Two batteries, 25.6 volt 6 amp-hour LiFePO4, about 150 watt-hours each, on the aft shelf of the external nose locker at the wing root. Mounting in sec 8.3. Either one alone runs the entire wing, so losing one is not a crisis. The sizing requirement, stated plainly: ride out a two-week total-overcast storm sequence on ONE battery, in reduced mode, with solar cut in half. One 150 Wh battery covers about 16 days of that case, so the pair carries the requirement with a whole battery of redundancy on top.

5. How much it makes, uses, and lasts (Southern Ocean)

The Southern Ocean is the hard case: short on sun, long on wind. There is a silver lining. Strong wind means the wing barely needs active damping there, so it uses very little power. Holding trim draws essentially nothing with the tab; only moving it costs anything. Per-device design values:

Device	Average draw
Computer and sensors	~0.15 W
Wind sensor	0.2 W
Radio beacon	0.05 W
Actuator movement, full active	0.4 W

The daily balance, with solar making about 10 Wh per day:

Wing mode	Uses per day	Result on solar alone
Full active, all damping on	About 20 Wh	Short each day; battery makes up the difference
Reduced, light damping	About 14 Wh	Small shortfall; battery covers it
Coarse, damping off but still trimming	About 7 Wh	Roughly break-even on solar alone
Feathered, resting	About 2.5 Wh	Solar makes several times what it needs

The last line is the important one. Feathered, the wing makes far more than it uses, even in the worst light. So low sun can never permanently drain it. If it runs low it drops toward feather, refills its batteries, and comes back. The batteries are only there to buy active-sailing time through dark patches.

6. Running low: shedding load and turning off damping

The wing watches its own battery level and steps itself down, the same way the whole boat does.

Battery level	What the wing does
High	Full active trim and damping, normal readings sent
Medium	Lighter damping, slower sensor checks, beacon-only readings
Low	Damping off. The wing hunts a little but still trims and steers; computer mostly asleep
Critical	Feather and sleep. Wake now and then. Drift east, which is progress

Turning damping off when low is safe and nearly free. Damping is just software smoothing. It is not what keeps the wing safe; the compliance link does that. And shedding trim entirely is safe: an unpowered tab holds its last position in light wind, where it cannot hurt anything, and in strong wind the compliance link lets it straighten, which is a feathered wing. With damping off the wing wanders a few degrees around its heading and loses a little speed. It still trims and steers. The Southern Ocean is windy, and damping is barely needed in wind. So you give up almost nothing by shedding it right where you are shortest on sun.

6.1 What the wing does when the radio link drops

A link drop is a comms failure, not a wing failure. The wing still has its own wind sensor, computer, and batteries. Going straight to feather on a 30-second dropout would stop the boat many times a voyage. So feather is the END of the ladder, not the first step.

Outage length	Response
Under 10 minutes	Hold the last commanded trim. Most dropouts are seconds
10 minutes to 2 hours	Self-trim on the wing's own wind sensor at a reduced setting, about 60 to 70 percent of the last commanded angle
Over 2 hours, or wing battery below reserve, or wind above the storm threshold	Feather, on the wing's own power ladder

How the middle rung works: it is the same trick a wind-vane self-steering gear uses. Hold a fixed angle to the measured wind, without knowing where the boat is going. The wing cannot overload anything because it is trimming to measured wind, and the boat keeps making way. Bench test: power the wing up, kill the radio, and watch it walk the ladder on its own clock. Folds into the section 10 tests.

7. Where everything sits on the wing (the map)

• The wing stands about 21 in above the deck at its bottom edge, and rises 6.5 ft.
• The bottom 2 ft, the root, wraps around the top of the mast and is the thickest, strongest part. All the heavy bits live here, low: the pod electronics, the backup cell, and the turning half of the transformer sit close to the turning axis.
• The two batteries ride just outside the skin, in the faired nose locker on the root leading edge, where the same low mass also balances the wing (Wing Construction Guide v9.6 Stage 10).
• The tab actuator is the one exception. It lives up at the boom root, 47 in above the wing's bottom edge, inside the small sealed fairing on the boom hardpoint, next to the flap it drives. The part is the Thomson Electrak MD (Trim Actuator Guide v9.6 sec 2). One sealed wiring conduit inside the trailing edge, built into the wing at Construction Guide Stage 10, carries its 24 volt power and signal up from the pod on the tinned 18/4 cable. The signal pair is CAN, to the pod's Nucleo-G431 through its CAN transceiver breakout, with a 120 ohm terminator at each end.
• No switched feed. The MD drops into sleep mode on its own after bus inactivity, under 2 mA, so its 24 V feed stays on all the time.
• The wind sensor is the Calypso CMI1017 ultrasonic unit (RS485), on a short arm ahead of the nose at the wing tip, head level, wired to the pod's MAX3485 breakout, the only thing on that breakout. Its cable runs down the trailing-edge conduit with the actuator cable (Mike, 2026-09-15). Both cables are pulled at the same time at Stage 10 of the Wing Construction Guide v9.6, because nothing can be added to a sealed wing later. The post unit is primary; this one is the cross-check.
• The lightning path runs outside the skin, on the side opposite the pod recess: a 1 x 1/16 in copper strap from the tip rod down the spar line, a short braid jumper to the boom-root clamping plate, and a clamp to the mast below the root (Lightning Guide v9.6 sec 5.1). Nothing of the pod's wiring touches it. The pod, its conduit and the packs stay on their own side of the wing, and the pod keeps its surge protection as before.
• The solar cells sit higher up on the two flat faces, in the middle of the wing, kept away from the sharp leading edge and away from the boom.
• The fixed half of the transformer and its little power driver live inside the fixed can, just under the bearing.

8. Building and mounting it, step by step

8.1 Order of work

1. Build the wing skin and root structure first, per the Wing Construction Guide v9.6 (Build Guide v9.6 sec 6.2 to 6.3 holds the design values). That also bonds the trailing edge wiring conduit in before the edge closes.
2. Plan the cell pockets and wiring channels into the skin while you are laying it up. Far easier than adding them afterward.
3. Fit the pod and the turning half of the transformer during wing assembly, on the bench, before the wing goes onto the boat. Pull the actuator wiring through the trailing edge conduit at the same sitting.
4. The batteries go into the nose locker at balance time (Construction Guide Stage 10). Their leads run through the locker's sealed saddle fitting and the short internal conduit to the pod, bonded in before the root closes.
5. The fixed half of the transformer is fitted to the can when the can is built (Build Guide sec 3.3).

8.2 Solar cells into the skin, so nothing can rip off

Use thin flexible back-contact cells, the kind with no metal lines on the top face. The rule is simple: the cells become part of the skin, flush, with no edge for the wind to grab.

Laying them in

1. Pick the cell zone on each face. A band about 10 in across and 24 in tall, centered on the wing face, starting about 8 in back from the leading edge and stopping well short of the trailing edge. That is about 2 sq ft per face.
2. Split the zone into four to six small cells, not one big sheet. If one ever fails it is a small loss, and small pieces cannot flap.
3. Make a shallow pocket. After the wing skin has cured, rout a recess about 1/16 in deep, the size of each cell, plus a thin channel for its wires running down toward the root.
4. Bed each cell into its pocket with clear adhesive, so its top face sits flush with the skin around it.
5. Run the thin wires down the routed channel to the root in 1/4 in OD sealed nylon tube (100 ft bought), and pot the wire exits so no water can track inside. At the pod, each face's string lands on its own 5 A MPPT board (8 to 28 V input, two bought), and each board feeds the 24 V bus through a DG7512 ideal-diode board (one bought, one to buy). That is solar electronics option A.
6. Fair the edges. Fill the tiny step around each cell with thickened epoxy and sand it flush, so the surface runs smooth from skin to cell with no lip anywhere.
7. Cover the cells with a clear, sun-proof film or coating that laps onto the skin all the way around, sealing the edge.

Now each cell is captured on every side and sits flush. A 60 kt wind has nothing to lift. Keep the cells off the front 6 to 8 in of the wing, which takes the wave slams.

Heat protection under each pocket

Lay TWO EXTRA PLIES of glass under each cell pocket during the skin layup, and keep a white-painted border around each pocket. Never let dark encapsulation run past the cell edge. Why. A solar cell absorbs 90 to 95 percent of the sun that hits it, against 20 to 25 percent for the white paint. That is four times the heat into the same surface. In windless tropical sun, a dark cell over H80 foam approaches the foam's softening range: H80 is good to about 158 F continuous, roughly 176 F short-term. Glass spreads heat about 5 times better than epoxy, and costs a scrap of cloth. GATE: soak one cell-on-coupon sample at 176 F and look for print-through or softening before committing the wing. Only if the coupon fails, add 0.005 in adhesive-backed COPPER foil under each pocket.

• Never aluminum powder in the epoxy. Filled epoxy barely conducts; foil spreads hundreds of times better.
• Never aluminum foil. It raises a corrosion question next to salt and a lightning bar that the boat does not need.

Edge protection

Simply OVERPAINT the film's lap border with the same white one-part Rust-Oleum Marine Topside the wing already wears. Twenty minutes, and it is the 90 percent fix. A routed rebate for the film edge was considered and skipped. The failure it guards against, the lap edge lifting under UV and salt, costs one cell string, and the wing's solar is about 10 Wh/day of backup against a 50 W hull feed. It is not a boat problem.

An alternative

If you prefer, the cells can instead be laid between the outer two glass layers during the skin layup, then covered with a clear coat. Same result: cells inside the skin, nothing on top to peel.

8.3 The two battery packs in the nose locker

Each battery is a 24 V (25.6 V nominal) 6 Ah LiFePO4 pack, about 150 Wh, 2.9 lb as bought. Confirm the pack dimensions before the locker plug is cut (Wing Construction Guide v9.6 Stage 10). The packs get no boxes of their own. Both ride on the aft shelf of the external nose locker on the root leading edge. The Wing Construction Guide v9.6 (Stage 10) owns the locker build: saddle, through-bolts, gasketed lid, drains, and vent. This guide owns the packs and their wiring.

Mounting and wiring

• Strap each pack to the shelf at bolted strap points. Nothing relies on glue alone, and a knockdown cannot shift a clamped pack.
• The shelf is sized so a future doubled 300 Wh pack still fits. Every pound of battery at that lever retires about a pound and a half of trim lead from the rack beside it.
• Keep the two separate. Each pack carries its own small BMS board, so a problem in one is sealed away from the other.
• Wire both batteries into the pod through one-way diodes, so either battery can feed the wing and a failed battery drops out on its own. Either one alone must be able to run everything.
• The leads leave the locker through the sealed saddle fitting and run in the short internal conduit to the pod tub. Bonded in before the root closeout, pull string proved, service loops at both ends.

Treat the locker as if it might get damp, because it will

• Gasketed lid.
• Two drain holes at the low point, countersunk on the outside.
• Breather vent.
• Boards conformal-coated.
• Everything up on its shelf or rack, never sitting on the locker floor, so the first water to arrive reaches nothing.

The cold-charging rule

LiFePO4 batteries must not be charged below freezing. Below 32 F the lithium plates onto the anode instead of going in, and the damage is permanent. Discharge is fine to -4 F. Fitted now, not deferred to lake testing that cannot produce Southern Ocean cold: a hard charge inhibit at 35 F, independent of software.

• Buy packs whose BMS datasheet STATES a low-temperature charge cutoff. Confirm before ordering.
• If a pack lacks it, a $5 bimetal snap disc, normally closed, opening below 35 F, goes in series with its charge line.

Thermostated heater pads under each pack are fitted, with one hard constraint written here so it is never violated. The pod heaters STAY: the main-bank heaters were deleted because the sea holds that pack above freezing, but these pod packs sit aloft in the nose locker and follow air temperature. Do not re-propose deleting them. Parts: two 24 V 10 W silicone pads, 2 x 2 in (bought), with a pair of 24 V XH-W3001 thermostats, one per pad. A 10 W pad running continuously is 240 Wh/day, against a wing budget of about 20 Wh/day and about 10 Wh/day of solar. So: THE PADS RUN ONLY ON HULL POWER THROUGH THE COUPLER. On wing power a 10 W pad would empty a 150 Wh pack in well under a day. Cold-charge heating is a hull-powered function, never a wing-battery one. With the hull dead, the packs simply wait for sun-warmth, protected by the inhibit. One check to make: the coupler's rating is not logged yet (sec 4.2), and two 10 W pads plus the pod's own few watts is about 20 W. The thermostats cycle the pads, so the average is lower, but confirm at the sec 10 coupler test that the pads warm the packs on the coupler alone. If they cannot, the choice is a bigger coupler or running the pads from the pod packs, which the rule above otherwise forbids; record the measured figure and put that choice to Mike. Nothing else in this guide changes.

8.4 The backup cell

One lithium primary D-cell: the OmniCel ER34615 with leads, 3.6 volt lithium thionyl chloride, about 19 amp-hours, with very long shelf life. In a sealed holder, potted, mounted anywhere dry in the pod. It feeds only the 3.3 volt awareness rail, through its own regulator and one-way diode, and switches in by itself when both batteries are gone. It keeps the computer, sensors, and radio beacon alive for about two weeks at beacon duty. The arithmetic: the sec 5 device table sums to about 0.4 W with everything awake (0.15 W computer and sensors, 0.2 W wind sensor, 0.05 W radio beacon). On the backup cell the pod is in its sleep ladder, computer and sensors duty-cycled to roughly half time, so the average draw is about 0.2 W. The cell holds about 68 Wh (3.6 V x 19 Ah), and 68 Wh / 0.2 W is about 340 hours, about 14 days. What it does not do, on purpose: it never powers the actuator. A single D-cell cannot source actuator current. It does not need to. Losing all trim power never puts the wing at risk: the compliance link is the fuse, and it feathers the wing in any wind that could hurt it. Mark the cell with the install date and replace it at the big pre-voyage refit.

Monthly conditioning load

Lithium thionyl chloride cells grow a film on the anode while sitting unloaded. That film is what gives them their shelf life, and it costs only a couple of percent of capacity a year. But it makes the FIRST current pulse sag, sometimes for seconds to minutes. If the beacon's transmit pulse drags the cell below the radio's brownout, the transmission fails until the film breaks down. The risk is neither lost capacity nor total failure. It is that first pulse. And the cell has no fat to give: about 68 Wh against the 0.2 W average duty-cycled draw (sec 8.4) is roughly two weeks, exactly the stated claim. The fix costs a resistor and a MOSFET: pull 10 to 50 mA for 30 seconds once a month, and log the voltage. The logged voltage IS the health trend.

8.5 The contactless coupler (charging across the bearing)

Two bought coils and their leads. Nothing wound, nothing tuned. The part is the 24 V inductive coil pair of sec 4.2 (bought on Amazon 2026-09-15; part number and rating to be logged at the next mail audit): one transmitting coil with its small driver board on the fixed side, one receiving coil on the turning side.

Mounting

• Transmitting coil: inside the fixed can, on a bracket in the can's center opening, face up, centered on the turning axis, fed 24 V from the hull through a sealed gland in the can wall.
• Receiving coil: on the underside of the turning mast plate, face down, centered on the axis, facing the transmitting coil across the air gap the coil pair's listing allows. Read that gap off the listing when the part arrives, and set the mounting to it. Ordinary build tolerance in offset and angle is fine at the power involved.
• Reference the gap off the bearing, not the deck. Bolt the fixed half to the can, which carries the fixed bearing ring, and the turning half to the mast plate, which carries the turning ring. That way the bearing itself keeps the two halves lined up, and normal deck flex cannot open the gap.
• Mount both on non-metal brackets (G10 offcut from stock), so the coil faces stay clear of surrounding steel. No separate standoffs are bought.

Wiring

• Fixed side: hull 24 V through the can-wall gland to the transmitting coil's driver board. Loop and clip the cable so nothing hangs in the drain path.
• Turning side: the receiving coil's output runs up into the pod to the wing charge controller, through a potted gland, and is treated there like a solar panel input.

Protection

Put surge protectors on both sides, and keep this circuit separate from the lightning bonding strap that already bridges the bearing, so a strike does not travel through the coils. The coil pair carries no weather rating of its own, so its driver board is conformal-coated and its lead exits potted. Both halves live in the most sheltered spot on the boat, inside the can under the labyrinth skirt.

8.6 Wiring and keeping water out

• All wire on the wing is marine tinned, all boards are conformal-coated, every wire exit through the skin or a box is potted, and every connection gets a little slack, a service loop.
• The actuator run: one four-conductor shielded cable, a 24 volt power pair plus a signal pair (CAN; Trim Actuator Guide v9.6 sec 3), shield grounded at the pod end only. The cable is tinned 18/4 shielded, 50 ft. It was never ordered and is on the buy list: Amazon B0D9JV83HJ, $36.99, 50 ft, tinned, shielded. The MOOKEERF bare-copper roll goes back. Pulled through the trailing edge conduit with the pull string left there at the wing build. Glanded where it leaves the pod and again where it enters the boom root fairing, with drip loops at both ends. The conduit is sealed at both fittings. The wind-meter cable shares it (Mike, 2026-09-15) and nothing else does; both cables are pulled at the same time at Stage 10, and the conduit is 1/2 in inside diameter or more so both pass with the pull string still in it. Check the conduit bore against the 18/4 cable before it is bonded into the wing; the 1/4 in OD tube used for the solar and locker wiring is too small for it.
• Remember what does and does not cross the turning joint. Only magnetism, through the transformer, and radio. No power or signal wire runs across the bearing, so there is nothing there to chafe or wind up. Data goes by the short-range radio.
• Give the pod, the nose locker, and the boom root fairing a tiny waterproof breather, so pressure can equalize without letting water in.

8.7 The pod enclosure

A sealed housing at the wing root, on the turning side, holding:

• the computer,
• the radio,
• the angle-sensor electronics,
• the two 5 A MPPT charge controllers (8 to 28 V boards, bought), one per face,
• the two DG7512 ideal-diode boards (one bought, one to buy),
• the pod watchdog (sec 8.8),
• the backup cell.

The two battery packs ride forward in the nose locker (sec 8.3). The actuator does not live here; it is up in the boom root fairing at the end of the wiring run. Build rules:

• Mount the pod to the root ribs and the spar.
• Mount every board on standoffs, up off the tub floor, so the first water to arrive reaches nothing.
• The tub's two low-point drain holes, built at Construction Guide Stage 10 and countersunk outside, carry that water out.
• One waterproof breather vent.
• All wires in through potted glands, with service loops.

8.8 The pod watchdog

The pod computer gets a wing-side hardware watchdog: the same Nano-plus-regulator recipe as the hull watchdogs. The pod software must pet the watchdog on schedule. If the petting stops, the watchdog power-cycles the pod computer's rail. Two build rules are pass/fail:

1. The pass element defaults ON. A dead watchdog changes nothing; the computer stays powered.
2. The action is a power cycle, never a latch-off.

So a hung pod reboots itself. While it reboots, nothing drives the actuator: the tab holds its last position, and in strong wind the compliance link lets it straighten. A healthy reboot resumes sailing. Section 10 has the bench test that proves both rules.

9. What happens when things fail

If this fails	What happens	Why it is OK
A solar cell tears or dies	Lose a little charging	Cells are flush and captured, so no debris reaches the trim parts. The transformer and batteries still charge and run the wing
All wing solar lost in a storm	No wing charging during the storm	In a storm the wing is feathered and barely using power. The batteries and the transformer cover it
The transformer or hull power fails	No charging from the hull	Wing solar and the batteries carry on
One battery or its board fails	Half the storage is gone	The other battery alone carries the design case: about 16 days of reduced-mode sailing through storm overcast
The trim wiring or the actuator fails	No active trim	The tab holds where it was. In strong wind the compliance link in the pushrod gives, the tab straightens, the tail streamlines, and the wing feathers. Light wind cannot hurt the rig
The pod computer hangs	The watchdog power-cycles its rail. The tab holds its last position while it reboots, then sailing resumes	The watchdog's pass element defaults ON and its action is a cycle, never a latch-off. So a dead watchdog cannot cut power, and a hung computer cannot stay hung
Both batteries flat and still no sun	No active trim	The wing is safe without power: the compliance link feathers it in any wind that matters, and it drifts east. The backup cell keeps sensors and radio alive so you still see its state
Backup cell also dead, or years old	If the batteries are flat too, the wing goes silent	Still safe: feather is the compliance link's job. The wing weathervanes edge-on and recharges on the next sun. Replace the cell at every refit so this stays theoretical
The whole pod goes dark	No active trim, no readings	The wing still feathers mechanically, the boat is safe, it just drifts until the sun returns

The single most important point: losing wing power is never a safety failure. The wing's safe state, feathered, is set by the compliance link and the hinge spring, and needs no power. And it is also the state in which the wing makes the most surplus solar. Everything above only protects performance and awareness.

10. Testing (folds into the section 11 campaigns)

Bench, power paths

1. Charge the wing from the hull through the transformer, and confirm it works at every wing angle through the full swing.
2. Confirm either battery alone runs the whole pod.
3. Confirm the one-way diodes cut out a failed battery.
4. Confirm the backup cell keeps sensors and radio alive with both batteries disconnected.

Bench, fail-safe

The two proof tests of the Trim Actuator Guide v9.6 sec 7, both mandatory, with the wing on the bench and again on the boat:

1. Test 1, the fuse opens: actuator rod locked at full extension, power off. Pull the tab toward straight with a hanging scale at the trailing edge. It moves at about 1.5 lbf and reaches near straight; the witness marks on the link do not move.
2. Test 2, the hinge spring centers the tab: pushrod disconnected, deflect the tab by hand and release. It returns to within 2 degrees of straight, both ways.

The old test (cut power, the tab springs straight) is deleted. With the link as the fuse, and with a self-locking actuator, it cannot pass and proves nothing.

Bench, watchdog

1. Pull the watchdog's Nano and confirm the wing still sails. The pass element defaults ON, so a dead watchdog changes nothing.
2. Force a software hang and confirm the result is one power cycle and a pod that comes back sailing on its own. Never a latch-off.

On the water and in the storm tests

After the component closure-verification battery and the high-wind buffet run (Build Guide sec 11.2; the build verifies watertight-at-any-angle by component flood and hose tests rather than a whole-boat inversion):

• Inspect every cell edge, every battery mount, and the fairing glands.
• Run one test with a cell deliberately lifted or removed, and confirm the wing still feathers and trims.

Sun test

Cover and uncover the cells, and confirm both faces feed the batteries and the charge controllers behave.

Coupler test

1. With the wing rotating slowly through full travel, log the charge current and confirm it stays continuous at every angle.
2. Then kill hull power and confirm the wing rides through on its own sources, with no reset anywhere.

11. Added parts list (wing power)

Item	Rough cost
Flexible back-contact solar cells, about 4 sq ft, plus clear encapsulation	$300
Two 24 V 6 Ah LiFePO4 packs (about 150 Wh, 2.9 lb each) with their BMS boards, bought; locker shelf mounting hardware	$300 (estimate; the order sheet has the bought price)
OmniCel ER34615 backup D-cell with leads, and holder	$30
24 V inductive coil pair bought on Amazon 2026-09-15; part number and rating to be logged at the next mail audit	$45
Pod watchdog (Nano, regulator, small parts, same recipe as the hull watchdogs)	$30
Solar electronics option A: two 5 A MPPT boards, 8 to 28 V (bought), and two DG7512 ideal-diode boards (one bought, one to buy)	in the line below
Pod pack heaters: two 24 V 10 W silicone pads, 2 x 2 in (bought), and two 24 V XH-W3001 thermostats	see Bill of Materials v9.6
Wing wiring conduit: 1/4 in OD sealed nylon tube, 100 ft (bought)	see Bill of Materials v9.6
Trailing-edge cable: tinned 18/4 shielded, 50 ft, on the buy list (Amazon B0D9JV83HJ, tinned, shielded). The bare-copper roll goes back	$36.99
Wiring, glands, conformal coat, and the solar electronics above	$220
Total	$925 (rough; the Bill of Materials v9.6 and the order sheet carry the bought prices)

Budget history and part substitutions are recorded in the central Changelog.