Keel Core

Keel Core is the construction method behind every Iron Keel kit. It replaces the largest, most print-intensive section of the hull with Schedule 40 PVC pipe — the same class of material used in industrial applications — and uses precision-engineered 3D-printed connectors to integrate it with detailed printed end caps, sails, and control surfaces.

The result is a model that's stronger, faster to build, and available at scales that would otherwise be impossible.

Why It Exists

Printing a five-foot submarine
is the wrong problem to solve.

A fully printed 6" scale hull is 100+ hours on the printer, warps under its own weight during long runs, and fills a box with twenty structural parts before you've even gotten to the fun bits — the sail, the bow sonar dome, the control surfaces.

The hull is a tube. It doesn't need to be printed. It needs to be strong, straight, and the right diameter. PVC already is those things — cheaper, stiffer, and more durable than anything that comes off a printer.

Keel Core stops fighting that fact. The tube is a tube. The details get printed.

How It Works

Three parts,
one hull.

1

The Core

Schedule 40 PVC pipe forms the primary hull structure. Industrial-grade, dimensionally stable, available in the right diameters at any hardware store.

2

The Coupler

Precision-engineered stepped rings bond the core to the printed hull sections. The fit is calibrated to real measured PVC — not nominal specs.

3

The Detail

3D-printed bow, sail, stern, and control surfaces provide the scale accuracy. The parts you actually look at get the full resolution treatment.

Iron Keel connector ring demonstrating the PVC-to-printed-hull interface

A completed kit is three pieces of engineering working together: the PVC handles the structural load, the printed caps carry the geometry, and the couplers make the transition invisible.

Assembly is a matter of trimming the PVC to length, seating the couplers, and bonding the caps. Ten to twenty parts. One afternoon.

The Numbers

Measurements,
not assumptions.

Every Keel Core size is built around real, measured PVC pipe. The 4" system is the current production baseline; the 3" and 6" systems scale from the same master geometry using dimension-driven connector rings.

4" System Specifications

Measured PVC OD	~114 mm
Measured PVC ID	~101 mm
Hull Scale	128.23% (from 3" master)
PVC Length	~22.62" (cut long, trim to fit)
Coupler — PVC side	100.85 mm
Coupler — Hull side	101.85 mm

Lessons learned

Always measure real pipe. Never trust nominal specs.
Scale hull geometry, not connector fits.
Connector rings must be dimension-driven, not scale-driven.
PVC variation requires calibration to the tightest ID.
Length must be scaled and trimmed, not assumed.
The 3" hybrid files are the master baseline for all builds.

Keel Core is what turns a 100-hour print job into a kit anyone can build in an afternoon — without compromising scale, accuracy, or durability.

See the fleet →

Printing a five-foot submarineis the wrong problem to solve.

Three parts,one hull.