SHIPYARD NEWBUILD AND REFIT DUCT FABRICATION
Marine duct is built for a saltwater environment inside confined spaces, and the project phase changes almost everything about how it is fabricated and delivered. Newbuild work is planned, sequenced, and drawn from a model. Refit work is measured, verified against the actual vessel, and delivered against a window that is often days long. JMC has fabricated for the Gulf Coast marine industry from Pascagoula since 2002.
Here is how those two phases differ, and what each one asks of a fabricator.
Published August 5, 2026 · JMC Fabrication
Three conditions drive marine duct construction. The environment is corrosive, so material selection leans toward stainless where the service and the specification call for it. The spaces are confined, so geometry is tight and pieces have to be sized to get through the access they will actually pass through. And the structure moves, so joints and supports have to tolerate vibration and hull flex rather than sitting still in a building.
That combination means marine duct is often not a scaled-down version of commercial duct. It is closer to the industrial end of the range: heavier, more welded, more transitions per foot, and detailed around structure rather than routed through open plenum.
Access is the constraint that surprises people who come from building work. A piece that fits the space perfectly is useless if it cannot get to the space, so section sizing on a refit is often driven by hatch and passageway dimensions rather than by fabrication convenience.
Same product, very different project mechanics:
| Attribute | Newbuild | Refit and repair |
|---|---|---|
| Source of dimensions | Drawings and model | Field measurement of the actual vessel |
| Schedule | Sequenced to the build plan, weeks to months | Vessel availability window, often days |
| Section sizing driver | Fabrication and handling efficiency | Hatch and passageway access |
| Change rate | Managed through the drawing process | High, as conditions are uncovered |
| What matters most | Consistency and sequencing | Turnaround speed and proximity |
Duct is one part of a broader marine fabrication scope:
- Marine HVAC duct: Supply, return, and exhaust duct in stainless or galvanized steel per the specification, detailed for confined shipboard routing.
- Ventilation and exhaust trunking: Machinery space, galley, and general ventilation, including heavier welded construction where the service or the spec requires it.
- Pipe spools alongside duct: Pipe spool fabrication is a core JMC capability, so a vessel package can source duct and spools from one shop instead of two vendors on two schedules.
- Custom steel assemblies: Supports, penetrations, sleeves, and fabricated assemblies in carbon, stainless, or galvanized steel as the package requires.
Refit work generates changes. Conditions get uncovered once a space is opened, dimensions differ from the record drawings, and a piece that was measured correctly last week no longer fits after adjacent work moves. Every one of those is a same-day or next-day fabrication request.
That is where being in Pascagoula matters more than shop capability. A shop an hour away can measure in the morning, fabricate in the afternoon, and have the piece back on the vessel before the window closes. A shop several states away cannot, no matter how good it is, because freight time is the binding constraint rather than fabrication time.
JMC's service area covers the Mississippi Gulf Coast plus Mobile, Alabama and New Orleans, Louisiana, which is the concentration of Gulf Coast marine work. The shop also takes on projects with military specification requirements.
- Vessel type and the phase, newbuild or refit
- The material the specification calls for, and the stainless grade if applicable
- Drawings, or notice that dimensions will come from field measurement
- Access constraints, including hatch and passageway dimensions that limit section size
- The vessel availability window for refit work, which sets everything upstream
- Any military specification or certification requirements the project carries
- Whether pipe spools are in scope alongside the duct
Does JMC fabricate marine duct?
Yes. Marine and shipbuilding is one of JMC's core markets, and the shop has fabricated for Gulf Coast marine work from Pascagoula since 2002, in carbon, stainless, and galvanized steel.
What is the difference between newbuild and refit duct work?
Newbuild dimensions come from drawings and a model and the schedule follows the build plan. Refit dimensions come from measuring the actual vessel, the change rate is much higher, and the schedule is set by the vessel availability window, which is often days rather than weeks.
Why does section size matter on a refit?
Because the piece has to reach the space. On a refit, section sizing is often driven by hatch and passageway dimensions rather than by what is convenient to fabricate, so those access dimensions belong in the RFQ.
Can JMC turn around a same-day change during a refit?
That is what the Pascagoula location is for. Fabrication and engineering share a building and the shop is close to the Gulf Coast yards, so a measurement taken in the morning can often come back as a finished piece the same day or next day, depending on the work and current shop loading.
Can JMC supply duct and pipe spools for the same vessel package?
Yes. Pipe spool fabrication is a core JMC capability alongside duct, so a vessel package can come from one shop on one schedule rather than from two vendors that have to be coordinated.
Does JMC handle military specification work?
Yes. The shop takes on projects with industrial and military specification requirements. Send the specification and certification requirements with the RFQ so they are priced into the scope.