Turnbuckles or rigging screws are used to tension the stays and shrouds on your boat’s rigging. They comprise a body into each end of which is screwed a threaded rod. One rod has a right hand thread, the other a left hand thread – when you rotate the body both rods move towards or away from the body at the same time, tightening or loosening the stay.
The lower rod connects to the chainplate and the upper rod connects to the rigging wire. They do this via various fittings including T- ends, jaws, clevis pins and machine swages. Whatever is used on your particular boat the critical point is articulation: If your joints don’t articulate they can break.
The body of the turnbuckle can be open or closed: A closed turnbuckle body, sometimes called a tubular turnbuckle, looks smooth and tubular but it can trap water and dirt in the lower end and this can lead to corrosion; it is also difficult to tell how much ‘bury’ there is left on the rod ends – always comforting to know your turnbuckles aren’t hanging on by a single thread. Closed body turnbuckles have a check nut on the threaded rod to lock it against the turnbuckle body – this avoids the need for cotter pins or ring pins to stop the turnbuckle unscrewing itself. For extra security you can run a piece of wire from one fork, through the hole in the turnbuckle body, to the other fork, to prevent turning.
My personal preference is for open body turnbuckles. Open body turnbuckles bare all – you can see the rods and how much of their thread is engaged. You need to pin them to stop the turnbuckle unscrewing and you then you need to tape over the pin to prevent it snagging on a sail. Bandit™ tape is ideal for this because it is non-adhesive and doesn’t leave a sticky mess when you remove it to adjust the rig
To adjust an open body turnbuckle you can turn it by hand or with a screwdriver inserted through the body opening, or you can use a spanner. With a tubular closed body turnbuckle that is too stiff to turn by hand you can use a special tool that fits into the hole in the centre of the body.
Whether you have open or closed body turnbuckles you need to think carefully about the material from which they are made. On the face of it stainless steel would appear to be most fit for purpose but you’d be wrong. Stainless threads suffer from a condition called galling, and it is pretty galling I can tell you, in which the threads jam when heavy load is applied. A much more satisfactory arrangement is a combination of a silicone bronze body and stainless threaded rods. This is a non-galling combination.
The threaded portion of the turnbuckle is typically twice the diameter of the wire it is intended to handle – 6mm wire would have an M12 thread or, in old money, ¼” wire would use a ½” thread turnbuckle. Check with the turnbuckle manufacturer for their specific recommendations.
That’s probably as much as anyone needs to know about turnbuckles.