A turnbuckle is a device used to adjust tension in standing rigging on sailboats and in garage doors openers. It consists of two screws, one with left-handed threads and the other with right-handed threads, that can be turned to adjust the length of the wire and hence the tension. Turnbuckles are the standard method for tensioning standing rigging. Deadeyes are an alternative technology that can be used to adjust tension in standing rigging, but turnbuckles are more commonly used. 

The usual fail point on a turnbuckle is the threads, which can become stripped or corroded over time. In my garage door opener installation, the fail point was a left handed bolt that snapped. However, the snapped bolt appeared corroded from the inside, a different color than the brass outside, and may have been defective. It certainly was not of high quality.  It was covered under warranty and the manufacturer representative told me that it wasn’t the first time the bolt had snapped while adjusting the lower limit of door closing.

Turnbuckles of high quality offer several advantages over other tensioning methods. They are designed to be loaded in straight pull, inline applications like garage door openers, and can be used to take up slack and apply tension to a rigging assembly. Turnbuckles are easy to adjust, and you can easily twist the turnbuckle body to expand or contract the length, without rotating the end fittings on both sides.  However, the shroud adjusters on Murrelet, a Macgregor 26x sailboat, are just as easy to adjust using a tool provided by the sailboat dealer. 

Turnbuckles are versatile and can be used in various industries such as oil and gas, construction, shipping, mining, and fishing. They offer a mechanical advantage thanks to the inclined plane, which is the threads on the screw, and as you turn the turnbuckle, you are moving along that incline. Turnbuckles on sailboats must be made of high-strength materials such as stainless steel, which makes them durable and corrosion-resistant. This is especially true on boats where masts are raised or lowered because turnbuckles are easily damaged during that process. 

Shroud adjusters, like those on Mac26x boats are not a kind of dead eye. Shroud adjusters connect the shrouds to the deck of a sailboat, offering an easy means of both attachment and adjustment. They are typically made with stainless steel clevis pins and rings. Dead eyes, on the other hand, are smallish round thick wooden (usually lignum vitae) discs with one or more holes through them, perpendicular to the plane of the disc. They are used in the standing and running rigging of traditional sailing ships. 

In conclusion, while turnbuckles of high quality offer advantages, those advantages are lost when the rig is lowered.This is because turnbuckles, unlike dead eyes and shroud adjusters are designed to be loaded only in straight pull, inline applications. They are expensive if of high quality but unfortunately low cost substitutes easily substitute for expensive ones because of attractive coatings. You only know that the turnbuckle is coated and of low quality when it snaps a left or right handed bolt like my garage door opener did. Then you can see that the inner part of the bolt is a different mettle, possibly aluminum, than the outer stainless or brass coating. The care needed to preserve a turnbuckle during a mast lowering is labor intensive and hence professionals replace them when doing that work. If a mast was recently raised, riggers will climb masts to preserve newer turnbuckles, doing work aloft rather than on shore.

The manufacturer of my boat pointed out that shroud adjusters are used on hang gliders and turnbuckles are considered inappropriate because of the twisting and turning of the rig during flight. On modern sailboats where masts are bended for sail shape - like wings of hang glider - and certainly on boats where the mast is dropped frequently, turnbuckles should be frowned upon. This is an example where the “cheap” item is superior to the “high quality” one.