Shipbuilding and marine fabrication
Weld monitoring for shipbuilding
High-volume welding under classification-society rules. The codes that apply, the four monitoring pain points that cost yards most, and which Therness signal addresses each.
Welding glossary
Throat thickness
Throat thickness is the shortest distance from the root of a fillet weld to its face, the internal dimension used in structural calculations rather than the leg length shown on a drawing.
- Symbol
- a
- Unit
- mm (in, in ASME and AWS practice) — measured on the weld cross-section, not the visible leg
- Formula
- For a standard equal-leg fillet weld deposited at the theoretical 45-degree profile, effective throat a = leg size z multiplied by the cosine of 45 degrees
Throat thickness is the shortest distance from the root of a fillet weld to its face, and it is the dimension every structural calculation for that weld actually uses — not the leg length that appears as the callout on a fabrication drawing. For a standard equal-leg fillet deposited at the theoretical 45-degree profile, the effective throat is the leg size reduced by a fixed geometric factor, which is why a modest-looking leg produces a noticeably smaller throat and why the two numbers are easy to confuse.
The distinction matters because the two dimensions are decoupled in a way that is invisible on a routine gauge check. A weld can measure the correct leg length and still carry a below-specification throat if its profile is concave, undersized in build-up, or deposited at the wrong angle — weld pool shape at the moment of solidification sets the actual profile, and the leg gauge alone does not reveal it. This is the practical reason laser or structured-light profilometry, which reconstructs the full cross-sectional profile rather than a single leg reading, is used where throat is the governing requirement.
Throat thickness is the fillet-weld counterpart to penetration depth on a groove weld: both are the internal, load-bearing dimension that a visual check of the external weld size does not confirm on its own. And where weld toe condition governs a joint’s fatigue life, throat thickness governs its static strength — a joint can satisfy one requirement and fail the other, which is why weld joint design treats them as separate checks rather than a single pass/fail on appearance.
Where you meet this term
- Fillet weld sizing calculations, where throat is the dimension used for shear and bending capacity, not the leg
- Fabrication drawings, where the callout is the leg size while the design calculation behind it is the throat
- Weld profile measurement by laser or structured-light profilometry, one of the few routes that measures it directly
- Butt weld design, where the comparable governing dimension is full or partial joint penetration depth rather than a throat
