How do I use a turnbuckle effectively?

[DaveC426913]

TL;DR Summary

Speciifically, how much thread needs to be ... threaded, to function within operational parameters?

I bought four stainless steel turnbuckles to replace the vernier adjusters on the shrouds of my sailboat.

The cotter pin diameter is 1/4" - the same as what I am replacing. I was told that that is the metric by which one judges the capability of a turnbuckle.

They have a 1 1/2" long thread on each end and so can range from 6" inches to 9". Naturally, their functional range is less than 9". But my question now is: how much less?

On a test run, one of them blew out and almost dismasted me.

I checked once safely back ashore - the threads are stripped.
The left one is brand new. The right one looks like its threads are all stripped.
That's after one hour of sailing.

Before I go back to the chandlery and complain it's a hardware fail, I have to be sure I didn't install them incorrectly.

I had made sure I had what I thought was enough turns. It was at least 5/8", if not 3/4" of an inch on each end. If that's not enough, that means the useful range of these turnbuckles tops out at 7 1/2" at best - barely 1 1/2" of range. I don't really have enough play in my shrouds to go much further than I have.

If I go a size up in turnbuckles, the next ones are too long - longer than the distance from tang to shroud swage, and I would have to remove and completely replace my shrouds.


What can I do?

 

[jack action]

Looking at typical turnbuckles, you can deduce that approximately twice the diameter should be enough:

source: https://duckduckgo.com/?t=ffab&q=turnbuckle&iax=images&ia=images

To have a more precise number:

Recommended thread engagement​

While making a connection, the bolted joints should be designed so that the screw breaks before the threads strip. For components with tapped holes, the length of engagement (the number of threads engaged between the tapped hole and the screw) should be adjusted to favor this criterion. Table 1 summarizes the general rule of thumb for thread engagement distance calculation.

Table 1: General rule of thumb for thread engagement distance calculation

Material	Minimum thread engagement distance
Steel	1.0 to 1.5 x base diameter of the screw or bolt
Cast iron, brass	1.5 to 2.0 x base diameter of the screw or bolt
Aluminum	2.0 to 2.5 x base diameter of the screw or bolt
Soft materials (like plastic)	The minimum thread engagement distance will be higher and require trials to establish the optimum joint strength.

And then for even more precision, you can do the math:

Where:

LE2 = Minimum thread engagement length (tensile strength and shear area ratio)
UTS_external = Ultimate tensile strength of external thread material
UTS_internal = Ultimate tensile strength of internal thread material
AS_s = Minimum thread shear area for external threads
AS_n = Minimum thread shear area for internal threads
n = # of threads per inch
LE = Length of thread engagement
d_1_min = Minimum major diameter of external thread
d_2_min = Minimum pitch diameter of external thread
D_1_max = Maximum minor diameter of internal thread
D_2_max = Maximum pitch diameter of internal thread
A_s = Tensile shear area
d = Basic major diameter of external thread (in, mm)

source: https://www.omniamfg.com/mechanical...ate-fastener-minimum-thread-engagement-length

 

[Baluncore]

I have been told that full strength is obtained with seven turns. But with fine threads, I prefer to use one outer diameter, the length of a nut.

The threads on the rods were probably rolled, not cut, and so will be rounded and under-size at the ends. You might need to cut off the rolled end with a cutting disc to get a constant full thread OD. That will allow closer contact between the screw ends, with good thread when short. If poor LH and RH threads at the ends meet, they will reduce the range of adjustment.

Lateral vibration of a tight stay in the wind will produce very high axial forces on the turnbuckle, due to the triangle of forces being long and thin.
This is also a special case since each lock-nut is working with the rope tension to strip the thread.

If you adjust the turnbuckles under load, without lubrication or an anti-seize compound, the stainless steel threads will probably gall. That may remove some of the thread OD material, just where it is most needed. That could generate the damage you show.

 

[erobz]

On a test run, one of them blew out and almost dismasted me.

It almost dismasted you or the boat???

 

[DaveC426913]

Looking at typical turnbuckles, you can deduce that approximately twice the diameter should be enough:

If that's the case, I was well within the operational range at 5/8"- 3/4".

 

[DaveC426913]

It almost dismasted you or the boat???

If it fell the wrong way, it might have dismasted me too!

 

[DaveC426913]

I have been told that full strength is obtained with seven turns. But with fine threads, I prefer to use one outer diameter, the length of a nut.

I was still well within that.

The threads on the rods were probably rolled, not cut, and so will be rounded and under-size at the ends. You might need to cut off the rolled end with a cutting disc to get a constant full thread OD.

Interesting you mention this. My next question was going to be: should the bolt be loose in its socket?
There's definitely a little play in these turnbuckles. I can wiggle them around a tiny bit, at almost any extension.

My naive thought when I observed this was 'Hey, doesn't that mean the threads are not fully overlapped? Isn't that weak?' But who was I to argue with marine-grade hardware?

That will allow closer contact between the screw ends, with good thread when short. If poor LH and RH threads at the ends meet, they will reduce the range of adjustment.

The screws don't touch in the middle. There's a hole for a lever and locking wire.
The shaft is 4" long and the screws have 1 1/2" of thread.

I guess I could cut 1/4" - 3/8" off, but that doesn't solve my problem, does it? I don't have the option of screwing the turnbuckle in more than it is now. All I would be doing is cutting off useless thread.

Lateral vibration of a tight stay in the wind will produce very high axial forces on the turnbuckle, due to the triangle of forces being long and thin.
This is also a special case since each lock-nut is working with the rope tension to strip the thread.

I think this is exactly what happened.

If you adjust the turnbuckles under load, without lubrication or an anti-seize compound, the stainless steel threads will probably gall. That may remove some of the thread OD material, just where it is most needed. That could generate the damage you show.

I adjusted them at-dock and only hand-tight. Nowhere near as tight as the shrouds are recommended to be.

In fact, one of my theories was whether not having the rigging tight enough could have caused enough wobble to wear the hardware.


My take-away here is that - despite being sold in a marine chandlery, where they tend not to cheap out on hardware - these are cheap turnbuckles, and failed for that reason.

 

[Baluncore]

Interesting you mention this. My next question was going to be: should the bolt be loose in its socket?
There's definitely a little play in it. I can wiggle it around a tiny bit, at almost any length.

If the stainless thread is a tight fit, it will gall.

My turnbuckles never have sufficient range. I would like one thread to be cut on a larger tube, sufficient to house the other thread, so the ends don't meet and stop. Another way would be to stagger the threaded rod axes, so they pass each other in the middle of the turnbuckle.

I would first assemble rigging by hand, then while sailing, take up floppy slack by hand, when the shroud is on the leeward side.

Take care, the term "shroud" has another meaning.

 

[DaveC426913]

Interesting. That turnbuckle I posted a pic of is selling for $75.

The ones I bought are $14.

In fact, my 20/20 hindsight is reminding me that the reason I didn't do this upgrade years ago is because I was looking at $75 turnbuckles, and this time my critical judgement failed me and I bought impulsively,.


This site has the exact size and type, and calls them "Economy". "600-lb working load"
https://ca.binnacle.com/Turnbuckles-&-Accessories/c147/p11499/Stainless-Steel-Economy-1/4"-Turnbuckle-Tubular-Jaw-&-Jaw/product_info.html

 

[Flyboy]

Lemme make sure I understand this setup correctly: these turnbuckles are adjusted at the dock/while moored and not while under load, and are being used to make fine adjustments before/after sailing?

Given the specified thread engagement depth and the diameter, and the quoted price, I think the hypothesis that it just overloaded the material is the most likely one.

 

[DaveE]

despite being sold in a marine chandlery, where they tend not to cheap out on hardware - these are cheap turnbuckles, and failed for that reason.

I always wanted "name brand" hardware on my boat for anything that really shouldn't break. You get what you pay for, there's lots of cheap stuff in that market because the good stuff is expensive. It's a small market with special requirements. I see a lot of turnbuckles like that used for deck railings and such in building construction; applications that can get away with poor quality strength wise.

I would also suggest going to a local boatyard and talking to a pro rigger. They know what fails and what doesn't. They see many more boats than we ever will.