Zoning a PCTC Drencher — Why Fixed First Needs Detection
FSS Code Ch.7 sets a PCTC drencher at 5.0 L/min/m² — whether 'Fixed First' suppresses or floods the deck depends on how precisely detection zones it.
A fixed water-based drencher on a PCTC vehicle deck is designed to deliver 5.0 L/min per square metre (FSS Code Chapter 7; IMO MSC.1/Circ.1430/Rev.3). IUMI's 2025 'Fixed First' guidance says release it early, before crew intervene. On a large undivided deck, whether that early release suppresses the fire or floods the ship depends almost entirely on how precisely the detection layer can zone it.
What 'Fixed First' actually asks
Fixed First means the fixed system is the primary response — activated promptly and safely before any manual firefighting, because reaching a fire among tightly stowed cars is hazardous and slow.
- Early activation aims to stop the fire before it bridges to adjacent vehicles and their battery packs; manual firefighting only supplements, to save lives.
- Foam carries a high 'Fixed First' threshold — it needs power to run and risks unnecessary release, so crews hesitate to commit it early.
- Modern PCTCs have large, undivided horizontal decks, not subdivided compartments — so a fire spreads across vehicles faster than crew can respond.
The number that makes zoning matter
At 5.0 L/min/m², the water volume scales with the area you release over — and on a PCTC that area is the whole deck unless detection narrows it.
Take a single undivided deck of roughly 5,000 m². At the 5.0 L/min/m² nozzle density that is about 25 m³ of water per minute — on the order of 750 tonnes over a 30-minute hold if drainage and freeing ports cannot keep pace. Accumulated firewater high in the ship is a free-surface and stability problem, not only a cargo-damage one. Releasing the whole deck to fight a fire under one vehicle is the expensive way to use the system.
What detection has to deliver to zone the release
To release the right drencher section instead of the whole deck, detection has to localise the event to a zone — with enough confidence to commit water.
- Zone-resolved localisation: which drencher section, not just 'a deck alarm'.
- Confidence high enough to commit — a false zone release wastes the system and adds water weight for nothing.
- Early enough that the zoned release lands in the pre-spread window — the off-gas and early-thermal phase, before the fire bridges zones.
- Stage classification, so the bridge knows whether to hold, zone-release, or escalate to a wider release.
Sources
- IMO — FSS Code Chapter 7 (fixed pressure water-spraying / water-based systems) and SOLAS reg. II-2/20.6.1.
- IMO MSC.1/Circ.1430/Rev.3 — Revised guidelines for the design and approval of fixed water-based fire-extinguishing systems for ro-ro spaces and special category spaces (5.0 L/min/m² nozzles; 2.0 L/min/m² monitors).
- IUMI — "Risk mitigation for the safe ocean and short-sea carriage of electric vehicles" (2025 update): the 'Fixed First' approach and foam limitations.
- ABS — Regulatory News: "Fire Safety Requirements for Ro-Ro Passenger and Cargo Ships" (2025).
- IMO Resolution MSC.555(108) — FSS Code Chapter 7 amendments (in force 1 January 2026).
- [VERIFY: the ~5,000 m² deck area and ~750 t accumulated-firewater figure are an illustrative calculation from the 5.0 L/min/m² density, not a vessel-specific measurement — adjust to the actual deck area, hold time, and drainage assumptions.]
Questions, answered
What is the 'Fixed First' approach for car-carrier fires?+
IUMI's 2025 guidance recommends the fixed firefighting system be activated promptly and safely before any manual intervention, because reaching a fire among tightly stowed cars is hazardous and slow. Early activation aims to stop the fire before it spreads between vehicles; manual firefighting only supplements the fixed system, to save lives.
How much water does a fixed deck drencher deliver?+
Under FSS Code Chapter 7 and IMO MSC.1/Circ.1430/Rev.3, drencher nozzles for ro-ro and special category spaces are designed for 5.0 L/min per square metre of covered area, and weather-deck monitor systems must provide at least 2.0 L/min/m². On a large undivided PCTC deck that scales into tonnes of accumulating firewater per minute.
Why does fixed firefighting depend on the detection layer?+
A drencher protects by zones. Releasing the whole deck floods cargo and adds free-surface water weight high in the ship; releasing the right zone needs detection precise and confident enough to commit. Without zone-resolved localisation in the early window, 'Fixed First' becomes whole-deck-flood-first — effective, but at a stability and cargo cost.
Why are PCTCs harder to protect than older car carriers?+
Modern PCTCs have large, undivided horizontal decks rather than subdivided compartments, so a fire can spread rapidly across vehicles before crew respond. That is the structural reason IUMI favours an early fixed-system release — and the reason detection that can zone that release, rather than trip the whole deck, matters so much.
Continue the thread
IUMI's "Fixed-First" Approach — What It Means on the Bridge
The 2025 IUMI best-practice update made the priority order explicit: activate fixed systems before any manual intervention. The reasoning is sharper than most operators have absorbed.
Integrating Water-Mist Lances With a Per-Vehicle Detection Layer
Draft SOLAS amendments under II-2/10.7.3.1 set functional requirements for water mist lances. Their effectiveness depends entirely on the detection layer above them.
Does the 2026 SOLAS Fire Rule Reach Existing Ships?
Yes. The SOLAS II-2/20 detection and video-monitoring amendments in force since 1 January 2026 catch existing tonnage at the first survey on or after 1 January 2028.