Backing nut
Securing the hull penetration.
The backing nut threads onto the male spigot of the skin fitting from the inboard side, clamping the fitting assembly against the hull. It is a structurally critical component — the correct across-flats dimension must be verified before installation to ensure proper tooling engagement and torque.
Material must match the skin fitting material to avoid galvanic coupling — mismatched metals in a salt water environment will accelerate corrosion of the less noble metal. Thread standard, nominal diameter, pitch and hand must all match the skin fitting specification exactly.
| Property | Description |
|---|---|
| Material | Must match skin fitting material to avoid galvanic coupling |
| Thread standard | Must match skin fitting thread specification exactly |
| Nominal diameter | Must match skin fitting |
| Thread pitch / TPI | Must match skin fitting |
| Thread hand | Must match skin fitting |
| Across flats (AF) | Distance across opposing flats — determines correct spanner size |
| Nut height / thickness | Bearing face depth — determines thread engagement length |
The backing nut works in conjunction with the backing plate to distribute clamping load and prevent hull distortion. Both components must be inspected together — a correctly torqued nut against a deteriorated backing plate provides no structural benefit.
Data points
Property Values
| Value | Type |
|---|---|
| Construction | |
|
Part No.
Part Number.
|
Attribute |
|
Surface Finish
Surface treatment or finish type.
|
Attribute |
|
Hull fitting material
The material from which the hull fitting body is manufactured, typically a corrosion-resistant alloy or composite suited to continuous immersion in sea water.
|
Attribute |
| Dimensions | |
|
Across flats (AF)
The distance measured across opposing flat faces of a nut or bolt head, determining the correct wrench or spanner size required for installation and removal.
|
Measure |
|
Nut height thickness
The distance measured between the two bearing faces of a nut, determining the thread engagement length and the correct socket or spanner depth required for installation.
|
Measure |
| Thread Specifications | |
|
Thread pitch
Distance between adjacent thread crests, measured parallel to the thread axis. Determines how far the fastener advances with each full rotation.
|
Measure |
|
Nominal diameter
Stated reference diameter of a thread, representing the outer diameter of the male thread. Used to identify and match threaded components but does not necessarily reflect the precise measured dimension.
|
Measure |
|
Threads per inch (TPI)
The number of complete thread crests occurring within one inch of thread length, used to express thread pitch in imperial fastener standards.
|
Measure |
|
Thread Standard
The specification system defining the thread form, profile angle, and unit convention — for example M (ISO metric), UNC (unified coarse imperial), or BSP (pipe threads).
|
Attribute |
|
Thread hand
The direction of the thread helix, determining whether the fastener tightens with a clockwise (right-hand) or anti-clockwise (left-hand) rotation.
Right-hand is the universal default; left-hand threads are only used in specific applications where rotational forces would otherwise cause the fastener to self-loosen.
|
Attribute |
| Lifecycle events | |
|
Commissioning Date
Date when the equipment or component was put into commission or installed on board.
|
Lifecycle |
|
Disposal Date
Date when the item was permanently removed or decommissioned.
|
Lifecycle |
|
Purchase date
Date item was purchased.
|
Lifecycle |
|
Decommissioning date
Date item was decommissioned.
|
Lifecycle |
|
Installation date
Date the item was installed. Note this may be a different date to the commissioning date.
|
Lifecycle |