Spreader End Cap on Shroud - Tension?

[Capt Ben]

Hi all,
I have a very simple rig. One set of well swept back spreaders. One set of uppers & one set of lowers shrouds. I just replaced my shrouds and found that the end of my spreaders has a piece that bolts down to the spreader it self and has the ability to lock down the upper shroud in the spreader.

So, how tight should the spreader end cap be? I can readily lock the shroud pending the torque applied to the bolts. BTW, 5/16" or 8mm 1x19 wire.
Thanks in advance for any advice...

[onestepcsy37]

the end cap should just 'capture' the upper shroud to prevent it from coming off the end of the spreader. the upper shroud should be free to move up and down through the spreader end cap when you tighten or loosen the turnbuckles.

[Jim Cate]

FWIW:

On our rig (fractional, two swept back spreaders plus diamonds at the topmast) if we do not clamp the shrouds firmly we get a nasty "gronk" noise as the boat rolls under way and the shrouds work up and down in the spreader ends..

So, in our case we set up the rig with the clamps just snug, sail for a bit, make any necessary adjustments to things, repeat as required to get the tuning just right and then go up and clamp the caps down tight. No noise, no apparent motion, seems to be quite stable over time.

Cheers,

JIm

[Capt Ben]

Thank you for the replies. My gut feeling is to follow onestepcsy37's suggestions. As my mast is tapered I'm able to put quite a bit of bend in the mast with my backstay which dramatically reduces upper shroud tension. It would make sense that the shroud then would need to slide through my spreader vs trying to adjust the spreader angle / bend.

[Paul Elliott]

I've found that if I don't attach the spreader tips fairly securely, over time they tend to work their way downwards and droop. That's not a good look, and not what you want structurally. The droop-inducing force no doubt depends on the geometry of your spreaders and shrouds.

[Andrew Troup]

I'm sure you realise, but others reading this may not: the spreader should be raised at the outboard end to bisect the angle the cap shroud makes

IOW the angle of the upper part of the shroud relative to the spreader should be the same as the corresponding angle of the lower part.

[daddle]

Originally Posted by Andrew Troup

I'm sure you realise, but others reading this may not: the spreader should be raised at the outboard end...

"Should" is a bit broad. Depends on the design. Some are level no matter what. One can probably determine what the spar builder intended by examining the spreader bracket closely.

[Paul Elliott]

Yes, it should bisect the angle of the top shroud. In addition to the cap shroud, I've also got an intermediate (lower) shroud running over my lower spreader tip, and obviously the angle on the lower can't also be bisected. I think the intermediate shroud is the one that puts the downward pressure on the spreader.

[Andrew Troup]

Originally Posted by daddle

"Should" is a bit broad. Depends on the design. Some are level no matter what. One can probably determine what the spar builder intended by examining the spreader bracket closely.

I agree 100% with your observation of practice, but I entirely disagree with the conclusion you draw from it, namely, they wouldn't do it if it was not optimal.

I'll stick with 'should', thanks.

[Andrew Troup]

Paul's point goes broader than the OP's request, but it's a question worth considering in a more general case.

Ideally one would set out a vector diagram, knowing the relative tensions in the cap and the lower shroud, work out the direction and magnitude required to 'poke' each shroud to the angular diversion measured, then align the spreader with the vector sum of these two diverting forces.

This would all be relatively easy, except for knowing the relative tensions. Doable, but not easy.

In practice it's not necessary to get the diagram anywhere near perfect: I would just do the diagram on the respective breaking loads of the two wires, because it's only the direction, not magnitude of the resultant vector which matters. If the relative sizes of the wires made no sense to me, I'd perhaps dig deeper.

As Paul infers, the shroud which is 'poked' more severely will dominate the vector diagram, so the spreader up-angle will be higher on 2-spreader rigs than single.

If the rig was built by a reputable and knowledgeable firm, they will have supplied the builder (who should have passed it to the owner) with a fore and aft view of the rig, showing the spreader angles.

If no such diagram is in circulation, my "ideally..." procedure is all a lot of trouble to go to, if it's a low stressed rig:

Most people would eyeball it and call it good, taking the angle dictated by the bracket as a starting point.

However, if that angle is level, unless the athwartships rigging is an unusual layout (eg B&R), the inference I would draw is that the rig builder did not understand the finer points of their trade.

[Snowpetrel]

Lock it tight. If it slips you can lose the rig.

[daddle]

Originally Posted by Andrew Troup

However, if that angle is level, unless the athwartships rigging is an unusual layout (eg B&R), the inference I would draw is that the rig builder did not understand the finer points of their trade.

Possibly you are more knowledgeable than the rig builders, however the evidence gained by a scan around the anchorage would indicate that rig builders are not as concerned as you are about the spreader angle with the shrouds. Most here have only a slight upward angle, nowhere near bisecting the shroud angle, perhaps only an aesthetic touch. Many are level. Mine are up only tiny bit because like some rigs the shrouds are discontinuous: having a fixed terminal at the lower shrouds. One steel hulk has a jaunty upward angle on her very cautious wide shroud plan and at that falls considerably short of bisecting the angle at the upper shroud ... having all the spreaders angles matching being perhaps more important to the famous designer.

* Things like spreader tips will not easily slip until the angle is off by about 10 degrees. There being considerable friction.

* Many spreaders are built sturdy and simply will not slip under any expected load ... one can stand at the end.

So .... readers should ask a rigger if they are concerned with their spreader angle rather than take pedantic and overly broad advice about bisecting angles from the internet.

[Nicholson58]

Originally Posted by Andrew Troup

I'm sure you realise, but others reading this may not: the spreader should be raised at the outboard end to bisect the angle the cap shroud makes

IOW the angle of the upper part of the shroud relative to the spreader should be the same as the corresponding angle of the lower part.

Perhaps a bit late to the table - but -
You are correct. Ideal arrangement in most cases make equal angles all around. If the angles are not equal then the outboard end must be fixed to the shroud or the system can fail. In my opininon, all riggers will tell you the spreader tip is to be fixed -no sliding allowed. My mizzen has outboard clamps and heavy rubber sleeves/insulators/galvanic cell elimination. My main has all shroud ends pinned with a heavy plate coupler dividing the lower from the upper and intermediates. I have never sailed a boat where the spreaders could travel along the shrouds. I have had boat where the shroud was not clamped but retained between stoppers fixed to the shroud cable, ths free to rotate through an eye.