Reverse Engineering Marine Propellers and Other Marine Machinery

Nikita ShuvalovJul 10, 2026

A propeller comes out of the water with a bent blade tip, a cavitation-eaten trailing edge or a crack at the root, and the boat’s owner discovers that the foundry that cast it closed long ago and no drawing of it exists. The blades are nothing but free-form surfaces: each one is a twisted, cambered foil whose skew and thickness, and on many designs its pitch, change from the hub to the tip, and none of it can be measured with a tape. Reverse engineering marine machinery of this kind starts with a 3D scan, and this guide follows a propeller through it: the scan, the master blade, the section table, what the foundry needs and where the naval architect’s work begins.

Why propellers are measured by scanning

A propeller blade has no flat faces and no straight edges to measure from. Its shape is described in the drawing office as a set of sections at radii from the hub, each section a foil with its own chord, pitch angle, camber and thickness, and the blade is the surface lofted through those sections. Recovering that description from a physical blade means measuring the whole surface densely enough to cut the sections back out of it, which is what a 3D scan does and a pitch gauge does not. A handheld scanner of the HandySCAN SILVER class, by its brochure, projects fourteen blue laser lines over a scanning area of 310 by 350 mm, is rated at an accuracy of up to 0.040 mm as a typical value for diameter measurements on a calibrated sphere, and is recommended for parts from 0.05 to 4 m. That range covers propellers from a tender’s two-blader to a small ship’s screw. A whole shaft line on a hauled-out boat, with its brackets, stern tube and rudder, is recorded with a terrestrial scanner of the Trimble X7 class, rated at a 3D point accuracy of 2.4 mm at 10 m (one standard deviation) by its datasheet, which serves the layout of the propulsion system rather than the casting of a screw.

Other marine machinery

The same work covers the rest of the machinery a scan can reach: a rudder and its stock, a shaft bracket, a stern tube flange, a keel cooler, a pump impeller. Anything cast or forged for a boat that no longer has its drawings is reverse engineered the same way, as far as the scan reaches, since a surface scan does not capture internal passages; the propeller is only the hardest case because every surface on its blades is curved. The reverse engineering page describes the service for any part, and the 3D scanning for parts page the scan on its own.

Scanning a propeller

A propeller is scanned either on the shaft, with the boat hauled out, or on a bench once it is pulled. Bronze and stainless propellers are often polished, and polished metal returns the laser lines poorly, so the blades are coated with a thin matting spray that wipes off after the scan; a fouled propeller is cleaned first, because the scan records the fouling as if it were metal. The scanner is moved over both faces of every blade, the hub, the bore and the keyway, with the mesh building on the laptop as it goes, so that a patch the lines have not covered is seen and filled before the scan ends. Scanning a four-bladed propeller of a metre or so usually takes part of a day; the mesh, cleaned of stray points, is the measured record of the propeller as it is, damage included.

The bore, the taper and the keyway are measured at their openings with gauges where the scan cannot reach the whole length, and the shaft taper is checked against the shaft itself when the shaft is accessible, because a replacement propeller that does not seat on the taper is scrap regardless of how good its blades are.

From the mesh to the CAD model

The mesh records one worn propeller, and the model has to be the propeller as it was designed, so the modeling work is deciding, surface by surface, what is wear and what is design. The first decision is which blade to trust: on a damaged propeller one blade is often intact or nearly so, and that blade’s surface, fitted as NURBS surfaces to the scan, becomes the master blade, which is then patterned around the hub axis to make the others. Where every blade is damaged somewhere, the master is assembled from the sound regions of several blades, which is possible because the blades were cast from the same pattern. The hub is modeled as a solid of revolution from the scan, with the bore, taper and keyway at their measured sizes.

From the master blade the sections are cut at the usual radial stations, and the pitch, chord, camber and thickness at each radius are tabulated, so the model is delivered both as geometry, in STEP, and as the table of sections that describes it. The model does not change the design: a change of pitch or diameter is the naval architect’s calculation, and for a different propeller rather than a replacement, the scan and the model are what the architect starts from.

What the foundry needs

A replacement propeller is cast from a pattern, or from a mould machined from the model, with the foundry’s own allowances for the shrinkage of the alloy and for the machining of the hub and the finishing of the blades. Those allowances are the foundry’s to add, which is why the model is delivered at the finished size and the foundry is told so. Manufacturing tolerances for finished propellers are the subject of ISO 484-1, for propellers of diameter greater than 2.50 m, and ISO 484-2, for propellers between 0.80 and 2.50 m, both in their 2015 editions; the BSI catalogue describes Part 1 as covering methods for measuring pitch and blade section thickness, accuracy classes, and tolerances on pitch, extreme radius and section thickness, which is why the sections are delivered with pitch and thickness per radius. Whether a cast propeller meets a tolerance class is the foundry’s and the architect’s check; the model is delivered as geometry.

Deliverables and price

The delivery is the cleaned mesh in STL, the CAD model in STEP with native formats on request, the table of blade sections, and, on request, drawings in DWG and PDF of the sections and the hub for the foundry and the machine shop. The model is checked against the scan before delivery, and the list that comes with it says which features follow the scan and which were restored to their design shape.

Reverse engineering of a single part starts at about $1,000 for the scan and the CAD model; a propeller with several damaged blades, a large propeller or a complete shaft line is quoted on its own. The exact price comes in a quote within one business day of your sending photographs, the diameter and the number of blades, and what the model is for. A propeller is usually scanned in a day, in the yard or on the bench, and the model usually follows in one to two weeks. We scan in all 50 states.

FAQ

Can a damaged propeller be reverse engineered?

Yes. The intact blade, or the sound regions of several blades, becomes the master blade in the model, and the damaged areas are restored to that shape. The scan also records the damage itself, which can go to the surveyor or the insurer as a record.

How accurate is the scan of a propeller?

A handheld scanner of the HandySCAN SILVER class is rated by its brochure at up to 0.040 mm as a typical value for diameter measurements on a calibrated sphere, with a volumetric accuracy of 0.020 mm plus 0.100 mm per metre under the brochure’s test conditions. The model is checked against the scan before delivery.

Do I get the pitch and the section table?

Yes. The sections are cut from the model at the usual radial stations, and the pitch, chord, camber and thickness are tabulated with the STEP file.

Can you change the pitch or the diameter?

No. Changing a propeller’s design is a naval architect’s calculation. The scan and the model are the record of the existing propeller the architect works from.

Can the foundry cast from your model?

Yes, for the geometry: the model is delivered at finished size, and the foundry adds its own shrinkage and machining allowances to make the pattern. The alloy and the tolerance class come from the client or the architect.

How much does it cost and how long does it take?

A single part starts at about $1,000 for the scan and the model, with a quote within one business day. A propeller is usually scanned in a day and the model usually follows in one to two weeks.