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post #1 of 25 Old 02-27-2010 Thread Starter
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do spreaders need to be bolted tight?

Hi all,
First boat is a p-15, used in good shape. Question is: the spreaders are screwed to the mast with two sheet metal screws via brackets, and are a little loose. Before I replace with slightly larger screws to tighten up, is this necessary--as the tension on the shrouds should hold them tight? Exposing my ignorance further, I would like to ask, what is the purpose of spreaders in the first place? The resultant force vector will be identical to a straight mast-to-chainplate angle anyway. The "bend" in the shrouds, to me, just introduces more play, and if one of the spreaders should buckle or fail, the shroud is now useless, and the mast unstable. Am I way off the mark?

thanks
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post #2 of 25 Old 02-27-2010
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The purpose of spreaders is to spread the shrouds. The mast is a pole that should stay vertical and to support it you have to pull it horizantally in four differen directions. But the boat is not that wide. Therefore to increase the horizantal pull of the shrouds spreaders are presnt. They increase the angle of pull on the mast.

Normally spreaders are pushed towarfds the mast by the shrouds but you have to have some fixing so that they are stable. If the screws are loose they will get more looser as the spreaders oscilate.

Originally spreaders were not perpendicular to the mast but with equal angle to the top and bottom parts of the shroud.which means the spreaders were inclined towards the top of the mast. The spreaders are kept perpendicular to the mast which means further load to move them.
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post #3 of 25 Old 02-27-2010 Thread Starter
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Originally Posted by celenoglu View Post
But the boat is not that wide. Therefore to increase the horizantal pull of the shrouds spreaders are presnt. They increase the angle of pull on the mast.
Thanks, I will definately tighten them up. Anything that is loose, and exposed to motion, will just get looser over time. Universal truth!

As you the comment above, that is what I don't understand. You can put bends in the shrouds to increase the angle off the mast, but then by doing so, you decrease the angle off the gunwale. Net effect, no change, except a longer shroud with more play. No matter how you angle or bend the shrouds, you're not widening the boat, and therefore not altering the ultimate force vector from the mast to the gunwale. I'm not sure if my question makes sense?
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post #4 of 25 Old 02-27-2010
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The spreaders should ideally bisect the angle that the shroud makes over them. If the spreader doesn't bisect this angle, there will be a torque on the spreader, and that can lead to the spreader failing. Spreaders are designed to resist compression quite well, but often not designed to resist torque.

The base of the spreaders should be tight to the mast.

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post #5 of 25 Old 02-27-2010
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In ideal physics world, no, using spreaders doesn't make a difference because the loads on the two shrouds are identical and the angles of the two shrouds to the mast are identical and the mast is perfectly vertical. Also, in ideal physics world, there are no waves, no air drag, no shock loading, and all spars are perfectly rigid.

In the real world, however, a variety of conditions might cause the pull of the shrouds to be not perfectly identical. Masts bend, sometimes suddenly. And the set up may not have been perfect in the first place. On a beat, the tension in the leeward shrouds drops significantly. If the shrouds connected at the masthead at a very acute angle, the horizontal component of the force at the top of the mast would be much less and the top of the mast would be much more likely to fall off to leeward (either suddenly or gradually).

The downside to spreaders is that, while they increase the horizontal component of the force at the masthead, they do so at the cost of increased tension at the deck and throughout the shroud. If you had a stumpy little mast you could get away without spreaders; spreaders are for taller rigs common to modern sloops (which use modern materials and building techniques that can withstand the higher loads).

Anyway, regarding screwing your spreaders into the mast. A better set up might be to through-bolt the brackets to the mast and just have the spreaders sit in the brackets. This probably won't wear away at the material of the mast as quickly as sheet metal screws would, and you wouldn't have the issue of tightening the screws or enbiggenating the hole once in a while. I recently replaced the small cast aluminum spreader brackets on my boat with a new set of large stainless brackets welded onto a plate with a nonconductive coating underneath. The two plates are held to the mast with four bolts. One sheet metal screw on each side keeps a compression post aligned athwartships inside the mast. It looks quite robust and secure.

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post #6 of 25 Old 02-27-2010
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"enbiggenating'- I'm going to add that to "bonkheads" and "fecal turbulator" on the "modern nautical terms" glossary list.
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"enbiggenating'- I'm going to add that to "bonkheads" and "fecal turbulator" on the "modern nautical terms" glossary list.
LMAO! I thought I heard it once on a middle-of-the-night infomercial by a young lady in a low cut dress!

I like the idea of bolting through the mast...but drilling the mast ranks right up there with taking the neck of a vintage guitar though... but should be relatively small holes...
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post #8 of 25 Old 02-27-2010
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If you are going to use larger sheet metal screws, you already have holes in the mast... might as well use them.
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One thing—file down the points of the screws if you have wiring or halyards running inside the mast and use the shortest screws that will do the job. Last thing you need is the halyards getting shredded on the screws you're putting into the mast.

Also, if you through-bolt, make sure to install a compression sleeve inside the mast to prevent the bolts from crushing the spar.

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Last edited by sailingdog; 02-27-2010 at 04:02 PM.
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post #10 of 25 Old 02-27-2010
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The spreaders create a rectangle, which is very rigid torsionally. It's the same principle that allowed colonials to build bridges using lumber that was shorter than the distances being spanned. Read up on covered bridge architecture for more discussion.

The spreaders, as SD said, are in compression, and very little force is required to hold them in place. That's why they were attached with sheet metal screws. If the holes have enlarged, you can replace the screws with larger diameter ones of the same length. Like SD said, dulling the points makes sense if there is anything inside the mast that can chafe. Through-bolting and such really will make the mast weaker, not stronger.

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