By MAURIPRO Sailing Specialists · Updated July 2026
What Is a Shackle and How Does It Relate to Standing Rigging?
A shackle is a U-shaped metal fastener closed by a pin, used throughout marine rigging to connect lines, fittings, and structural hardware. In standing rigging, shackles are commonly used to attach turnbuckle eye terminals to chainplates, deck plates, or toggle forks. The shackles meaning in a nautical context is straightforward: they are load-bearing connectors that must be rated, inspected, and properly moused (secured against unscrewing under vibration). A single failed shackle in a shroud circuit can lead to dismasting.
The define shackle question often arises among new sailors, but the hardware itself is familiar: a bow shackle (also called an anchor shackle) has a rounded omega-shaped bow for multidirectional loading, while a D-shackle — also called a clevis-style shackle — has a narrower profile suited for in-line loads. The eye terminal on the Sta-Lok S20 ET-78 is designed to mate directly with a D-shackle or toggle, keeping the load axis aligned through the turnbuckle body.
The meaning of shackling in rigging refers to the act of connecting two fittings using a shackle — a step that requires correct pin orientation, full thread engagement, and mousing wire or seizing thread to prevent accidental release.
What Is the Sta-Lok Supajust Turnbuckle Used For?
Core Function in Standing Rigging
The Sta-Lok Standing Rigging - Turnbuckle Supajust - Eye & Blank - For Threads: 7/8 in (S20 ET-78) serves as the tensioning element in a shroud or stay circuit. The eye end connects to the chainplate via a shackle or toggle; the blank end accepts a Sta-Lok or compatible wire terminal. Rotating the barrel threads the two ends together, increasing tension in the stay. The Supajust design allows this rotation to be performed while the terminal remains attached to the wire — a significant practical advantage over conventional open-body turnbuckles that must be removed for adjustment.
Eye & Blank Configuration Explained
The "Eye & Blank" designation describes the two terminal ends. The eye end is a fixed loop — forged or machined — that accepts a shackle, clevis pin, or toggle. The blank end has no preformed terminal; instead, it accepts a Sta-Lok swaged or swageless wire fitting that threads directly into the barrel. This configuration is common on cap shrouds, forestays, and backstays where the lower end requires a shackle connection and the upper end terminates directly onto the wire. The 7/8-inch thread size (S20 specification) places this fitting in the mid-to-large sailboat category — suitable for vessels where staying loads are significant.
How to Choose the Right Supajust Turnbuckle Size
Selecting the correct Sta-Lok turnbuckle depends on wire diameter, breaking load requirements, and the thread pitch of your existing or planned terminals. The S20 ET-78 is the 7/8-inch thread variant of the Supajust range. Matching the thread size across your entire rigging circuit — turnbuckle barrel, wire terminal, and toggle — is critical. Mismatched threads cannot be safely forced together and will strip under load. The shackle shank diameter of any shackle used with the eye terminal should be sized to match the eye bore; oversized shanks will not close properly, and undersized shanks introduce point loading.
| Boat / Application | Typical Wire Diameter | Compatible with S20 ET-78? |
|---|---|---|
| Jeanneau Sun Odyssey 440 | 8–9 mm (5/16–3/8 in) | Verify thread match with Sta-Lok terminal |
| Beneteau Oceanis 46.1 | 9–10 mm (3/8 in) | Yes — confirm S20 terminal series |
| Catalina 42 | 8 mm (5/16 in) | Yes — standard mid-size cap shroud application |
| Swan 48 (Racing) | 10–11 mm (3/8–7/16 in) | Yes — confirm with rigger for load rating |
When in doubt about sizing, cross-reference the S20 thread specification against your existing Sta-Lok terminal part numbers before ordering. See the compatibility chart above for common applications.
Installing the Sta-Lok Supajust Eye & Blank Turnbuckle
Step-by-Step: Connecting the Eye Terminal with Shackles
- Inspect the eye bore: Confirm it is free of corrosion or deformation before connecting any shackle.
- Select the correct shackle: Use a marine-grade 316 stainless D-shackle or bow shackle rated above the shroud's breaking load. Check that the shackle shank diameter fits cleanly through the eye without excessive play.
- Thread the shackle pin fully: Engage all threads before loading. Do not use pliers — hand-tight plus one full turn is correct, then mouse with stainless seizing wire through the pin hole.
- Attach the blank end terminal: Thread your Sta-Lok wire terminal into the blank barrel end. Follow Sta-Lok's published assembly torque values — do not estimate.
- Tension the stay: Rotate the Supajust barrel to tension. The Supajust barrel design allows rotation without disassembly, so intermediate tensioning during tuning is straightforward.
- Lock the barrel: Use the Sta-Lok locking nut or seizing wire across the barrel slots per manufacturer specification to prevent unthreading under vibration.
Clevis Pins vs. Shackles at the Eye Terminal
Some sailors prefer a clevis pin and toggle arrangement over a shackle at the chainplate connection. A clevis provides a lower-profile, in-line load path and is less prone to toggling fatigue than a shackle under cyclic loading. The eye terminal on the S20 ET-78 is compatible with both arrangements — confirm bore diameter against your toggle or chainplate pin size before final installation.
Why Buy the Sta-Lok Supajust from MAURIPRO?
MAURIPRO sources the Sta-Lok Standing Rigging - Turnbuckle Supajust - Eye & Blank - For Threads: 7/8 in (S20 ET-78) directly, ensuring you receive genuine Sta-Lok hardware with the material certification and dimensional tolerances the brand is known for. Counterfeit or off-brand rigging hardware is a documented hazard in the marine market; shackles shackles and turnbuckles from uncertified sources frequently lack the grain structure and surface finish that marine-grade 316 stainless requires for long-term corrosion resistance in saltwater environments.
Standing rigging is not the place to economize on component origin. The Supajust system has a proven service record among offshore cruisers and club racers alike because its in-situ adjustment feature reduces the number of disassembly cycles — each of which is a potential point of human error or thread damage. Order through MAURIPRO and get the correct fitting the first time, backed by a team with deep standing rigging expertise ready to assist with sizing, compatibility, and installation questions.
Frequently Asked Questions
What is a shackle in sailing rigging, and how does the Sta-Lok Supajust use one?
A shackle in sailing rigging is a U-shaped metal connector with a threaded or pin closure used to link rigging components securely. The Sta-Lok Turnbuckle Supajust Eye & Blank (S20 ET-78) uses an eye fitting that functions similarly, connecting the turnbuckle to chainplates or other standing rigging terminals on 7/8 in thread systems.
What does the 'Eye & Blank' configuration mean on the Sta-Lok Supajust turnbuckle?
The Eye & Blank configuration on the Sta-Lok Turnbuckle Supajust S20 ET-78 means one end features an open eye fitting for pin connection to a chainplate or deck fitting, while the opposite end is blank, accepting a Sta-Lok swage or mechanical terminal. This gives riggers flexibility when terminating 7/8 in threaded standing rigging wire.
What size wire or rod does the Sta-Lok Supajust S20 ET-78 accommodate?
The Sta-Lok Turnbuckle Supajust Eye & Blank S20 ET-78 is designed for 7/8 in thread diameter standing rigging applications. Always verify the specific wire or rod diameter compatibility with Sta-Lok's sizing charts before installation, as thread size and wire diameter must match the terminal end fittings for safe working load ratings.
How do you tension the Sta-Lok Supajust turnbuckle when tuning standing rigging?
The Sta-Lok Turnbuckle Supajust S20 ET-78 features Sta-Lok's tool-free Supajust barrel, allowing rigging tension adjustment by hand without a screwdriver or spike. Once the desired tension is set, the locking mechanism secures the adjustment. MAURIPRO recommends verifying forestay and shroud tension with a calibrated rig tension gauge after final adjustment.
Is the Sta-Lok Supajust Eye & Blank suitable for offshore or bluewater sailing?
The Sta-Lok Turnbuckle Supajust Eye & Blank S20 ET-78 is manufactured by Sta-Lok, a brand recognized for offshore-grade standing rigging hardware used on cruising and racing yachts worldwide. Its tool-free tensioning and positive locking system make it appropriate for offshore passages where deck access and ease of rig adjustment are critical priorities.
Questions? We're Here to Help
Have questions? Chat with us! Our MAURIPRO rigging and sailing specialists are available to help you find the right solution for your boat and sailing style.
Explore our selection: Sta-Lok Standing Rigging - Turnbuckle Supajust - Eye & Blank - For Threads: 7/8 in (S20 ET-78) and related sailing gear at MAURIPRO.
