An Introduction to Naval Architecture for Evaluating a Seastead

Section 0

First principles: floating, weight, and stability

Before the seven headline concepts, five ideas form the foundation of everything else.

Buoyancy and displacement

Archimedes’ principle: a floating body pushes aside water, and the water pushes back up with a force equal to the weight of the displaced water. A vessel floating at rest therefore displaces exactly its own weight. Seawater weighs about 64 lb/ft³ (1,025 kg/m³), so a vessel weighing 27,500 lb must have about 27,500 ÷ 64 ≈ 430 ft³ of its structure below the waterline. That volume is the displacement; the depth to which it sinks is the draft; the distance from waterline to deck is the freeboard.

The weight budget is merciless

A design floats at the waterline where total weight equals total buoyancy. Every battery, inverter, solar panel, watermaker, tool, and person is on the ledger. Naval architects track a lightship weight (the empty vessel) plus a deadweight allowance (everything added), always with a margin, because real projects reliably come out heavier than the drawing.

Centers and initial stability

Three imaginary points govern whether a hull stands up or capsizes:

G — the center of gravity: the weighted average position of all mass.
B — the center of buoyancy: the centroid of the submerged volume.
M — the metacenter: where the buoyancy force’s line of action crosses the vessel’s centerline when heeled a small amount.

The distance GM (metacenter minus center of gravity) is the measure of initial stability. Heel the ship a small angle φ and it experiences a righting moment of approximately Δ · GM · sin φ. Crucially, the lever arm BM = I_wp / ∇: the inertia of the waterplane area divided by displaced volume. Waterplane inertia grows with the square of distance, which is why spreading buoyancy far apart (as a trimaran or semi-submersible does) buys enormous stability from very little waterplane area.

Righting moment ≈ Δ · GM · sin φ BM = I_wp / ∇ Δ = displacement (mass) · I_wp = waterplane second moment of area · ∇ = displaced volume

Free surface effect

Liquid sloshing in a partially filled tank shifts to the low side when the vessel heels, effectively raising G and subtracting from GM. Large unfilled tanks are stability poison. Solid cargo bolted in place (like battery packs) has no free surface — one reason batteries make excellent low ballast.

Reserve buoyancy and subdivision

The watertight volume above the waterline is the reserve that keeps you afloat when things go wrong. Dividing the underwater volume into multiple airtight compartments means one puncture floods one compartment, not the whole vessel.

Applied to this design Batteries placed low in the three legs put G as low as physically possible — ideal for stability, with no free-surface penalty. Each leg having multiple airtight compartments means any single-point hull breach leaves two-thirds of buoyancy and all three legs’ structure intact. The wide triangular stance (corners ~25 ft from the center) makes the waterplane inertia — and therefore GM — very large even though the waterplane area is tiny. That combination (tiny area, huge inertia) is the signature of the whole design and recurs in every section below.

Section 1

Resonant roll period (and its siblings)

Any floating body is a collection of springs and masses. Push it down and it bobs back; heel it and it rolls back. Like any spring–mass system it has a natural period: the time it takes to complete one free oscillation after a disturbance.

T = 2π √( I_effective / k ) Natural period = 2π × √( inertia ÷ stiffness ) — true for heave, roll, and pitch alike

For roll, the stiffness is the righting moment per degree of heel (Δ · GM) and the inertia is the mass moment of inertia about the roll axis, plus the added inertia of the water that must be dragged along (typically 10–30% extra for hulls, more with deep appendages). A convenient classic form uses the radius of gyration k_r:

T_roll = 2π · k_r / √( g · GM ) Small sailboats: 3–5 s · Ships: 8–18 s · Wide multihulls: short and stiff

Why resonance matters

Open-ocean wave energy is concentrated roughly in the 5–14 second band (Caribbean trade-wind seas: ~6–9 s; long-range swell: 10–14 s). If a vessel’s natural period sits inside that band, ordinary waves pump it rhythmically and the motion builds until damping limits it. Hull damping is weak — a damping ratio of only 0.05–0.10 is typical — so resonant amplification can reach 1/(2×0.07) ≈ 7×. A “moderate” sea can produce violent rolling.

The designer’s levers are:

Tune the period out of the wave-energy band — change GM or inertia.
Add damping — bilge keels, active fins, anti-roll tanks, heave plates.
Manage encounter frequency — speed and heading change how often waves hit you. Heading into the waves shortens the encounter period; running with them lengthens it. The dangerous condition is when the encounter period matches the natural period, and following seas add sneaky failure modes (surf-riding, broaching, parametric rolling).

Deep dive: encounter frequency

In deep water, the frequency at which crests pass a moving vessel is ω_e = ω_w − (ω_w²/g) v cos μ, where μ is the angle between heading and wave travel (μ=0 head seas). This is why the same sea feels completely different on different courses, and why a speed/heading change is the cheapest “stabilizer” aboard.

Heave and pitch obey the same law

Vertical bouncing (heave) is a spring–mass system whose stiffness is set by the waterplane area; pitching is set by the waterplane inertia. Soft vertical springs mean long heave periods and gentle accelerations — the heart of the next section.

Applied to this design The tiny waterplane makes heave and pitch very soft (long periods — good), while the wide leg spacing keeps roll stiffness strong. Expect a comparatively quick, firm roll (wide multihull character) but gentle vertical motion — the opposite of a rolling barge. When the three helical mooring screws are set and the platform is pulled down 3 ft, the tethers replace the waterplane as the spring: heave, roll, and pitch all become extremely stiff and the platform essentially tracks the local mean sea surface. Parked mode and underway mode are dynamically two different machines, and both should be analyzed separately.

Section 2

Small waterplane area

The waterplane area A_wp is the area enclosed by the vessel’s outline at the waterline. It is the piston area of a hydraulic spring: push the vessel down by dz and buoyancy increases by ρ g A_wp dz.

k_heave = ρ g A_wp T_heave = 2π √( (m + m_a) / (ρ g A_wp) ) m_a = added mass of entrained water — often comparable to or larger than the vessel mass for submerged forms

The waterplane area acts as a spring of stiffness ρgA_wp supporting the vessel mass (plus added mass).

Reading the spec sheet: deriving A_wp from the design’s own numbers

This design states that a 1-foot change in water level equals about 1/7 of total buoyancy, with 27,500 lb of buoyancy at the design waterline. That single sentence fully determines the waterplane area:

Worked example — waterplane area

Buoyancy gained per foot of immersion: 27,500 ÷ 7 ≈ 3,930 lb per foot.
Since each cubic foot of seawater weighs 64 lb, the area is 3,930 ÷ 64 ≈ 61 ft² (about 5.7 m²), split across three legs (~20 ft² each).

Handy rule of thumb: ≈330 lb per inch of sinkage. Add a 1,000 lb tender of gear and the seastead settles about 3 inches. A comparably-sized houseboat barge might have 200–400 ft² of waterplane — this design is roughly 5–10× softer.

What small waterplane buys you

Long natural heave/pitch periods — the vessel barely notices waves shorter than its period; it rises and falls slowly, like an oil platform.
Low vertical accelerations — the quantity that drives seasickness (motion sickness onset climbs sharply around 0.05–0.1 g of vertical acceleration) and fatigue of people and equipment.
Weak wave forcing — wave pressure fluctuations decay exponentially with depth (amplitude ∝ e^−2πz/λ). Buoyancy generated well below the surface lives in nearly still water.

What small waterplane costs you

Load sensitivity — 330 lb per inch means weight discipline and careful distribution; asymmetry shows up as visible heel or trim.
Stability must come from spacing, not area — GM survives only because the legs are far apart (see Section 0) and G is kept low.
Varying freeboard — fuel, water, and guest lists visibly change the waterline; reserve buoyancy margins must absorb it.
Deck clearance management — the structure between the legs must clear wave crests, or take slamming loads.

Added mass: the hidden half of the ride quality

When a body accelerates in water, it must accelerate a parcel of water too. That “extra inertia” is added mass, and for broad, flat, submerged surfaces moving normal to their faces it can be several times the structure’s own mass. This is exactly what heave plates exploit: bolt-on horizontal plates near the bottom of each leg are edge-on to forward motion (almost no drag) but face-on to heave (large added mass and damping), deliberately lengthening the heave period and killing resonant buildup.

Evaluator’s caution — validate the ride claim numerically Using static numbers only (m ≈ 12,500 kg, A_wp ≈ 5.7 m²), the heave period computes to a short ~2.9 s — which sounds bad. The design’s ride quality therefore rests heavily on added mass from the heave plates and submerged fins: with effective mass 4–9× lightship, the same formula yields a comfortable ~6–9 s. Which side of that line you land on depends on actual plate sizes and depths. This is the single most important number to verify with a proper seakeeping analysis (strip theory, CFD, or model tests) before committing to the build.

Section 3

Drag: moving through the water

Push a hull through water and the water pushes back. Total resistance is the sum of several physically distinct taxes:

Friction drag — viscosity dragging the wetted surface. Proportional to wetted area S; the price you pay on every knot, forever.
Form (pressure) drag — flow that separates leaves a low-pressure wake behind the body. Minimized by rounded leading edges and long, gradual tails.
Wave-making drag — energy radiated away as gravity waves. Paid only by things near the surface, and it explodes with speed.
Induced drag — the cost of generating lift (relevant if any element works as a foil).
Interference & appendage drag — junctions, struts, pipes, ladders, plates: small items, real sums.

R_F = ½ ρ v² S C_f C_f = 0.075 / (log₁₀ Re − 2)² ITTC 1957 friction line · Re = vL/ν (seawater ν ≈ 1.05×10⁻&sup6; m²/s)

F_n = v / √( g L ) Froude number — the master variable for wave-making. Displacement hulls hit a “wave wall” near F_n ≈ 0.4–0.5.

Friction grows gently with speed; wave-making grows explosively once the vessel nears its hull-speed regime.

The seastead’s trump card: submerged buoyancy makes almost no waves

Wave-making drag is a surface phenomenon. The pressure field of a moving submerged body fades exponentially toward the surface, so a deeply submerged pontoon generates essentially zero wave drag regardless of speed. Only the slender waterline-piercing columns pay the wave toll. This is why semi-submersibles and SWATH ships can carry heavy payloads at respectable speeds without planing — and why this design’s foil-shaped legs, with only small cross-sections at the waterline, are fundamentally low-wave-drag.

Shape choices that reduce drag

Rounded leading edge presented forward (tolerates small heading errors gracefully).
Legs aligned with the direction of travel — far kinder than the crossed cylinders of a conventional semi-submersible.
Heave plates mounted edge-on to the flow.
Smooth, fouled-free surfaces: slime and barnacles can double friction drag. Antifouling strategy is a propulsion decision, not a cosmetic one.

Evaluator’s caution — audit the appendages Small items broadside to the flow add up fast. Illustrative check: a ~2-inch electrical conduit running down the trailing edge of each leg into the water is a cylinder in crossflow (C_d ≈ 1.0–1.2). Three such conduits, ~12 ft submerged each, present ~6 ft² of frontal area — roughly 1,000 lb of drag at 8 knots, comparable to the entire bare-hull friction of the vessel. Fairing the conduit into the trailing edge (or using a streamlined conduit) is cheap insurance. Run the same audit over ladder rungs, plate brackets, and anodes.

Worked example — order-of-magnitude power budget

Assumptions (illustrative): ~600 ft² submerged wetted surface (lower halves of three legs plus plates), form factor 1.3, appendage allowance 30%, modest wave drag from the small waterline intersections, propulsive efficiency 60%.

Speed	Total resistance (est.)	Power at shaft	Electrical input
5 kn	~250–300 lb	~4 hp (≈3 kW)	~5 kW
8 kn	~700–800 lb	~18 hp (≈13 kW)	~22 kW

Energy reality check: 25% of 27,500 lb in LiFePO4 ≈ 3,100 kg → roughly 400 kWh. That supports ~80 hours at 5 kn (~400 nmi) or ~17 hours at 8 kn (~135 nmi). The roof of a 44-ft equilateral triangle is ~840 ft² (≈78 m²) → ~15 kWp of solar → roughly 70–90 kWh/day in the Caribbean — enough to cruise several hours daily at 5 kn indefinitely, or to recover a sprint in a day. All figures are order-of-magnitude, but they show the design’s energy story is coherent — provided appendage drag is kept honest.

Deep dive: why model tests work at all

You cannot match both Reynolds and Froude numbers on a scaled model. Naval architecture’s elegant workaround (Froude’s method): test at the correct Froude number so wave-making scales properly, compute the model’s friction analytically from the ITTC line, subtract it, scale the remainder up by the cube of length ratio, then add back full-scale friction. Every tow-tank result you will ever read uses this trick.

Section 4

Wind drag

Air is 1/800th as dense as water, but it blows far faster than any current and pushes on everything above the waterline. For a lightweight seastead, wind is a first-order load, not a footnote.

F_wind = ½ ρ_air v² A C_d ρ_air ≈ 0.00238 slugs/ft³ · A = projected area · C_d = shape coefficient (see Section 7)

Dynamic pressure cheat sheet

Wind speed	Dynamic pressure q = ½ρv²	Force on 100 ft² at C_d=1.2
10 kn	0.34 lb/ft²	41 lb
20 kn	1.36 lb/ft²	163 lb
30 kn	3.05 lb/ft²	366 lb
40 kn	5.42 lb/ft²	650 lb
60 kn	12.2 lb/ft²	1,465 lb
90 kn	27.5 lb/ft²	3,300 lb

Worked example — the living area as a sail

The 44 ft × 7 ft wall presents ~308 ft². As a bluff box (C_d ≈ 1.2): at 60 kn the force is 12.2 × 308 × 1.2 ≈ 4,500 lb; at 90 kn, ~10,000 lb — against a 27,500 lb vessel. Wind alone can shove the whole seastead hard enough to dominate mooring design. Remember also that force scales with v²: a 40% gust increase doubles the load.

What wind actually does to a vessel

Leeway — steady drift downwind, balanced by the underwater lateral resistance of the legs.
Weathervaning — if the windage center (above water) and the lateral resistance center (below water) don’t align, the vessel yaws until they do. Designers deliberately place these centers to choose the vessel’s preferred attitude.
Mooring load — wind + current + wave drift, summed, is what the anchors must hold. Illustrative check: a 2-knot Caribbean current pushing on ~80 ft² of submerged frontal area adds ~1,000 lb before any wind blows.

Reducing the bill

Minimize projected area and round corners where practical.
Use porous surfaces: open grating walkways and railings pass air (and waves), with effective drag scaled by their solidity ratio.
Shield windage behind other structure — a dinghy tucked behind the house sees a fraction of the free-stream wind.
Plan the storm posture: orient the smallest face to the wind, secure loose items, and get on the moorings before the blow, not during it.

Applied to this design The 7-foot house on a 27,500-lb platform is a modest sail for its size; flush-mounted solar adds little; the aluminum grating walkway is both wave-transparent and largely wind-transparent; the RIB rides sheltered behind the house underway. The main unavoidable item is the house itself — worth computing its exact moment arm against the underwater lateral center so the seastead lies predictably at anchor. When parked, the helical-screw tension legs convert wind load from a motion problem into a static anchoring problem, which is exactly the right trade.

Section 5

Active stabilizers

Passive design sets the natural periods and damping (Sections 1–2). Active stabilizers go further: they sense the motion and apply counter-forces, phased ahead of the wave, to cancel it. Choosing among them is a trade of speed requirement, power, weight, complexity, and failure modes.

Type	Works at zero speed?	Typical roll reduction	Costs & cautions
Bilge keels (passive)	Yes	15–25%	Tiny drag penalty; nearly foolproof.
Anti-roll tank (passive/active)	Yes	30–60%	Narrowband — must be tuned to the roll period; weight placed high; free-surface penalty if badly designed.
Active fins	No (needs flow)	60–90% underway	Useless at anchor; drag; vulnerable to impact; a jammed fin is a hazard — need fail-safe feathering.
Gyroscopic (CMG)	Yes	50–80%	Heavy; significant spin-up power; maintenance; excellent retrofit option.
Thruster-based	Yes (limited authority)	Yaw/surge mainly	Great for course-keeping, station-keeping, and multi-vessel coordination; rarely enough authority for wave-frequency roll.
Tension mooring (parked)	Parked only	>90% (all axes)	The ultimate stabilizer is not being free to move. Requires seabed anchorage and pretension management.

Control essentials

Sense with an IMU (rate gyros + accelerometers), filter, and lead the phase — the counter-force must arrive before the wave crest does.
Respect actuator saturation and rate limits; a stabilizer that clips is worse than one that’s absent, because partial cancellation at the wrong phase amplifies motion.
Fail safe: any active surface must default to a neutral, low-drag state.

Applied to this design Six fixed rim-driven thrusters with independent per-leg power give genuine differential-thrust authority: yaw control in harbor (reverse one side, forward the other), course-keeping underway, and — elegantly — coordinated motion damping when two seasteads share a connecting walkway, with both computers working the thruster pairs to quiet the walkway. What the design deliberately lacks is wave-frequency roll/pitch actuation; instead it leans on geometry (Section 2), heave plates (passive added mass and damping), and — when parked — tension legs, which outperform any active system ever built. If underway comfort ever falls short, the retrofit paths (gyro, or active fins faired into the legs) exist, at known power and weight prices.

Section 6

Semi-submersible platforms

A semi-submersible concentrates buoyancy in deeply submerged bodies (pontoons or lower hulls) and connects them to the payload on top via slender columns that pierce the waterline. The recipe delivers three wins at once:

The buoyancy hides from the waves. Wave-induced pressure and orbital water motion decay exponentially with depth, so submerged displacement volume sits in nearly quiescent water.
The waterline is tiny. Small waterplane area means long natural periods and soft spring forces (Section 2).
Stability comes from spacing, not area. Widely separated columns give enormous waterplane inertia — hence large GM — from almost no waterplane area (Section 0).

Semi-submersible anatomy: deck on slender columns, buoyancy deep, heave plates below.

A proven family tree

Offshore drilling & production semis — workhorses of the oil industry since the 1960s, moored for years in open ocean.
Floating wind turbines — semi-sub and spar platforms now host multi-megawatt turbines; the motion criteria are strikingly similar to habitability criteria.
SWATH ships (Small Waterplane Area Twin Hull) — passenger ferries and research vessels that keep operating in seas that stop conventional hulls, at the price of meticulous weight control.

The design levers

Column spacing ↑ stability, ↑ structural bending moments and steel weight.
Pontoon depth ↓ wave excitation (exponentially), ↑ cost and access difficulty.
Heave plates — added mass and viscous damping; longer period, less resonance.
Waterline crossings — the splash-zone welds are fatigue-critical and corrosion-prone; detail design lives or dies here.
Variable load — ballast systems traditionally compensate for changing weights; fixing heavy items (batteries) low simplifies the problem enormously.

The tension-leg variant (a mini-TLP at anchor)

Pre-tensioned vertical tethers pull the platform down, making the tethers the vertical spring instead of the waterplane. Heave, roll, and pitch become extremely stiff — the platform tracks the mean sea surface almost perfectly. Classic TLP practice keeps the tether-stiffness natural period very short (well below wave periods) and guards against slack tethers in extreme troughs, which cause snap loads.

Applied to this design This seastead is a textbook small semi-submersible: three foil-shaped pontoons (the legs), minimal waterline intersection (~61 ft² total), heave plates for added mass and damping, and stability from the wide triangular column spacing. Its two operating modes map cleanly onto offshore practice:

Underway / free-floating = semi mode. Soft vertical springs, long periods, modest wave drag.
Parked = mini-TLP mode. Three helical screw pairs with motors between them provide tethers; pulling down 3 ft stores 3 ft × 3,930 lb/ft ≈ 11,800 lb of pretension (~3,900 lb per corner). The 3-ft margin is sized against micro-tidal Caribbean range plus protected-water wave setup — sound logic, provided the sites truly never see swell or storm surge that could exceed the margin and slack a tether. Verify per-site; slack-then-snap is the one failure mode this scheme must never be allowed.

Section 7

Coefficient of drag due to shape

All drag laws funnel through one dimensionless number:

C_d = F / ( ½ ρ v² A ) The shape’s entire aerodynamic/hydrodynamic personality, compressed into one number

The trap: C_d is meaningless without knowing which reference area A it uses — frontal projected area, planform (chord) area, or wetted surface all appear in the literature. Always check the fine print before comparing numbers.

A field guide to shape coefficients

Shape	C_d (approx.)	Reference area / notes
Streamlined body (fineness 3–5)	0.04–0.10	frontal — the benchmark for “slippery”
NACA foil section, axial flow, 0° AoA	0.006–0.012	chord × span (2-D section data)
Same, converted to wetted-area basis	0.003–0.005	÷ by (wetted/chord ≈ 2.2–2.6) — same physics, different bookkeeping
Sphere	0.47	frontal
Cylinder, axis across flow	1.0–1.2	frontal (subcritical Re)
Cube, face to flow	≈1.05	frontal
Box / building / house	0.8–1.4	frontal
Flat plate normal to flow	1.1–1.3	frontal
Foil section in crossflow (broadside)	1.0–1.4	frontal (thickness × span) — the mooring-load case
Automobile	0.25–0.40	frontal
Parachute	1.3–1.5	frontal

Why shape matters so much

Separation is the enemy. Bluff shapes force flow to detach early, leaving a broad low-pressure wake: pressure drag dominates. Streamlined shapes keep flow attached to a long, gradual tail, shrinking the wake to nearly nothing.
Rounded noses forgive attitude errors. A blunt face is efficient only exactly head-on; a rounded leading edge degrades gracefully at small yaw angles — valuable for anything that won’t always point perfectly into the flow.
Reynolds number shifts the numbers. Around Re ≈ 2–5×10⁹ cylinders and spheres undergo the “drag crisis”: the boundary layer turns turbulent, separates later, and C_d drops abruptly. At vessel scales you are almost always past the crisis — use the high-Re values.
Junctions and stubs leak drag. Leg-to-deck corners, exposed conduit, ladder recesses, and plate brackets each add interference drag; fillets and fairings buy it back.

Applied to this design The legs are NACA 0035 sections presented nose-first: in axial flow they operate near the bottom of the entire C_d table, and the rounded leading edge tolerates crab-angle operation. The 0.5-ft trailing-edge truncation (to fit the shipping container) leaves a small blunt base — a modest penalty at these scales, and a candidate for a cheap fairing cap. The unavoidable bluff body is the living-area prism (C_d ≈ 1.1–1.3 in wind), which argues for minimizing its frontal profile and rounding external corners where practical. For mooring and current loads, remember the legs in crossflow are bluff bodies (C_d ≈ 1.0–1.4 on frontal area) — the same elegant shape that slips forward resists sideways flow like a wall, which is exactly what you want for holding position.

Section 8

Pulling it together: reading this seastead design

Every major feature of the design maps onto one or more of the concepts above. Use this table as a bridge between the feature list and the questions a naval architect would ask.

Design feature	Concept	Question it obliges you to answer
Three foil-shaped legs, blunt edge forward	§3 drag, §7 shape	What is total resistance at cruise and sprint? Are all appendages faired?
Waterplane: 1 ft = 1/7 of buoyancy (≈61 ft²)	§2 small waterplane	What is the real heave period once added mass is included?
Wide triangular leg spacing	§0 stability, §1 roll	What are GM and the roll period, upright and damaged?
Bolt-on heave plates, low on each leg	§1 damping, §2 added mass, §6 semi	How much added mass/damping do they actually contribute?
Batteries (25% of displacement) low in legs	§0 stability	Where is G with batteries full, half, and what if one leg’s pack floods?
Multiple airtight compartments per leg	§0 reserve buoyancy	What is the damaged-waterline and residual GM after losing any one compartment?
Grating walkway and railing	§2 wave passage, §4 wind porosity	What wave climate passes through unimpeded, and what solidity ratio did you assume for wind?
Six rim-driven thrusters, differential steering	§3 propulsion, §5 active control	Is installed thrust ≥ resistance at sprint speed plus wind margin? What is the control bandwidth?
No through-hulls; conduit on trailing edge	§0 flooding, §3 appendage drag	Has the conduit’s crossflow drag been faired or accounted for?
Helical-screw tension legs, 3 ft pull-down	§6 TLP mode	Can any site condition (swell, surge, tide) exceed 3 ft and slack a tether?
Dinghy tucked behind the house	§4 wind shielding	What is its exposed windage at anchor vs underway?
Two-seastead walkway with coordinated thrust	§5 active stabilization	What relative-motion criterion must the control loop meet, and with what sensor suite?

Section 9

The evaluator’s checklist

Twelve questions that expose almost any weakness in a seastead proposal. If a designer can answer all twelve with numbers — not adjectives — the design deserves serious attention.

Weight ledger: What is the lightship weight, the deadweight budget, and the margin? Where is the center of gravity, vertically and longitudinally?
Stability: What are GM and the full righting-arm (GZ) curve, intact and with the worst single compartment flooded?
Natural periods: What are heave, roll, and pitch periods — computed with real added mass — and how do they compare to the site’s wave spectrum?
Load sensitivity: How many inches per 1,000 lb, and what is the plan for asymmetric loading?
Reserve buoyancy: How much watertight volume sits above the design waterline, and what damage scenario defines the minimum?
Windage: What is the total projected area inventory with component C_ds, and what is the survival-wind design point?
Resistance & power: What is total drag at cruise and sprint — including appendages — and does installed thrust cover it with margin?
Energy balance: Do solar input, battery capacity, and hotel loads close the budget in a cloudy week?
Mooring: What combined wind + current + wave-drift load must the anchors hold, and what is the tether slack margin in the worst forecast site conditions?
Structures: Are slamming, green water, splash-zone fatigue, and corrosion each addressed with a named design load case?
Habitability: What are predicted vertical accelerations at the berths (motion sickness begins around 0.05–0.1 g), and noise levels from thrusters and inverters?
Verification: Which claims rest on calculation, which on CFD or model tests, and which on hope? (Hope is not a marine standard.)

Recommended verification sequence for this design

Hydrostatics + intact/damaged GZ curves from the actual 3-D model.
Seakeeping prediction (strip theory or CFD) of heave/roll/pitch periods and responses in Caribbean trade-wind seas and swell — the make-or-break analysis for the “soft ride” claim.
Resistance estimate with a full appendage audit (conduit!, plates, brackets), then a tow-tank or self-propulsion check if the budget allows.
Wind-load estimate of the topsides (CFD or wind tunnel) for mooring sizing and weathervane behavior.
Tether analysis per anchorage: pretension, slack margin, and cycle fatigue of the helical screw connections.

Appendix

Glossary

Added mass (m_a): The inertia of water that must move with an accelerating body; often comparable to the body’s own mass for broad submerged surfaces.
A_wp (waterplane area): The area enclosed by the hull at the waterline; sets heave stiffness (ρgA_wp).
B, G, M: Center of buoyancy, center of gravity, metacenter — the three points of initial stability.
Displacement (Δ, ∇): The weight of the vessel (equal to the weight of water displaced), or equivalently the displaced volume.
Draft / freeboard: Depth of the hull below the waterline / height of the deck above it.
Froude number (F_n): v/√(gL); the governing similarity parameter for wave-making.
GM (metacentric height): Initial stability measure; righting moment ≈ Δ·GM·sin φ.
Heave plate: Horizontal submerged plate adding heave added-mass and damping; edge-on to forward motion.
Reynolds number (Re): vL/ν; the governing similarity parameter for viscous (friction) effects.
Rim-driven thruster: Propulsor whose motor is a ring integrated into the duct around the impeller — no shaft, no hull penetration, compact footprint.
SWATH: Small Waterplane Area Twin Hull — a ship built on the same principle as a semi-submersible.
Tension leg (TLP): Mooring with pre-tensioned vertical tethers that make heave/roll/pitch extremely stiff.
Waterplane inertia (I_wp): Second moment of the waterplane area; sets pitch stiffness and, via BM = I_wp/∇, much of GM.