|Topic Review (Newest First)|
|05-26-2007 11:23 AM|
Originally Posted by Goodnewsboy
|05-26-2007 12:52 AM|
|patrickbwells||The boat next to mine has those same devices, only they are directly under the spreader, rather than above. This leads me to believe that they are in fact rollers.|
|05-26-2007 12:18 AM|
Rollers or no rollers, the key element to remember where spreaders are concerned is that the angle that the spreader makes with the upper and lower portions of the shroud must be equal. That is, the spreader must bisect the angle formed by the shroud as it passes over the spreader end.
Failure to observe this rule can lead to a heavily loaded spreader folding up or down against the mast, usually with loss of the rig.
|05-25-2007 11:30 PM|
|sailingdog||That's what I thought they were.... to help prevent chafe. I think the clamps are to prevent them from moving the spreaders down, which would be bad.|
|05-25-2007 10:52 PM|
Are you sure those aren't to keep the foresail from catching on the spreaders?
I seem to recall I've see something similar on smaller boats around here: little plastic discs with a slot and a center hole.
|05-25-2007 08:09 PM|
I wonder if the increased tension should go on the innerstays. This holds the mast basically erect. As force comes on the outerstays the spreader is forced in on the mast producing a bend to leeward. This has the effect of making the sail fuller in stronger winds just when you want the opposite.
If they are raked back to any extent they can push the mast forward at that point flattening the sail to some degree but how much depends on the exact position of the spreaders and rake. This may be small in comparison.
If your stoppers have an effect it may be to limit the tension achievable in the upper outer stays. To a degree this may allow the top to fall off depowering the main. However I suspect that you would want the tensions in top and bottom to be much the same to stop from bending the spreader. So I doubt they do much.
I had a problem with a former boat with some odd standard rigging for that class, namely no inner stay, save one running form the outer end of the spreader to the base of the mast.
In racing we always had trouble in heavy weather, until I finally worked out that the mast was bending the wrong way ie bending out in the middle to leeward.
I changed it to the rig you show with greater tension on the inners than the outers and it made a huge difference.
I guess to some extent it depends how stiff the mast is the spreader rake etc. I just worked on what I wanted the mast to do and figured it out rather than having any special knowledge.
|05-25-2007 05:16 PM|
Originally Posted by BarryL
|05-25-2007 04:51 PM|
Are you sure they are 'stoppers' and not just rollers to help the sail move during a tack? My last boat had the rollers, my new boat does not. I don't really notice a difference.
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|05-25-2007 03:54 PM|
|tenuki||oh, and the reason it's bad for your spreaders to creep up is that once they get high enough the loads start changing from pure compressive to lateral and will bend that spreader the rest of the way up like a twig for the same reasons I described above. They wouldn't creep up under tension, since a small component of the force under tension pushes them down, but as G said, if they are loose on the lee side they can creep up.|
|05-25-2007 03:43 PM|
From an engineering standpoint I would guess they are there to keep the stays from sliding upwards as G described. Stays only 'push out gently' on the shrouds, they generally don't/can't supply the sorts of forces you are thinking about in the directions you are thinking to change the tensioning. Those forces are rather large and I honestly can't think of a way you could change wire tension short of adding more shrouds and attached everything at the spreader. but that would reduce tension on the side you put more wires on, ie the same force would now be spread between the two wires. The question would then be why do that, since you already have shrouds at that point attached near the foot of the spreader. If the spreader was actually able to be strong enough to change the tensions between the upper and lower half of the wire those forces would have to go somewhere. Unfortunately, if the spreader was rigid enough to do that it would act like a giant torque lever and probably bend/break your mast. Spars are not designed to take anything but compressive forces. Look at the way the spreader is attatched to the mast, it looks like you could just bend it down, right, not a lot of up and down strength. But you can push it into the mast directly with a great deal of force, right?
The mast holds largely compressive loads (straight down), the shrouds are under direct tension in the direction of the wire. and the stays have a small compressive load pushing them into the mast. None of the spars can stand much 'sideways' forces but can take tremendous compressive forces, and of course ropes and wires can't stop any sideway force, ie they are pure tension. I'm not at my home computer (photoshop) so i'll have to wait to draw the diagram.
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