Hull and structure
A developable hull form chosen for what it lets you build and load, not for what looks fastest on a page.
A section chosen so the deck could be flat.
The envelope is 18.00 × 3.80 × 3.40 m with a small R200 radius at the deck corner and a large R1100 at the bilge. That asymmetry is the whole point: it yields a flat deck 3.40 m wide, which is what lets a 2.50 m hatch land on flat plate with margin instead of wrapping over the turn of the shell.
Longitudinally the hull is 3.00 m of entrance, 10.00 m of parallel middle body and 5.00 m of run. The payload sits in the parallel section, where the hull is fullest, the sections are constant, and construction is cheapest.
Plating is developable throughout — single-curvature corners, no compound curvature anywhere. Every plate can be rolled on one axis or formed cold, which keeps the vessel inside the capability of a general fabrication yard rather than a specialist submarine builder.
| 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 |
Scantlings
8 mm plate, 300 mm stiffening, eight frames.
The shell is 8 mm plate on stiffeners at 300 mm pitch with a 184.9 mm web, against a required section modulus of 45.6 cm³. Eight ring frames at 12 mm with a 220 mm web — 513 kg in total — carry the transverse strength.
The frames sit on the compartment bulkhead stations, so frames and bulkheads are the same structure. Cheaper to build, structurally cleaner, and it keeps every frame clear of every deck opening. A frame crossing a hatch is the kind of defect that only surfaces when someone tries to cut the hole.
Material is 5083-H116 aluminium at 215 MPa yield. Design stress is 70.3 MPa at a safety factor of 3.06; first yield depth is 30.4 m and collapse 54.7 m, against an operating depth of 20 m.
| 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 |
Weight and buoyancy
Solved, not estimated.
Submerged displacement is 82.94 t against a light ship of 46.19 t including a 10% margin, 19.75 t of fixed ballast and 17.0 t of cargo. Sealed volume is 68.20 m³; tanks run 34.71 m³ gross and 31.93 m³ net at 0.92 permeability.
KB is 1.909 m and KG 1.300 m, giving BG 0.609 m and a righting moment of 0.882 t·m per degree. Longitudinal centres of buoyancy and gravity are both at 8.431 m.
Surfaced, the vessel carries 17.4% reserve buoyancy on 14.10 m³ of main ballast, with 320 mm of dry deck. Because ballast is solved as part of the model rather than added at the end, changing the payload fit re-solves it instead of invalidating it.
Appendages
X-form planes, twin screws.
Control is by an X-form set at station 15.45 m, 1.10 m span and 1.55 m chord. The X arrangement gives redundancy a cruciform does not: any three surfaces can still control the vessel in both planes if one jams. On an uncrewed hull with nobody aboard to clear a fault, that is worth the extra control complexity.
The planes also sit inside the hull envelope in plan, so the vessel can sit on a cradle or be lifted in strops without the appendages taking load — which is what makes the travel-lift handling concept work.
Load cases
The boundary is designed for a fault, not for the sea.
Pressure balance means the compartment shell's design load is chosen rather than imposed. Naming the cases explicitly matters more here than on a conventional hull, because the whole architecture rests on which one governs what.
| Case | Differential | Condition | Governs |
|---|---|---|---|
| Normal | ±0.15 bar | The compartment tracks sea pressure to within the control band throughout the dive. | Governs nothing. An order of magnitude inside the design case. |
| Fault | +0.50 bar | Charge control has failed high and the relief valve has lifted. The compartment is over-pressurised against the sea. | Governs dog sizing and dog count. Uplift on one cover is 162.5 kN. |
| Design | 1.00 bar | The structural design differential — twice the relief setting and 6.7 times the control band. | Governs plate thickness and stiffener scantlings on the compartment shell. |
| Emergency | 2.011 bar | Total loss of pressurisation at operating depth: the compartment at one atmosphere with the full head outside it. Survival only. | Governs hatch cover thickness and stiffening. The cover is driven onto its seat at 66.6 t, and the dogs are unloaded. |
Safety factors
On the design case the allowable stress is yield divided by 2.00 — 107.5 MPa on the 215 MPa yield of 5083-H116. On the emergency case it is yield divided by 1.25, or 172.0 MPa. The emergency factor sits deliberately close to yield: that case is survival of a fault that should not occur, not continued operation, and pretending otherwise would buy weight the vessel cannot spare.
And it is still proof-tested to the full head
Every compartment is hydrostatically tested to 3.017 bar — 1.5 times the complete unbalanced head at operating depth — off the vessel, individually, before it is installed. The pressure-balance system reduces the load the structure carries on an ordinary dive. It does not reduce what the structure is tested to.
Frames and boundary
Eight frames, and none of them cross an opening.
The ring frames are section-matched to the hull, 12 mm thick with a 220 mm web, and they sit on the compartment bulkhead stations — which are also the stations the four-point compartment mounts land on. The load path from a loaded compartment into the hull is therefore direct, and the same piece of structure does both jobs.
That placement is deliberate. A frame crossing a 2.50 m deck opening is the kind of defect that only surfaces when somebody tries to cut the hole, and by then the arrangement is committed.
The compartment shell is 8 mm plate on 300 mm stiffening with a 184.9 mm web, sized on the design case and checked against the emergency case at a reduced factor.
Bring us the payload and we will size it.
We run your fit through the engineering model and come back with real numbers for it, not a generic figure.