HD-18
An 18-metre uncrewed diesel-electric submersible built around three dry, pressure-balanced payload compartments that load on the surface through the deck.
Overview
A hull shaped by what has to fit through the deck.
The envelope is 18.00 × 3.80 × 3.40 m with an R200 radius at the deck corner and R1100 at the bilge. That combination is deliberate: it yields a flat deck 3.40 m wide, so a 1.15 × 2.50 m hatch lands on flat plate with margin rather than overhanging the shell.
Displacement is 82.94 t submerged and operating depth is 20 m. The parallel middle body runs 10 m of the 18 m length, so payload volume sits where the hull is at its fullest and the ends are left to do their job.
| Length overall | 18.00 m |
| Beam | 3.80 m |
| Depth moulded | 3.40 m |
| Deck corner radius | R200 |
| Bilge radius | R1,100 |
| Flat deck width | 3.40 m |
| Entrance · parallel body · run | 3.00 · 10.00 · 5.00 m |
| Prismatic coefficient | 0.819 |
| Wetted surface | 273.5 m² |
| Envelope volume | 182.7 m³ |
Payload
Three sealed compartments, pressure-balanced.
Three compartments of 3.50 × 2.80 × 1.45 m — 14.21 m³ each, 42.63 m³ total — on four-point flange mounts, with SubConn/MCBH penetrations already in the boundary and dry 1-atm junction boxes inside.
The compartments are charged with air to sea pressure when submerged. The structure therefore carries a 1.00 bar design differential instead of the full 2.011 bar head, inside a ±0.15 bar control band with relief at 0.5 bar.
Balancing the pressure is what makes the volume affordable. The boundary is sized for 1.00 bar rather than the full head, so the structure stays light, the compartments stay dry at 1 atm, and payload integrators can work to ordinary topside practice inside them.
| Configuration | 3 sealed pressure-balanced compartments |
| Compartment envelope | 3.50 × 2.80 × 1.45 m |
| Volume per compartment | 14.21 m³ |
| Total payload volume | 42.63 m³ |
| Compartment pitch | 3.60 m · stations 2.70 to 13.50 m |
| Cargo deadweight | 17.0 t |
| Payload modules | 6 · 2 per compartment |
| Module envelope | 2.44 × 1.09 × 1.10 m · 4 mm skin |
| Module clearances | 60 mm wide, 60 mm long, 350 mm standing |
| Cargo per module | 2,833 kg |
| Lift mass per module | 3,071 kg |
| Module lifts | 6 scissor tables · 1,670 mm stroke, 250 mm collapsed |
| Reference cargo density | 399 kg/m³ |
| Mounting | Four machined flange mounts per compartment, landing on ring-frame stations |
| Connectors | SubConn / MCBH — 44 bulkhead and 44 cable ends, dry 1-atm junction boxes |
| Principle | Compartments charged with air to sea pressure when submerged |
| Design differential | 1.00 bar |
| Control band | ±0.15 bar · ±1.49 m of depth |
| Relief setting | 0.5 bar |
| Proof test | 3.017 bar — 1.5× the unbalanced head |
| Boundary plate | 8 mm on 300 mm stiffening |
| Design-case stress | 70.3 MPa · SF 3.06 |
| Loss-of-pressurisation stress | 141.4 MPa · SF 1.52 |
| Material | 5083-H116 · 215 MPa yield |
| Pressurised volume | 50.737 m³ · compartments and trunks |
| Compartment mass, each | 2,027 kg |
Deck access
Six hatches, dogged submarine-style.
Each hatch gives a 1.15 × 2.50 m clear opening, hinged on the outboard long edge, opening to 95° and balanced on two gas springs. Fourteen dogs per hatch at 19.31 kN bear on a machined seat with a 12 mm nitrile O-ring.
At operating depth the seated load is 653.6 kN — 66.6 tonnes — pressing the hatch onto its seat. The closure is fail-safe by geometry: the deeper it goes, the harder the sea holds it shut.
Opening condition
Surface only. Nothing on this vessel opens under water, and no mission profile on this site assumes otherwise.
| Hatches | 6 upward-hinging, dogged, seated plug closures |
| Clear opening | 1.15 × 2.50 m |
| Cover | 1.35 × 2.70 m · 8 mm plate on 186.5 mm webs |
| Dogs per hatch | 14 at 19.3 kN |
| Seal | 12 mm nitrile O-ring on a machined seat, 100 mm land |
| Opening angle | 95° |
| Balancing | 2 gas springs · 13.2 kN manual effort |
| Uplift, relief valve lifted | 162.5 kN — carried by the dogs |
| Seated load at depth | 653.6 kN · 66.6 t — carried by the seat |
| Opening condition | Surface only |
| Access trunks | 6 · 1.15 × 2.50 × 0.47 m |
Power and endurance
750 kWh submerged, a diesel for everything else.
A 750 kWh LFP pack — 600 kWh usable at 80% depth of discharge — drives the submerged leg: 486 km at 4 kn, 65.6 hours. A 150 kWe generator with 3,960 L of fuel covers transit: 4,528 km at 6 kn. Declared total range is 5,014 km.
The air plant moves 142.78 m³ of free air per cycle from 13 × 50 L bottles at 300 bar, recharging in 12.85 hours.
| Battery pack | 750 kWh LFP |
| Usable energy | 600 kWh at 80% depth of discharge |
| Diesel generator | 150 kWe |
| Fuel | 3,960 L · 3,326 kg · pressure-compensated |
| Specific fuel consumption | 0.212 kg/kWh |
| Quasi-propulsive coefficient | 0.68 |
| Propellers | 2 × 0.80 m diameter on 1.90 m centres |
| Control planes | X-form, 1.10 m span, 1.55 m chord |
| Hotel load, submerged | 2.0 kW |
| Hotel load, payload active | 8.0 kW |
| Snorkel duty | 30% at the transit condition |
| Submerged range | 486 km at 4 kn · 65.6 h |
| Diesel transit range | 4,528 km at 6 kn · 407.5 h |
| Declared total range | 5,014 km |
| Sprint speed | 12 kn · 79.1 km · 3.56 h |
| Bus power at sprint | 168.6 kW |
| Service life | 500 h between overhauls |
| Free air per cycle | 142.78 m³ |
| Compartment charge | 100.70 m³ |
| Main ballast blow | 42.07 m³ |
| Bottles | 13 × 50 L at 300 bar |
| Stored free air | 195 m³ |
| Compressor | 15 m³/h |
| Recharge time | 12.85 h |
| Design margin | ×1.35 on one full cycle |
| Charged-air mass | 123.4 kg — carried in the weight statement |
Structure
8 mm plate on 300 mm stiffening.
Eight ring frames at 12 mm with a 220 mm web, sited on the compartment bulkhead stations so that frames and bulkheads are the same parts — cheaper as well as structurally cleaner, and it keeps every frame clear of every deck opening.
Fixed ballast is 19.75 t, solved rather than assumed, giving BG 0.609 m with longitudinal centres coincident. Surfaced, the vessel carries 17.4% reserve buoyancy and 320 mm of dry deck.
| Shell plate | 8 mm |
| Stiffener pitch | 300 mm |
| Stiffener web | 184.9 mm |
| Ring frames | 8 at 12 mm, 220 mm web |
| Section modulus required | 45.6 cm³ |
| Operating depth | 20 m · 2.011 bar gauge |
| Yield depth | 30.4 m |
| Collapse depth | 54.7 m |
| Submerged displacement | 82.94 t |
| Light ship | 46.19 t incl. 10% margin |
| Fixed ballast | 19.75 t · 23.8% of displacement |
| Closure residual | 0.00 kg |
| KB | 1.9088 m |
| KG | 1.2997 m |
| BG | 0.609 m |
| Righting moment | 0.8817 t·m per degree |
| LCG − LCB | 0.0000 m |
| Main ballast tank volume | 14.10 m³ |
| Installed tankage | 34.71 m³ gross |
| Surfaced reserve buoyancy | 17.4% |
| Freeboard, surfaced | 320 mm of dry deck |
| Tank permeability | 0.92 |
Mast and communications
Three stages, its own hatch, a pressure-tight gland.
The mast stows at 1.6842 m inside a watertight trunk on the centreline and extends 4.693 m, putting the masthead 3.81 m above water at 1.20 m snorkel depth. It has a dedicated hinged access hatch carrying the gland, rather than sharing a payload opening.
Stated deviation — the long-range link
Approximately 50 bps usable at 10 km, with 1.5–3 s latency. That is telemetry and waypoint command. It is not remote control, and describing it as remote control would be a misstatement.
| Stages | 3 |
| Stowed length | 1.6842 m |
| Stroke | 4.693 m |
| Snorkel depth | 1.20 m |
| Masthead above water | 3.81 m |
| Trunk | 0.60 m wide, watertight, with a dedicated hinged access hatch |
| Gland | Pressure-tight, in the mast hatch |
| Links carried | Satellite broadband, cellular, satellite short-burst data, long-range LPWAN |
| Long-range link | Approximately 50 bps usable at 10 km, 1.5–3 s latency — telemetry and waypoint command, not remote control |
The complete specification
Ninety-eight rows across eleven groups, every one generated from the V10 master model.