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Turnbuckle thread depth for full strength?

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slim
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USA
113 Posts

Initially Posted - 12/21/2015 : 18:00:18

Minimum thread depth for full strength on stay wire turnbuckles? I have always been told that the minimum thread depth for full strength is one diameter for full engagement but i have also been told that three threads is enough. I have a short forestay and was wondering if my knowlege about this is correct? Better to find out on the trailer then on the water right? Any thoughts on this from you engineer types? Good hard data would be nice to calm my worry bug down.

Thanks

1978 - C25 - standard rig - Fixed keel #1040

Edited by - slim on 12/25/2015 11:31:57

Dave5041
Former Mainsheet Editor

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Response Posted - 12/21/2015 : 18:09:22

I can't offer up hard data, but 1 diameter is the rule I'm familiar with. There is no way that three threads would be equivalent to the rod diameter. Standard nuts usually have a thickness of about one diameter. Three threads would also give you no warning of it backing out. I go with at least 3 threads visible beyond the engagement for visual checks.

Dave B. aboard Pearl
1982 TR/SK/Trad. #3399
Lake Erie/Florida Panhandle

slim
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Response Posted - 12/21/2015 : 18:27:25

My trust meter is saying the same as yours Dave. Also let's not forget about vibration. Still one diameter on a 5/16 bolt is not much to look at.

1978 - C25 - standard rig - Fixed keel #1040

Dave5041
Former Mainsheet Editor

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Response Posted - 12/21/2015 : 20:47:25

No, but the only advantage of having one part stronger than the other is that you know where its going to break.

Three threads visible is my minimum, a carry over from engine work where excess is undesirable. I like to have an inch or more; vibration can really make for a bad day.

Dave B. aboard Pearl
1982 TR/SK/Trad. #3399
Lake Erie/Florida Panhandle

dasreboot
Admiral

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803 Posts

Response Posted - 12/22/2015 : 05:05:15

if its that short, put an extender on it. one of those fittings with an eye on one side and a fork on the other.

Todd Lewis
Eowyn 87 TR/WK C25 #5656
ARWEN 84 TR/SK C25 #4031
www.mainsailsailingschool.com

pastmember
Master Marine Consultant

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2402 Posts

Response Posted - 12/22/2015 : 09:19:27

Buy a toggle and add it to the stem fitting.

Frank Hopper

Stinkpotter
Master Marine Consultant

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Djibouti
9076 Posts

Response Posted - 12/22/2015 : 11:05:46

Are we talking about open-body (bronze) turnbuckles or closed barrel stainless ones with "lock" nuts (that don't)? If the latter, how old is the rigging? If original, put the former with new wire high on your list. (Safer--"Search" for more on that here--use "turnbuckle open" in the C-25 forum.) Then you want enough of the bolt going through so the cotter pin hole is usable, which might call for a toggle to add to the length.

Dave Bristle
Association "Port Captain" for Mystic/Stonington CT
PO of 1985 C-25 SR/FK #5032 Passage, USCG "sixpack" (expired),
Now on Eastern 27 $+!nkp*+ Sarge

Edited by - Stinkpotter on 12/22/2015 11:20:25

awetmore
Master Marine Consultant

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1144 Posts

Response Posted - 12/22/2015 : 19:53:39

It needs to extend far enough into the turnbuckle that you can fit the safety cotter pin. That also means you'll have full thread engagement.

If the forestay is too short then just order a toggle, that will extend it by about two inches. They look like this:

Alex W
Seattle, WA
Express 37 "re-Quest"
previously owned 1984 Catalina 25 "Lutra"

Edited by - awetmore on 12/22/2015 19:56:26

Steve Milby
Past Commodore

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Response Posted - 12/23/2015 : 06:12:40

This raises the question of why the forestay turnbuckle seems to be too short. Adding an extension might solve the immediate concern, but it probably changes the geometry of the rig. If the forestay turnbuckle seems to be too short, there are only three likely causes. Either the forestay is too short, the backstay is too short, or the turnbuckles are improperly adjusted.

The standard rig forestay is supposed to be 29' 10". I suggest you begin by measuring the length of your forestay. You can do that with the mast up by attaching a 50' long tape measure to the jib halyard, and then raise it to the masthead. Check the measurement to the end of the forestay. To be accurate, you need to add to that measurement the length of the shackle on the end of the jib halyard, and the knot that attaches the shackle to the halyard.

If the forestay is the correct length, then you need to measure the backstay length in the same manner. If your boat has a single backstay, the length should be 32' 3". If it has a split backstay, the length should be 26' 4".

If the backstay is too short, then you should add an extension toggle to the backstay. If the forestay is too short, then that's the one that should be extended.

If both are the correct length, then the problem is the way they are adjusted. To correct the adjustment, you need to tie a piece of line to the main halyard, take all the slack out of it and then tie it to a stern cleat, to ensure that the mast won't fall when you disconnect the backstay turnbuckle. Then, unscrew the turnbuckle barrel completely. Then, begin to screw it back onto the screws so that the screws on both ends have an equal number of threads.

Then you should do the same with the forestay turnbuckle, to make sure that the screws on both ends have an equal number of threads. That should get the length of the forestay and backstay back to factory specs.

When that's done, you should tune the rig. Tuning it correctly will ensure that it is erect, that the stays are adjusted to the correct tension, and that the mast is raked to the correct angle fore and aft so that the boat will have a comfortable amount of weather helm.

Steve Milby J/24 "Captiva Wind"
previously C&C 35, Cal 25, C25 TR/FK, C22
Past Commodore

Edited by - Steve Milby on 12/23/2015 06:31:16

slim
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USA
113 Posts

Response Posted - 12/23/2015 : 07:34:32

Thanks for all the help guys i think i can make a judgement call on this now and feel pretty good about it. All of your suggestions have been great and very helpfull.

1978 - C25 - standard rig - Fixed keel #1040

islander
Master Marine Consultant

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4018 Posts

Response Posted - 12/26/2015 : 09:58:05

From Boat US..(quote)

(Turnbuckles should have sufficient thread inside the barrel --at least 3/4" - and cotter pins to prevent their coming loose. (Be sure and wrap fresh tape around the cotter pins when you're done.)

Scott-"IMPULSE"87'C25/SR/WK/Din.#5688
Sailing out of Glen Cove,L.I Sound

Edited by - islander on 12/26/2015 10:00:19

Stinkpotter
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Djibouti
9076 Posts

Response Posted - 12/28/2015 : 21:24:57

That's mainly why I asked about open versus closed bodies. With the original stainless closed-body (barrel) turnbuckles, the only thing that prevents loosening or even losing a stay is a lock-nut on the bolt being tight enough to bind on both the body of the turnbuckle and the threads on the bolt. An open-body turnbuckle allows installation of cotter pins through holes in the bolts and wrapped back around the turnbuckle, locking them absolutely.

Also, the bronze open-body will not suffer crevice corrosion or galling with the bolts, as will the stainless steel of the barrel types. And you can see your thread depth, since the bolt will be through the threads and into the center of the turnbuckle.

Closed-body turnbuckles were standard, so their existence raises the question of whether the standing rigging is original to the boat. That would make it awfully old. CD sells exact replacement stays and shrouds with bronze open-body turnbuckles.

Dave Bristle
Association "Port Captain" for Mystic/Stonington CT
PO of 1985 C-25 SR/FK #5032 Passage, USCG "sixpack" (expired),
Now on Eastern 27 $+!nkp*+ Sarge

Edited by - Stinkpotter on 12/28/2015 21:46:37

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