Doc #137 — Cook Strait Link

---

RECOMMENDED ACTIONS (BY URGENCY)

First week:

1. Place ferry fuel allocation under national fuel rationing authority (Doc #53). Ferry operations are a strategic national priority — not a commercial decision.
2. Immediately reduce service frequency to conserve fuel: move from 8–12 daily crossings to 3–4, running only full or near-full sailings.
3. Implement cargo prioritisation (Section 4): food, medical supplies, and essential industrial inputs only. No private vehicles except by permit.
4. Begin inventory of all Cook Strait-capable vessels in the Wellington–Marlborough region.

First month:

5. Assess all rail-capable vessels — NZ’s rail ferries (Interislander’s vessels have rail-capable decks) are uniquely valuable because they connect the North Island and South Island rail networks without requiring cargo handling at each end.⁴
6. Identify all large sailing vessels, coastal traders, fishing vessels, and barges in the Wellington–Marlborough–Nelson region capable of strait crossings.
7. Establish a Cook Strait Transport Authority with scheduling and prioritisation power.
8. Begin sail-assist retrofit assessment for existing ferry or coastal vessels (Section 5).

First year:

9. Begin construction or conversion of sail-powered Cook Strait cargo vessels (Section 5, Doc #138, Doc #141).
10. Develop tug-and-barge operations using smaller powered vessels to move unpowered barges across the strait — more fuel-efficient per tonne of cargo than running full ferries below capacity.
11. Establish coastal feeder services: smaller vessels bringing South Island cargo to Picton from Lyttelton, Timaru, Bluff, and Dunedin, for consolidated strait crossing.
12. Investigate electric or hybrid propulsion options using NZ’s grid power (Section 5.3).

Years 2–5:

13. Commission first purpose-built sail-powered inter-island cargo vessels.
14. Develop alternative crossing points if warranted (Section 6).
15. Phase ferry service down in proportion to declining fuel availability; phase sail and electric services up.

---

ECONOMIC JUSTIFICATION

The cost of losing the link

The inter-island connection is not a single industry — it is the connective tissue of a geographically divided country. Quantifying its value requires estimating what NZ loses without it.

Food supply: The South Island produces roughly 55–65% of NZ’s dairy output and the majority of its grain, lamb, and beef.⁵ Under nuclear winter, with pastoral production declining 30–60% (Doc #74), the North Island becomes even more dependent on whatever South Island food surplus exists. If the strait link fails, the North Island must feed ~4 million people from its own reduced production — achievable but with a much thinner margin and requiring faster, more disruptive agricultural restructuring.

Manufacturing and industrial supply: The North Island holds NZ Steel (Glenbrook — Doc #89), most of NZ’s engineering workshops, the majority of pharmaceutical stocks, and the government. South Island industries (including the Tiwai Point aluminium smelter, engineering workshops in Christchurch, and agricultural processing) depend on North Island inputs.

Person-years of ferry operation vs. alternatives: Maintaining a single ferry on a reduced schedule (2 crossings per day) requires a crew of approximately 30–40 per vessel (operating in shifts).⁶ This moves roughly 500–1,500 tonnes of cargo per day. Replacing this throughput with small sailing vessels carrying 10–30 tonnes each would require 15–50 one-way crossings per day — impractical in the early phases, though a sail fleet develops over time to supplement and eventually replace powered ferries.

The honest assessment: There is no near-term substitute for powered ferries. The transition to sail is measured in years, not months. Ferry fuel allocation is therefore one of the highest-priority uses of NZ’s diesel reserves — arguably second only to agricultural machinery and emergency services. Every litre of diesel burned moving a ferry across Cook Strait moves more cargo than any alternative use of that litre.

Fuel cost

At 3,000–5,000 litres per crossing, fuel cost depends heavily on service frequency. At reduced service — 1–2 crossings per day rather than the pre-event 8–12 — the strait link consumes approximately 3,000–10,000 litres of diesel daily, or roughly 1–3 million litres per month. Against NZ’s estimated diesel stocks of 400–600 million litres (Doc #53), this is 0.2–0.8% of total diesel per month — a genuinely modest draw.⁷

The per-crossing cost is modest, but diesel has many competing critical uses. Agriculture, emergency services, fishing, generators, and military operations all draw on the same diesel pool. Doc #53 models total fuel stocks (all types) lasting 6–24 months under strict rationing, with diesel consumed after petrol (which degrades faster). The ferry’s 1–3 ML/month draw competes favourably on a per-litre basis — every litre moves more cargo across Cook Strait than almost any alternative use — but the total diesel pool is finite and shrinking. Doc #53 plans for biocide treatment of diesel stocks to prevent microbial degradation, extending usable shelf life to 2–5 years with proper storage, so degradation is not the binding constraint it might appear.⁸

Realistic planning horizon: A reduced ferry service is sustainable for 1–5 years depending on how aggressively other sectors transition away from diesel and how quickly alternative fuels (wood gas, biodiesel) displace diesel in agriculture and land transport. If those transitions succeed and free up diesel, the ferry could run for the longer end of that range. If they stall, diesel competition forces the ferry service down sooner. Mechanical failure of the ferry fleet (Section 1.3 estimates 3–10 years without imported spares) is the other constraint — whichever hits first ends powered service. Sailing vessel alternatives must be developed in parallel regardless.

---

1. THE EXISTING FERRY FLEET

1.1 Current operations

As of 2025–2026, Cook Strait ferry services are operated by two companies:⁹

Interislander (KiwiRail): - Operates between Wellington and Picton - Fleet has included vessels such as the Kaitaki (the largest, approximately 22,000 gross tonnes, capacity ~1,600 passengers and ~60 trucks or equivalent rail wagons) and smaller vessels - Crossing time: approximately 3 hours 20 minutes - KiwiRail’s ferries include rail-capable decks, connecting the North Island Main Trunk Line to the South Island Main North Line — this is a unique capability

Bluebridge (StraitNZ): - Operates between Wellington and Picton - Smaller vessels than Interislander’s largest - Crossing time: approximately 3 hours 30 minutes - Road freight and passengers only (no rail capability)

Combined fleet capacity: Approximately 3,000–5,000 lane metres of vehicle deck space per day at full service levels. This translates to roughly 3,000–8,000 tonnes of cargo per day depending on cargo type and loading efficiency.¹⁰

1.2 Fuel dependency

All current ferries are diesel-powered. Marine diesel consumption varies by vessel size, speed, loading, and sea conditions. Larger vessels are more fuel-efficient per tonne of cargo moved but consume more total fuel per crossing. Reducing speed reduces fuel consumption roughly in proportion to the cube of the velocity ratio — slowing from 18 knots to 14 knots reduces fuel consumption by approximately 40–50%, at the cost of extending crossing time from ~3.5 hours to ~4.5 hours.¹¹

Implication: An early fuel-saving measure is to reduce ferry speed. This requires no engineering modification — the vessel runs at reduced throttle — but extends crossing time and reduces daily throughput if the number of crossings is held constant. Combined with reduced service frequency, fuel consumption can be cut to approximately 20–30% of pre-event levels while maintaining 1–3 crossings per day. The lower end of this range assumes maximum speed reduction (to ~14 knots) and minimum crossings (1/day); the upper end assumes moderate speed reduction and 2–3 crossings per day.

1.3 Maintenance and spares

Ferry maintenance depends on specialised marine engineering — propulsion systems, navigation electronics, hull maintenance, safety systems. NZ has shipyard capability in Auckland and Lyttelton, and some maintenance can be performed at the Wellington and Picton ferry terminals. However, critical spares (propulsion components, control electronics, safety equipment) are largely imported. As spares deplete, ferries will eventually become unreliable and then unserviceable.¹²

Estimated operational life without imported parts: Uncertain, but probably 3–10 years with careful maintenance, cannibalisation of parts between vessels, and engineering improvisation. The variability is large because it depends on which components fail first and whether NZ engineering can fabricate substitutes. Major propulsion failure (crankshaft, gearbox, propeller shaft) on a vessel with no available replacement effectively retires that vessel.

Implication: The ferry fleet is a depleting asset. Every year of operation brings it closer to retirement. Planning for sail and electric replacements must begin immediately, even though the ferries will remain the primary service for years.

---

2. COOK STRAIT: PHYSICAL CHARACTERISTICS

2.1 Geography

Cook Strait at its narrowest point is approximately 22 km wide, between Cape Terawhiti (North Island) and Perano Head (South Island).¹³ The ferry route between Wellington and Picton is longer — approximately 92 km — because it enters the Marlborough Sounds through Tory Channel or Queen Charlotte Sound to reach Picton.¹⁴

The strait connects the Tasman Sea to the west with the Pacific Ocean to the east. The land on both sides rises steeply, funnelling wind through the gap.

2.2 Weather and sea conditions

Cook Strait has a well-deserved reputation as one of the rougher stretches of coastal water in the world. Key hazards:¹⁵

• Wind: The prevailing westerly and north-westerly winds are funnelled and accelerated through the strait. Gale-force winds (34+ knots) occur on approximately 40–50 days per year. Southerly storms are particularly dangerous, producing steep, short-period waves.
• Tidal currents: Strong tidal flows through the strait, reaching 4–5 knots in narrow sections. These currents interact with wind-driven waves to produce confused, steep seas — particularly when wind opposes tide.
• Wave conditions: Significant wave heights of 2–4 metres are common; 6–8 metre seas occur during storms. The short fetch from some directions produces steep, closely spaced waves that are harder on vessels than the longer-period ocean swells of the open Tasman.
• Visibility: Fog is relatively infrequent but Wellington and the strait experience low cloud and reduced visibility, particularly in winter.

Implications for sail-powered vessels: Cook Strait is crossable under sail — it has been crossed under sail for centuries, by Māori waka and European sailing vessels alike. But it requires capable vessels and experienced crews. The weather window approach (crossing during settled conditions, waiting out storms) is viable for sail vessels but reduces reliability compared to powered ferries that operate in most conditions.

2.3 Tidal considerations

Tidal currents in Cook Strait are significant — up to 4–5 knots in constricted areas. A sailing vessel making 5–6 knots through the water can be making 1 knot or 10 knots over the ground depending on whether the tide is foul or fair. Passage planning must account for tidal timing (Doc #12 covers tide prediction). The optimal crossing strategy for sailing vessels is to depart timed to carry a fair tide through the narrowest section.¹⁶

---

3. CARGO PRIORITISATION

3.1 The sorting problem

Under reduced service, not everything currently crossing the strait can continue to cross. Cargo must be triaged. The principle is: prioritise goods that are essential, that cannot be sourced locally on the destination island, and that have high value relative to their weight and volume.

3.2 South-to-North priority cargo

Priority	Cargo type	Rationale
1	Refrigerated/frozen food (dairy, meat)	North Island food deficit
2	Grain and seed	Canterbury is NZ’s primary grain-growing region
3	Aluminium (Tiwai Point)	Critical industrial material; North Island manufacturing needs it
4	Wool and fiber	Textile raw material for North Island processing
5	Coal (West Coast)	Industrial fuel for NZ Steel and other processes
6	Timber (Nelson, West Coast)	Construction material

3.3 North-to-South priority cargo

Priority	Cargo type	Rationale
1	Medical supplies and pharmaceuticals	South Island hospitals and clinics
2	Steel products (Glenbrook)	South Island engineering, farming, construction
3	Machinery and spare parts	Agricultural and industrial maintenance
4	Fuel (if Marsden Point refinery operating)	South Island fuel supply
5	Government communications / printed material	Administrative and information distribution
6	Manufacturing inputs	Supporting South Island industry

3.4 What stops crossing

• Private motor vehicles (walk-on passengers only, except essential service vehicles)
• Non-essential consumer goods
• Bulk commodities that can be sourced on the same island
• Tourism (the concept no longer applies)

3.5 Passenger policy

Under reduced service, passenger crossings should be restricted to:

• Essential workers relocating as directed by workforce allocation (Doc #145)
• Medical transfers
• Government personnel
• Technical specialists needed on the other island
• Military and emergency services

This is a significant restriction on freedom of movement. It requires enforcement and will be unpopular. The justification is that every passenger displaces cargo, and cargo — particularly food — saves more lives than passenger convenience. As the sail fleet develops and capacity increases, passenger restrictions can be relaxed.

---

4. TRANSITION PATHWAY

4.1 Phase 1: Rationed powered service (months 0–12+)

Continue ferry operations at reduced frequency and speed. Target: 1–2 crossings per day using the most fuel-efficient vessel(s), prioritising cargo over passengers. At this service level, diesel consumption is approximately 1–3 million litres per month — modest per-crossing, but drawing on a diesel pool that serves many competing critical uses (see Fuel cost section above). The duration of Phase 1 depends on how quickly other sectors transition off diesel.

Key measures: - Reduce speed to minimum practical (14–16 knots, extending crossing to ~4–5 hours) - Run only when cargo demand justifies a crossing (no empty or near-empty sailings) - Consolidate onto one operator’s vessels if this improves fuel efficiency - Begin tug-and-barge trials: a smaller powered vessel towing an unpowered barge across the strait uses less fuel than a full ferry and can carry bulk cargo (grain, coal, timber) that does not require roll-on/roll-off capability

4.2 Phase 2: Mixed fleet (years 1–5)

As purpose-built sailing cargo vessels come online and tug-and-barge operations mature, the powered ferry service is progressively supplemented. Both fuel competition and mechanical degradation push the ferry toward reduced service over this period. The mix might look like:

• 1 powered ferry crossing per day (high-priority cargo, rail wagons, heavy vehicles)
• 1–2 tug-and-barge movements per day (bulk cargo)
• 2–6 sailing vessel crossings per day as the fleet grows (general cargo, passengers)

The powered ferry remains essential for heavy and time-critical cargo. Rail-capable ferry crossings are particularly valuable because they avoid the need to unload and reload cargo at each end — rail wagons roll on in Picton and roll off in Wellington (or vice versa) without handling.

4.3 Phase 3: Predominantly sail (years 3–10+)

As diesel becomes too scarce to justify ferry use or ferries become mechanically unserviceable — whichever comes first — sail becomes the primary mode. A fleet of 10–20 sailing cargo vessels, each carrying 10–40 tonnes, making weather-dependent crossings, provides aggregate throughput of perhaps 100–500 tonnes per day. The performance gap is severe: this is roughly 2–15% of the powered ferry service’s pre-event throughput of 3,000–8,000 tonnes per day. Sail vessels are also weather-dependent (crossing on approximately 60–75% of days attempted vs. ~95% for powered ferries), cannot carry rail wagons or heavy vehicles, and require cargo handling by crane rather than roll-on/roll-off. The throughput is sufficient for essential inter-island supply only if cargo is aggressively prioritised and lower-priority goods stop crossing.

Electric-powered vessels (Section 5.3) may supplement or replace the sail fleet if NZ’s electrical grid and battery/cable technology permits.

---

5. ALTERNATIVE PROPULSION

5.1 Sail-assist retrofit

Existing ferry or cargo vessels can be fitted with sail-assist rigs — auxiliary sails that reduce fuel consumption without eliminating the engine. Modern commercial demonstrations (Norsepower rotor sails, rigid wing sails) have shown 10–30% fuel savings on cargo vessels, though these savings vary significantly with route, wind conditions, and vessel type.¹⁷

For the Cook Strait ferries, sail-assist is complicated by their size and tophamper (superstructure, vehicle decks). A more practical approach may be fitting sail-assist to smaller coastal cargo vessels or purpose-built hybrid sail-motor vessels. Sail-assist retrofit requires: steel or aluminium fabrication for mounting structures (achievable at NZ shipyards in Auckland or Lyttelton), sail or rotor construction (canvas sails from NZ-woven fabric or rigid aerofoils from timber and plywood — Doc #138), rigging hardware (wire rope — Doc #52, blocks, winches), and naval architecture assessment of stability under sail loads. The engineering is within NZ’s existing marine capability, but each retrofit is a significant workshop project requiring 2–6 months per vessel depending on complexity.¹⁸

5.2 Purpose-built sailing vessels for Cook Strait

Cook Strait presents specific design challenges compared to open-ocean sailing:

• Short crossing: 22 km at the narrows, 92 km via the Picton route. Vessels do not need ocean-going range — they need reliability and cargo capacity for a day crossing.
• Strong tidal currents: Vessels need sufficient power (sail area) to make headway against adverse tides.
• Rough conditions: Must handle steep, confused seas. Flat-bottomed barge types are inappropriate — vessels need adequate displacement, freeboard, and stability.
• Loading and unloading: Port infrastructure at Wellington and Picton is designed for roll-on/roll-off ferries, not sailing vessels. Sailing cargo vessels will likely use crane loading/unloading at wharf, unless alternative berths with different handling are developed.

Suggested vessel type: A gaff-rigged ketch or schooner of 15–25 metres LOA, carvel or clinker planked in radiata pine on frames of pohutukawa, puriri, or rata (NZ’s strongest native hardwoods, traditionally used in boat construction — Doc #24), carrying 10–40 tonnes of cargo, with a crew of 3–6. Designed for day crossings in moderate conditions, with the ability to heave-to or shelter in the Marlborough Sounds if weather deteriorates. See Doc #138 for design details and materials.

Crossing time under sail: Highly variable. In fair conditions (moderate breeze, fair tide), a well-sailed vessel might cross the narrows in 3–5 hours. In poor conditions (light wind, foul tide, or heavy weather requiring a longer route), 8–16 hours or an overnight passage. Averaging across conditions, a sailing vessel might complete a one-way crossing on 60–75% of days attempted, with the remainder requiring delay for weather.

5.3 Electric propulsion

NZ’s electrical grid is 85%+ renewable (hydro, geothermal, wind) and is expected to continue operating (Doc #67, Doc #65). This creates a possibility that is not available to most countries: electrically powered inter-island vessels.

Battery-electric ferry: Modern battery-electric ferries operate on short routes (up to ~10 km) in Scandinavia and elsewhere.¹⁹ The longest operational battery-electric ferry routes as of 2025 are roughly 10–15 km; Cook Strait at 92 km (Picton route) is 6–9 times longer, and even the 22 km narrows crossing is at the outer edge of current battery technology for a vessel of useful cargo capacity. A battery-electric ferry for the full Wellington–Picton route would require an extremely large battery bank — on the order of 5–15 MWh for a vessel carrying 200–500 tonnes — which is probably beyond NZ’s battery manufacturing capability, though it could potentially be assembled from stockpiled EV batteries or imported cells.²⁰

A shorter crossing — Cape Terawhiti to Arapaoa Island or the entrance to Tory Channel (~22 km) — is more feasible for battery-electric propulsion. This would require developing new port infrastructure on both sides, but the energy requirement drops to perhaps 1–3 MWh, which is within the range of repurposed EV battery packs.

Overhead catenary or cable ferry: Impractical for Cook Strait due to water depth (up to 300+ metres in places), current strength, and shipping traffic.²¹

Hydrogen fuel cell: Potentially viable if NZ develops hydrogen production from electrolysis (using surplus grid electricity — Doc #63). However, the dependency chain is long: electrolysers require nickel or platinum-group electrodes and potassium hydroxide electrolyte; hydrogen storage requires pressure vessels (steel fabrication to withstand 30+ bar — Doc #89); fuel cells require proton exchange membranes that NZ cannot manufacture; and none of this infrastructure currently exists at marine scale in NZ. This is a Phase 4–5 development pathway, not a near-term solution.

Honest assessment: Electric propulsion for Cook Strait is technically plausible but requires engineering development beyond NZ’s current capability. It is a Phase 3–5 aspiration, not a Phase 1–2 solution. In the near term, diesel ferries and sailing vessels are what NZ has.

---

6. ALTERNATIVE CROSSING POINTS

6.1 Why consider alternatives?

The Wellington–Picton route is the established crossing, with port infrastructure, road and rail connections, and navigational aids in place. Any alternative crossing point starts with no infrastructure. However, there are reasons to consider alternatives:

• The Marlborough Sounds section of the route (70 of the 92 km) adds distance and time that a narrows crossing avoids
• Alternative points might offer shorter water crossings
• Redundancy — if Wellington’s port is damaged or inaccessible, an alternative is needed
• Different weather exposure — some routes may be more sheltered

6.2 Potential alternative crossings

Cape Terawhiti to Arapaoa Island / Tory Channel entrance (~22 km): The shortest water crossing. Both sides are remote with no existing port infrastructure. The North Island side (Cape Terawhiti) is connected to Wellington by rough road. The South Island side would connect into the Marlborough Sounds network. Feasible for small sailing vessels or a short-range electric vessel, but requires infrastructure development for any significant cargo throughput.

Kapiti Island corridor — Paraparaumu to Pelorus Sound: Longer crossing (~35–40 km, estimated from LINZ nautical charts) but Paraparaumu has some existing wharf infrastructure and road connections. Not a significant improvement over the Wellington–Picton route.

Whanganui/Wairarapa coast to Kaikoura coast: These east-coast alternatives involve longer open-water crossings (60–100+ km) with no sheltered approaches. No advantage over the existing route.

Nelson as South Island terminal: Nelson has a commercial port and road connections. A Wellington–Nelson route (~100 km of open water through western Cook Strait, estimated from LINZ nautical charts) avoids the Marlborough Sounds but is longer and more exposed to Tasman weather. May be useful as an alternative if Picton’s approach through Tory Channel is blocked or if Nelson’s connections to the West Coast and Christchurch are more important than Picton’s.

6.3 Assessment

The existing Wellington–Picton route remains the best option. Its advantages (port infrastructure, road and rail connections, relatively sheltered approach through the Sounds) outweigh the extra distance. Alternative crossings should be developed only as backup or for specific use cases (short-range electric vessels at the narrows, supplementary small-vessel crossings). Building new port infrastructure at undeveloped locations consumes resources that are better spent maintaining and adapting existing facilities.

---

7. MĀORI MARITIME TRADITION

Cook Strait — Te Moana-o-Raukawa — was crossed by Māori for centuries before European contact. Waka (canoes), including large ocean-going waka hourua (double-hulled sailing canoes), made regular crossings carrying people, trade goods, and kumara.²² The strait is crossable without powered vessels, using locally built craft and traditional navigation.

Iwi with strong maritime traditions in the Cook Strait region — including Ngāti Toa Rangatira, Te Āti Awa, Rangitāne, and Ngāi Tahu — hold practical knowledge of the strait’s currents, weather patterns, and landing points that complements and in some cases exceeds modern navigational data. Partnership with these iwi in developing the post-ferry sailing service is both practically valuable and appropriate given their centuries of Cook Strait seamanship.

Waka hourua design principles — twin hulls for stability, shallow draft, sail and paddle propulsion — are relevant to Cook Strait cargo vessel design, particularly for smaller vessels carrying 1–5 tonnes. Modern waka hourua, built for cultural voyaging programs, demonstrate that traditional designs can be constructed with contemporary tools and materials.²³

---

8. OPERATIONAL FRAMEWORK

8.1 Scheduling under sail

A sail-dependent Cook Strait service cannot operate on a fixed timetable. It operates on a weather-window basis: crossings depart when conditions are suitable and cargo is loaded, not at scheduled times. This requires:

• A meteorological observation capability at both terminals (basic weather station, barometer, wind measurement — not dependent on satellite data)
• An experienced sailing master making go/no-go decisions
• Cargo staging facilities at both terminals to hold freight until a crossing is available
• Cold storage at both terminals for perishable cargo (essential for food shipments — can be powered from the grid)
• Communication between terminals (landline telephone via the submarine cables, or HF radio — Doc #128)

8.2 Safety

Cook Strait has claimed vessels throughout its history of use — most notably the Wahine (1968, 53 lives lost) and numerous smaller craft.²⁴ A transition to smaller, sail-powered vessels increases the risk per crossing compared to modern ferries. Mitigation:

• Weather discipline: Do not sail in conditions beyond the vessel’s capability. The economic pressure to maintain cargo flow must not override safety. A lost vessel and cargo represents a far greater setback than a delayed crossing.
• Crew competence: Cook Strait sailing requires experienced crew. Training programs should begin immediately, drawing on NZ’s existing sailing community (NZ has an active blue-water sailing culture with yacht clubs, offshore racing programmes, and the Hauraki Gulf and Marlborough Sounds cruising fleets) and Māori maritime knowledge.²⁵
• Vessel standards: Even improvised vessels must meet basic seaworthiness standards — adequate freeboard, stability under load, watertight integrity, ground tackle (anchors) for emergency.
• Search and rescue: Maintain SAR capability in the strait. This may transition from powered vessels to shore-based observation and response.

8.3 Communication across the strait

Submarine telecommunications cables cross Cook Strait, connecting the North and South Island networks.²⁶ These cables are not dependent on satellite systems and should continue functioning as long as the terminal equipment on each side has power (grid-supplied). They are the primary communication channel between islands and should be protected as critical infrastructure.

If submarine cables fail, HF radio (Doc #128) and visual signalling provide backup. Maintaining communication across the strait is essential for coordinating cargo, scheduling crossings, and managing the inter-island supply chain.

---

CRITICAL UNCERTAINTIES

Uncertainty	Range	Impact
Diesel fuel availability for ferries	1–5 years at 1–2 crossings/day, depending on competing diesel demands	Per-crossing cost is modest, but diesel pool serves agriculture, emergency services, fishing, military — ferry competes for a shrinking resource
Ferry mechanical reliability without imported spares	3–10 years before major failures	The other binding constraint — whichever hits first (fuel or mechanical failure) ends powered service
Sail vessel construction rate	2–5 vessels per year after initial ramp-up (lower end assumes 1–2 boatyards with trained crews; upper end assumes 3–5 yards plus community boatbuilding — Doc #141)	Determines replacement capacity
Cook Strait weather under nuclear winter	Possibly stormier (increased temperature gradient); uncertain	Affects sailing vessel crossing frequency
Electric propulsion feasibility	Depends on battery availability and grid capacity	Could extend powered service indefinitely if achievable
South Island food surplus magnitude	Depends on nuclear winter severity (Doc #74, Doc #76)	Determines how much cargo actually needs to cross
North Island food self-sufficiency potential	Partial — depends on agricultural restructuring	If North Island can feed itself, strait link is less critical
Submarine cable integrity	Cables are robust but not indefinite	Loss would degrade inter-island coordination

---

CROSS-REFERENCES

• Doc #1 — National Emergency Stockpile Strategy: fuel stocks and allocation framework
• Doc #8 — National Asset and Skills Census: identify maritime workforce, vessels, boatbuilders
• Doc #12 — Tide Tables: tidal prediction for Cook Strait passage planning
• Doc #33 — Tires: vehicle transport constraints affecting road connections to ferry terminals
• Doc #52 — Wire Rope: rigging and cable for sailing vessel construction
• Doc #53 — Fuel Allocation and Drawdown: diesel prioritisation including ferry allocation
• Doc #65 — Hydroelectric Maintenance: grid power for electric vessel charging and port operations
• Doc #67 — Transpower Grid Operations: grid reliability for electric propulsion
• Doc #74 — Pastoral Farming Under Nuclear Winter: South Island food production estimates
• Doc #89 — NZ Steel Glenbrook: Continuity: North Island steel supply crossing south
• Doc #100 — Harakeke Fiber Processing: natural fiber rigging and sailcloth for sailing vessels
• Doc #128 — HF Radio Network: backup communication across the strait
• Doc #136 — NZ Port Operations: Wellington and Picton port infrastructure
• Doc #138 — Sailing Vessel Design (NZ Materials): vessel designs for inter-island trade
• Doc #139 — Celestial Navigation: navigation for strait and coastal passage
• Doc #140 — Coastal Trading Network: feeder services connecting to strait crossing
• Doc #141 — Boatbuilding Techniques: construction methods for replacement vessels
• Doc #143 — Powered Vessel Propulsion: engine maintenance and conversion for ferry fleet
• Doc #145 — Workforce Reallocation: personnel transfers between islands

---

FOOTNOTES

---

1. KiwiRail annual reports and Interislander service information. https://www.greatjourneysofnz.co.nz/interislander/ — Passenger and vehicle figures are approximate annual totals based on pre-2020 levels; recent years have been affected by fleet issues and the iReX replacement programme. Bluebridge (StraitNZ): https://www.bluebridge.co.nz/ — Combined, the two operators have historically provided 8–12 daily crossings. Exact figures vary seasonally.↩︎

2. Stats NZ Subnational Population Estimates. https://www.stats.govt.nz/ — The North Island held approximately 77% of NZ’s population as of 2023 estimates. The proportion has been slowly increasing for decades due to urbanisation in Auckland, Hamilton, and Tauranga.↩︎

3. Marine diesel consumption for Cook Strait ferries is not publicly reported in detail. The estimate of 3,000–5,000 litres per crossing is based on published fuel consumption rates for comparable ro-ro ferries (150–300 tonnes of fuel per day for large vessels operating multiple crossings). See: IMO Fourth GHG Study 2020, and comparable vessel fuel consumption data. The Kaitaki (largest Interislander vessel, ~22,000 GT) would be at the upper end; smaller Bluebridge vessels at the lower end.↩︎

4. The Interislander rail ferry capability connects NZ’s rail network across Cook Strait. Rail wagons are loaded at the Interislander terminal in Picton and unloaded in Wellington (or vice versa), avoiding the need to tranship cargo between road and rail at each end. This capability is unique to the Interislander fleet and was a primary reason for KiwiRail’s ferry operations. See KiwiRail corporate information.↩︎

5. DairyNZ regional statistics and Ministry for Primary Industries (MPI) Situation and Outlook reports. Canterbury, Southland, and Otago collectively account for a large share of NZ’s dairy production, and Canterbury is NZ’s primary arable region (wheat, barley, oats). The South Island also holds the majority of NZ’s sheep flock. Exact production shares vary by year and product.↩︎

6. Crewing levels for NZ inter-island ferries vary by vessel and regulatory requirements. Maritime NZ safe crewing requirements specify minimum crew based on vessel size, passenger capacity, and route. A large passenger/vehicle ferry typically requires 30–50 crew including deck, engineering, catering, and safety personnel. Under a cargo-only, reduced-service model, crewing requirements would be lower — perhaps 15–25 per vessel — since passenger safety and catering staff would not be needed.↩︎

7. NZ diesel stock estimates per Doc #53: approximately 400–600 million litres in-country (bulk terminals, commercial reserves, distributed storage). Diesel is more stable than petrol in storage — 6–12 months without additives, substantially longer with biocide treatment to prevent microbial growth. Doc #53 plans for biocide treatment as a standard fuel management action. Diesel shelf life with proper treatment and sealed storage is typically 2–5 years, though quality testing is advisable before use in precision engines. See: Ministry of Business, Innovation and Employment (MBIE), Energy in New Zealand reports. https://www.mbie.govt.nz/↩︎

8. NZ diesel stock estimates per Doc #53: approximately 400–600 million litres in-country (bulk terminals, commercial reserves, distributed storage). Diesel is more stable than petrol in storage — 6–12 months without additives, substantially longer with biocide treatment to prevent microbial growth. Doc #53 plans for biocide treatment as a standard fuel management action. Diesel shelf life with proper treatment and sealed storage is typically 2–5 years, though quality testing is advisable before use in precision engines. See: Ministry of Business, Innovation and Employment (MBIE), Energy in New Zealand reports. https://www.mbie.govt.nz/↩︎

9. KiwiRail annual reports and Interislander service information. https://www.greatjourneysofnz.co.nz/interislander/ — Passenger and vehicle figures are approximate annual totals based on pre-2020 levels; recent years have been affected by fleet issues and the iReX replacement programme. Bluebridge (StraitNZ): https://www.bluebridge.co.nz/ — Combined, the two operators have historically provided 8–12 daily crossings. Exact figures vary seasonally.↩︎

10. Lane metre capacity and cargo throughput estimates are based on published ferry specifications and standard ro-ro cargo density assumptions. One lane metre accommodates approximately one heavy vehicle (truck or trailer) of 15–25 tonnes. Actual throughput depends on cargo mix — cars, trucks, and rail wagons have different weight-to-space ratios.↩︎

11. The relationship between vessel speed and fuel consumption follows approximately a cube law for displacement hulls: fuel consumption is proportional to speed cubed. This is well-established naval architecture. See: Molland, A.F., Turnock, S.R., and Hudson, D.A., “Ship Resistance and Propulsion,” Cambridge University Press, 2011. The specific savings depend on hull form, propulsion efficiency, and sea conditions, but a 20–25% speed reduction typically yields 40–50% fuel savings.↩︎

12. Marine engine and propulsion system spares for NZ’s ferry fleet are sourced internationally. Major components (crankshafts, cylinder liners, turbochargers, gearboxes, propeller shafts, control electronics) are manufactured in Europe, Japan, or Korea. NZ’s marine engineering workshops (Auckland, Lyttelton) can perform significant maintenance and some fabrication, but complete replacement of major drivetrain components requires imported parts. See also Doc #91 (Machine Shop Operations) for general fabrication capability.↩︎

13. LINZ (Land Information NZ) nautical charts for Cook Strait. Chart NZ 463 and associated charts. The strait is approximately 22 km wide at its narrowest point. Maximum depth exceeds 300 metres in the main channel. https://www.linz.govt.nz/products-services/charts↩︎

14. The Wellington–Picton ferry route is approximately 92 km, of which approximately 70 km is within the Marlborough Sounds (Queen Charlotte Sound and Tory Channel). The open-water crossing of the strait proper is approximately 22 km. Route distances from LINZ nautical charts and ferry operator published information.↩︎

15. Cook Strait weather and sea conditions: MetService marine forecasts for Cook Strait. https://www.metservice.com/ — Also: Laing, A.K., “New Zealand wave climate,” NZ Meteorological Service Scientific Report, various editions; Gorman, R.M. et al., “Hindcast of ocean waves around New Zealand,” NZ Journal of Marine and Freshwater Research, 2003. Gale frequency estimates are approximate and vary by source and definition.↩︎

16. Tidal currents in Cook Strait are documented in LINZ tidal stream atlases and the NZ Nautical Almanac. Maximum tidal streams in the narrows can exceed 4 knots during spring tides. Passage planning for sailing vessels in Cook Strait has been practised since Māori settlement and is well-documented in NZ cruising guides, e.g., Royal Akarana Yacht Club cruising guides and NZ Coastguard publications.↩︎

17. Wind-assisted ship propulsion (WASP) technologies: Norsepower Oy, “Rotor Sail Technology,” https://www.norsepower.com/ — Demonstrated 5–30% fuel savings on commercial cargo vessels depending on route and conditions. Also: International Windship Association, various publications on modern sail-assist technologies. These technologies require engineering capability to retrofit, including structural analysis, fabrication, and integration with existing vessel systems.↩︎

18. Sail-assist retrofit timelines are estimates based on comparable marine engineering projects. Actual duration depends on vessel type, rig design, and workshop capacity. NZ’s marine engineering capability is concentrated in Auckland (several shipyards including Babcock NZ, McMullen & Wing) and Lyttelton (dry dock and marine engineering facilities). See also Doc #141 (Boatbuilding Techniques) for fabrication methods.↩︎

19. Battery-electric ferries: The MF Ampere in Norway (2015) was the first fully electric car ferry, operating a 5.7 km route with a 1,000 kWh battery. Since then, numerous battery-electric ferries have entered service on routes up to ~10 km. See: Bellona Foundation, “Electric ferries report”; Corvus Energy, commercial references. Longer routes remain challenging due to battery energy density and weight.↩︎

20. Battery capacity estimation: a vessel carrying 200–500 tonnes of cargo across 92 km at 10–12 knots requires propulsive power on the order of 1–3 MW for 7–9 hours, implying 7–27 MWh of battery capacity (accounting for efficiency losses and safety margins). Even at the lower end, this exceeds currently operational battery-electric ferry installations. Reducing the route to the 22 km narrows crossing (~2 hours at 10 knots) reduces the requirement to approximately 2–6 MWh — more feasible but still substantial.↩︎

21. LINZ (Land Information NZ) nautical charts for Cook Strait. Chart NZ 463 and associated charts. The strait is approximately 22 km wide at its narrowest point. Maximum depth exceeds 300 metres in the main channel. https://www.linz.govt.nz/products-services/charts↩︎

22. Cook Strait crossings by Māori: Best, E., “The Maori Canoe,” Government Printer, Wellington, 1925 (reprinted). Also: Anderson, A., “The Welcome of Strangers: An Ethnohistory of Southern Maori,” University of Otago Press, 1998. Māori crossed Cook Strait regularly for trade, warfare, and seasonal migration. The strait was a connector, not a barrier, in the Māori world.↩︎

23. Modern waka hourua: Te Toki Voyaging Trust and other organisations have built and sailed ocean-going waka hourua using both traditional and modern construction techniques. The waka hourua Te Matau a Māui and others have made Pacific voyages demonstrating the viability of traditional designs. https://www.tetokivoyagingtrust.org/↩︎

24. The TEV Wahine sank in Wellington Harbour on 10 April 1968 after encountering Cyclone Giselle in Cook Strait, with the loss of 53 lives. It remains NZ’s worst modern maritime disaster. Earlier losses include the SS Penguin (1909, 75 lives lost) near Cape Terawhiti. See: Ministry for Culture and Heritage, “Wahine disaster,” NZ History, https://nzhistory.govt.nz/↩︎

25. NZ has a strong sailing culture — the country has produced multiple America’s Cup campaigns, round-the-world sailors, and maintains yacht clubs with blue-water racing programmes in most coastal cities. The Royal New Zealand Yacht Squadron, the Cruising Association, and regional clubs collectively represent a significant pool of offshore sailing experience. Yachting New Zealand, https://www.yachtingnz.org.nz/↩︎

26. Multiple submarine fibre-optic telecommunications cables cross Cook Strait, connecting the North and South Island networks. These are domestic trunk cables owned and operated by providers including Spark and Chorus. Cable locations are charted by LINZ. Physical damage from anchor dragging or seismic activity is the primary risk. See LINZ nautical charts and Chorus network infrastructure documentation.↩︎