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Spreaders.

anoccasionalyachtsman

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It's an old (1980 Hr352. Yes it's a poor photo as others were either of bad angles or I was unable to crop to fit the upload limit..

What you say makes some sense, but in any case the ability to support the upper middle of the mast will require the spreader to remain horizontal and thereby reliant on the clamps...(which are not that impressive...)

So eitherway, worth getting the spreaders just horizontal with the clamps tight with the light wires reasonably taut?
As @Ceirwan says, definitely not horizontal and the clamps are doing there job!
 

Laminar Flow

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When a spreader does not bisect the angle to the shroud it receives an additional bending load and is no longer under pure compression. They are not designed for that and may fail with potentially catastrophic results.

That said, in multi spreader rigs when there are multiple shrouds running over the end one has to sometimes compromise and choose an "in-between" setting and beef up the spreader.

In regards to the pic of the Halberg unless there is some means of tensioning the 5mm wire, then it really is only there to hold up the spreader, as I seriously doubt that a simple clamp could take the potential 2.4t this wire could hold.

I have seen similar arrangements on boats with an additional staysail, but this would require a more secure transfer of load to the deck, either by having a shared terminal at the end of the spreader or by running the intermediate shroud all the way through. It would also require some form of backstay.
 

anoccasionalyachtsman

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That said, in multi spreader rigs when there are multiple shrouds running over the end one has to sometimes compromise and choose an "in-between" setting and beef up the spreader.
Only one shroud changes angle at the end of each spreader, and that's the only angle that needs to be bisected The other(s) can pass straight through unclamped and don't call for any compromise.

In regards to the pic of the Halberg unless there is some means of tensioning the 5mm wire, then it really is only there to hold up the spreader,
You're kidding?

as I seriously doubt that a simple clamp could take the potential 2.4t this wire could hold.
A simple clamp, holding important things up around the world.

 

Laminar Flow

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Cute. These clamps are not recommended for stainless, never mind 1x19. Even when used for galvanized steel cable it should be only used in looped applications, such as with a thimble, and in alternating groups of three.

I have designed and built a few rigs and have even used such clamps on occasion.

Only one shroud changes angle at the end of each spreader, and that's the only angle that needs to be bisected The other(s) can pass straight through unclamped and don't call for any compromise.
That is only correct if all the spreaders were the same length, which is not a given, or, as in the sampled HR, if it were say, a shroud for a intermediate headstay. I know that with some certainty having designed a very similar rig for a large cruising cutter.
I would suggest you read up on that. Larsson&Eliasson, Principles of Yacht Design would be a good start.
 
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Allan

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Cute. These clamps are not recommended for stainless, never mind 1x19. Even when used for galvanized steel cable it should be only used in looped applications, such as with a thimble, and in alternating groups of three.

I have designed and built a few rigs and have even used such clamps on occasion.


That is only correct if all the spreaders were the same length, which is not a given, or, as in the sampled HR, if it were say, a shroud for a intermediate headstay. I know that with some certainty having designed a very similar rig for a large cruising cutter.
I would suggest you read up on that. Larsson&Eliasson, Principles of Yacht Design would be a good start.
When dissecting the angle was mentioned earlier I went out side and looked at our rig. As you say the first spreaders are longer than the second spreaders. As two shrouds go through and exit at different angles what angle should the spreader be set at?
Allan
 

pandos

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Thanks for the replies. Just to add. There is an adjuster on the end of the 5mm wires.

I have seen a similar rig on a newer 352 with 8mm wire in place of the 5mm and this and the top shroud terminated at the spreader ends so no worries about a clamp slipping. As I write this it occurs to me that the reason for the light wires must be to provide lateral support in circumstances where in this design the spreaders cannot move upwards or drop...

As to the comments that the designer was happy...he later changed to the more modern rig (described above)and later to a taller mast with two spreaders.

The comment about bissecting the angle has me a bit puzzled as I think that would require the spreaders to be well above horizontal... But I'll certainly give it some thought...

I'll have a look tomorrow at the current arrangement...but I'll be leaving the dyneema strop in place and adding one on the other side thereby removing the potential danger in the event of the clamps not holding...

Decent wind forecast for 60mile trip Friday and Sunday so fingers crossed...
 

Laminar Flow

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The comment about bissecting the angle has me a bit puzzled as I think that would require the spreaders to be well above horizontal... But I'll certainly give it some thought...
That is correct, if the spreader bisected the angle, as it should, it would be above horizontal.
 

Laminar Flow

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When dissecting the angle was mentioned earlier I went out side and looked at our rig. As you say the first spreaders are longer than the second spreaders. As two shrouds go through and exit at different angles what angle should the spreader be set at?
Allan
You use the average medial angle.
 

pandos

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You use the average medial angle.
In the single spreader rig, Selden say to put both ends of the spreader the same distance from the point where the shrouds connect,

If I understand this correctly I need to have the angle between the mast and the spreader the same as the angle between the upper part of the shroud and the spreader end.
 

anoccasionalyachtsman

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In the single spreader rig, Selden say to put both ends of the spreader the same distance from the point where the shrouds connect,

If I understand this correctly I need to have the angle between the mast and the spreader the same as the angle between the upper part of the shroud and the spreader end.
Yes, or the angle between spreader and shroud equal above and below.
 

reeac

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You use the average medial angle.
Yes, or the angle between spreader and shroud equal above and below.
I think of medial angle as a medical term and I don't know which set of angles you would be averaging.
The fundamental requirement is, by setting the spreader symmetrically between the upper and lower lengths of the shroud, to get the vertical components of the shroud tension in each to cancel out, thus putting the spreader in pure compressiion.
 

Allan

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Earlier in this thread, the figure of 10 degrees was mentioned. I took this to be the minimum angle between the mast and the top shroud. If that is the at the top of a single spreader rig and below the spreader the shroud is parallel to the mast the turn at the shroud must be 170 degrees, so the spreader should be raised by 5 degrees to go into pure compression. Does that sound correct?
Allan
 

reeac

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Earlier in this thread, the figure of 10 degrees was mentioned. I took this to be the minimum angle between the mast and the top shroud. If that is the at the top of a single spreader rig and below the spreader the shroud is parallel to the mast the turn at the shroud must be 170 degrees, so the spreader should be raised by 5 degrees to go into pure compression. Does that sound correct?
Allan
Sounds right to me as long as the lower shrouds are vertical ie. the spreader length including half the mast breadth is equal to half the beam of the hull measured between chainplates. I have to admit that I don't know if this is always the case.
 

anoccasionalyachtsman

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Earlier in this thread, the figure of 10 degrees was mentioned. I took this to be the minimum angle between the mast and the top shroud. If that is the at the top of a single spreader rig and below the spreader the shroud is parallel to the mast the turn at the shroud must be 170 degrees, so the spreader should be raised by 5 degrees to go into pure compression. Does that sound correct?
Allan
Yes, quite right. In practice though, getting the angles equal by eye is really easy - added to which you'll normally find that the socket is already angled correctly.
 

Laminar Flow

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Earlier in this thread, the figure of 10 degrees was mentioned. I took this to be the minimum angle between the mast and the top shroud. If that is the at the top of a single spreader rig and below the spreader the shroud is parallel to the mast the turn at the shroud must be 170 degrees, so the spreader should be raised by 5 degrees to go into pure compression. Does that sound correct?
Allan
Yes, 10 degr, or there abouts, is the minimum angle between mast and shroud. This varies in practice and a (much) greater angle is always desirable. The spreader must bisect the angle at the outboard end of the spreader. If the angle of the shroud were 160 (at the spreader) degr then the shroud angles above and below the spreader tip would be 80 degr to the spreader.

In practice this is not easy to get accurately right, when the mast is down, due to the inevitable degree of stretch. If you can, it is better to fine tune once the mast is up, but you are not likely to lose your rig if you are out by a degree or two.

To be sure, a horizontal spreader is not correct, but as one sees occasionally, a droopy one is just plain wrong.

Lastly, the safety margins in a cruising rig are pretty substantial and often to the order of 3 or more.
 

Laminar Flow

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I think of medial angle as a medical term and I don't know which set of angles you would be averaging.
The fundamental requirement is, by setting the spreader symmetrically between the upper and lower lengths of the shroud, to get the vertical components of the shroud tension in each to cancel out, thus putting the spreader in pure compressiion.
Sorry, I was quoting & translating from a different language and if you substitute "mean" for medial it makes more sense:
As in mean average angles.
I'm not sure how the length of a shroud enters into it as due to the different angles the upper and lower lengths are quite different and even in a single spreader rig the spreaders are not a 50% mast height either.
 

Laminar Flow

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Sounds right to me as long as the lower shrouds are vertical ie. the spreader length including half the mast breadth is equal to half the beam of the hull measured between chainplates. I have to admit that I don't know if this is always the case.
In my experience, the spreaders, whether single or multi, are usually shorter than half beam chain plate to mast.
 

pandos

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Yes, or the angle between spreader and shroud equal above and below.
I think Alan in #33 is correct.

Selden say to put the end of the spreader the same distance from mast fitting as the inner end.

Therefore an isosoles triangle is formed, with the spreader as the base.

So the angles from the mast to spreader and the angle of spreader to shroud should both be the same...or about 85 degrees if the shroud top to mast angle is 10 degrees (180-10/2)!


The rig is now tight, but the spreaders look horizontal , presumably caused by all of the stretch in the shroud being taken out at the base, so I guess it will be a bit of tinkering by slackening the clamps and taking up on the 5mm wires..?

Thanks for all the insights... (Ps I drafted this hours ago but had no coverage so it remained unsent)
 

anoccasionalyachtsman

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I think Alan in #33 is correct.

Selden say to put the end of the spreader the same distance from mast fitting as the inner end.

Therefore an isosoles triangle is formed, with the spreader as the base.

So the angles from the mast to spreader and the angle of spreader to shroud should both be the same...or about 85 degrees if the shroud top to mast angle is 10 degrees (180-10/2)!


The rig is now tight, but the spreaders look horizontal , presumably caused by all of the stretch in the shroud being taken out at the base, so I guess it will be a bit of tinkering by slackening the clamps and taking up on the 5mm wires..?

Thanks for all the insights... (Ps I drafted this hours ago but had no coverage so it remained unsent)
Firstly, I think that having the spreaders a couple of degrees down from their ideal position isn't worth losing sleep over. I did a quick calc and it's not until they're three degrees wrong that they'd even think about sliding even if there was no clamp. My keelboat has no clamps so the only thing that keeps the spreaders angled is the socket! And don't tell anyone, but they're only just above horizontal.

Secondly, it won't be quite that simple. You have some clamps in the spreader tips which you may be able to access from outside. Other types require the tip to have a retaining screw removed and the tip then extracted to get to the clamp. To do that you need to slacken the cap shroud significantly.

First thing to do then is find out what sort of tip clamps you have. If they're internal I'd leave it as it is.
 
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