Should Spreaders Clamp In Upper Shrouds Tightly?

[BigEasy]

Yes Scott, I think that is good advice for Dino--check out the specs from the manufacturer for your rig.

When I set up my rig a few years ago, I hand tightened the spreader end clamp lightly--didn't really torque it tightly. No way am I going up the mast to loosen those clamps (double spreader rig) everytime I adjust the rig!

 

[kloudie1]

Quick thumbnail example of why you want the tips tight.. The shroud loading wants to compress the spreader but because the shroud angles toward the mast the compression load is angled such that the tip of the spreader will want to rotate downward. As long as the tip clamps the shroud wire, everything stays straight.. If the wire can slide freely through the tip, that downward force on the tip will allow the load to push the tip down.. The down load is not huge.. My thumbnail says about 55-60 pounds down with a shroud load of 200 pounds and the shroud making a 30 degree angle with the mast tip.. Most masts will not have the angle that great, so the down force (at spreader tip)decreases as that angle decreases.. When you include impact loads from a flogging main and lots of wind, there would be a potential of cracking or tearing the spreader base..It does not take a lot of clamp force to hold a wire with 50-60 pounds on it.. so ya may be ok with what you are doing .. but if I were considering heavy weather sailing, I would want my tips clamped well.. My spreader tips are not able to move by design..

 

[Chief RA]

My tips are able to move by design. The huge 1/4" tabs welded to my mast that the spreaders slide over and are bolted to are what provides the horizontal location of the spreader. I think there are at least 2 or more ways to secure the spreader tips and it depends on the type of spreader/cap that you have. Scott and I have just wire holding our shrouds in place, yet the pictures show another method of using a bolt and cap. Based on this discussion: I do not believe there is only one way to hold the shrouds. Chief

 

[kloudie1]

Another consideration.. say your rigging tension results in 0.5% "stretch" in the shrouds.. If you measure the length of the wire from the spreader tip (set at exactly 90deg to the mast) to the end at the mast tip, you can multiply that number of inches by .005 and get the distance that you should raise the spreader tip so that the spreader returns to 90 deg (in the vertical) once the wire is tensioned.. so if the shroud is 100 inches from the spreader tip to the mast top, and you know you are going to tension to .5% stretch, then the tip would be moved upward by (100" X .005) = 0.50inch.. so you'd raise the tip by a half inch with the top of the shroud hand tight before clamping the wire in the tip.. There have been references on this forum to rig tension by measuring stretch, so you should use those instead of my example numbers..

 

[Scott T-Bird]

Kloudie, not that it matters to me, but it seems to me that clamping the shroud at the spreaders makes it more complicated to adjust the rig. Big Easy indicates that the clamps are finally tightened AFTER final rig adjustments. It seems to me that you have to climb the mast to do that. That doesn't sound too unreasonable, but what if you want to make adjustments? Suppose you want to tension the shrouds. When you turn the turnbuckle for tension, thus shortening the shroud, aren't you drawing the tip down? Maybe it isn't enough to worry about, but you are still creating the forces that you say you are trying to avoid.

With my rig, friction would tend to draw the tip down when tensioning the shroud in the same manner, but "working" the rig in rough weather isn't going to have that effect. Tension in the shroud above the spreader just as equally provides an upward force at the tip. The ears set the spreader at the proper angle (slightly raised) in equilibrium when the rig is "working" under stress. There is no need to worry about the spreaders drooping even when the shroud isn't clamped.

 

[Johnb]

There is no need to worry about the spreaders drooping even when the shroud isn't clamped.

I worry without any real need! It does seem to me though that if the spreader tips are not secured to the stay then Murphy's law will come into effect and they will potentially randomly work their way either up or down. Once they are longer bisect the angle at stay the tendency to continue moving in the same direction will only increase.

One inch of movement at the end on a 50 inch long spreader would only produce an angle of 1.15 degrees at the mast. There is almost certainly enough slop to allow that with no bending moment in the spreader, so I don't see wire stretch being much of an issue.

If the tip of the spreader were to move enough for the attachment at the mast to become stressed the forces are going to be large with 4 to 5 feet of leverage and then I would expect damage.

 

[justsomeguy]

snip-

I worry without any real need! -snip

Most folks do.

 

[Jackdaw]

Here is how you find the shroud/spreader attach point EXACTLY.

With the mast down and the shroud in the top socket, pull the shroud tightly down the length of the mast until it crosses the mid-point of the spreader at the base. THAT is the exact mathematical bisection of the angle. Hold or mark this spot, and tighten 1/2 inch ABOVE this point into the spreader tip. The extra 1/2 inch allows for wire stretch.

 

[Salty Canuck]

Just a thought, but what clamping would be allowed if the cables in a shroud were replaced with the no-stretch polymer rope. I'm sure that clamping a rope with metal would not be dynamically identical to a clamped wire. Differences in crushability, etc.
Rod

 

[Salty Canuck]

polymer vs. wire shrouds

Just a thought: if you replace a wire shroud with a no-stretch polymer rope, would you clamp it to the same extent? I suspect that polymer rope will not tolerate the clamping forces that could be applied to SS cable. Thoughts?
Rod

 

[Salty Canuck]

Sorry for the double entry. I thought I lost the first one. Rod

 

[stvnm]

The old rigging on my h23 was clamped and the wire showed an obvious bent. The rigger at West marine NC who made the new one recently and the rigger who commissioned the boat insisted that the wire should be allowed to slide to let forces even out and avoid stressing the wire.
Steven

 

[justsomeguy]

The old rigging on my h23 was clamped and the wire showed an obvious bent. The rigger at West marine NC who made the new one recently and the rigger who commissioned the boat insisted that the wire should be allowed to slide to let forces even out and avoid stressing the wire.
Steven

I'm going to take issue with these "riggers". Have you been sailing your boat much with the tips like this?

 

[rjenglan]

I recently purchased new upper side stays from Rudy at D&R Marine and had the same question. His response was to thread the keeper wire so that it held the stay against the spreader but allowed it to move freely. To fasten the stay tightly against the spreader could mean bending the spreader as the mast stresses the stays.

 

[Jackdaw]

This thread is so full of dis- or mis- information that I don't know where to start. OK maybe here.

From the rigging guide of the world's largest maker of masts.

https://dl.dropboxusercontent.com/u/955501/Selden Hints and Advice.pdf

 

[stvnm]

Opinions based on different experiences a rigging have their value and interest. However, rather than making some blanket judgment based on the most authoritative source and blindly apply it to all cases I'd be interested in the physics supporting the design and engineering of the particular rigging and why loose or tight is better or even indispensable.

Peace,
Steven

 

[Jackdaw]

Opinions based on different experiences a rigging have their value and interest. However, rather than making some blanket judgment based on the most authoritative source and blindly apply it to all cases I'd be interested in the physics supporting the design and engineering of the particular rigging and why loose or tight is better or even indispensable.

Peace,
Steven

An interesting point, but I'd hardly call it 'blindly'. The engineering behind my statement that they should all be tight is widely understood and supported. I'd be interested in ANY engineering-based opinion on why you should leave them loose. If that is what you are implying, I'd be interested in yours.

 

[Kingjim91]

Hmmm ... my spreader tips have an open slot to accept the shroud. The shroud is held in place with monel wire criss-crossed over the shroud and secured. The angle of the spreader is fixed by the ears and doesn't move when the spreader is fixed by thru bolts. It seems to me that there is no reason to fix the shroud to the end cap so that it can't slide through the slot (but the monel wire does tend to create friction that generally holds the shroud in place). When the mast is in place, tensioning the shroud fixes the spreader in line with the angle of the ears. If the shroud was fixed, tensioning the shroud at the deck would pull the spreader down, wouldn't it? I want neither upward or downward pressure at the end of the shroud. I want only inward pressure as the shroud is tensioned.

I have to go with the advice to check the specs of your own boat. There are many design variations.

Similar to Scott, my C-30 has spreaders (wooden - I made new ones over the winter) with just a U-shaped groove at the end, and a U-shaped stainless strap to protect the wood from wear and prevent splitting. No clamp. It's also safety wired. It seems like the brackets attaching the spreaders to the mast hold them at the right angle for rigging. If the angle was off a little, I could loosen the shrouds and nudge the spreader ends with a pole, but that hasn't happened.

 

[Scott T-Bird]

I'm sure that instructions for individual spars are going to differ. I don't have Selden, I have Kenyon and the spreader tips don't have a captive end. It does appear that the original poster has a captive end, so I won't argue that he shouldn't clamp the end based on the information that has come to light in this thread.

However, as an engineer, I would suggest that the sequence and procedure should be followed carefully. As Big Easy indicated, his instructions are to leave the clamp loose until all final rigging adjustments are completed. Only then, clamp the shroud in place.

I don't know how you can do that with the mast on the ground. Jackdaw, I read your procedure and it left me with a lot of questions. You can't possibly mark your shroud accurately with just hand tension. Then you seem to give it a half-inch just for what? wiggle-room? Besides that, how do you clamp it half-inch shorter without having to apply an even greater amount of tension? Perhaps the spreader has a half-inch of wiggle so you artificially push the end of the tip towards the top of the mast? ... I don't know. My spreaders don't seem to have that much wiggle when they are bolted to the ears. You seem to indicate that you just get it close for clamping the wire and then accommodate the "stretch" when tuning. I think that this leaves a lot of room for error.

What if you get the measurement wrong before you clamp the shroud? Then you can only equalize the tension on both sides of the spreader by distorting the spreader. That's why Big Easy's instructions indicate that the shroud should be free to align during rigging, before clamping it down after all final adjustments. In my mind, this can only be accomplished when the mast is in place and the shrouds connected to the chainplates and tensioned. That necessitates that the mast must be climbed to clamp the spreader tips.

This seems more complicated to me than simply capturing the shroud and allowing equalized tension to occur naturally. I agree that friction isn't desirable. A better design would utilize a frictionless fitting so that there are no tangential forces on the spreader tip, up or down. The only force that is wanted to keep the spreader in proper alignment is the parallel force in compression along the length of the spreader.

The objective in all cases is to have equalized tension in the wire from top to bottom. By clamping the wire, there is an increased potential for the forces to differ on opposite sides of the clamp, which would then lead to more potential for wire failure.

 

[Scott T-Bird]

BTW ...

I seem to have an unlimited amount of time to contemplate this issue ... see my post in AAS.