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Old 25-04-2013, 16:47   #1
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Mast spreaders, are they under lots of pressure ?

I changed my spreaders and brackets just a few days ago and I used rivets for the spreader brackets. The brackets seem quite well stuck to the mast, however I am scared the bracket will break off the mast while sailing and come off (the rivets I put were aluminum, bracket is stainless).

Is there a lot of pressure on the spreaders ? enough that might cause the brackets to break off ?
Also if the spreaders were to come off the mast what would happen ?I mean the shrouds are still attached im imagining I would still be able to sail right ?

Last year I have had several issues with losing one of my shroud turnbuckles and at the end of the season my mast came toppling down because of a turnbuckle ... so I am trying to avoid mast related problems ... as much as possible.
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Old 25-04-2013, 16:55   #2
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Re: Mast spreaders, are they under lots of pressure ?

The spreaders are under lots of load but mostly in compression, so pushing against the mast. I think rivets are okay but not the aluminium ones. Switch them to stainless steel (One by one you can do it with mast standing) and use TefGel to isolate the different metals.
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Old 25-04-2013, 17:02   #3
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Re: Mast spreaders, are they under lots of pressure ?

Andrei, if you loose a spreader you will likely loose the mast as well, and for sure you would not be able to sail with the missing spreader on the windward side.

I agree that replacing the rivets with stainless or monel would be a good idea.

Cheers,

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Old 25-04-2013, 17:12   #4
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Re: Mast spreaders, are they under lots of pressure ?

McMaster Carr sells some nice s/s, high strength, rivets. The ones that don't leave a hole for water to enter are nice.

You will need a serious rivet gun for 1/4" stainless rivets... something like this :

Astro Pneumatic 1426 1/4-Inch Heavy-Duty Hand Riveter - Amazon.com
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Old 25-04-2013, 17:14   #5
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Re: Mast spreaders, are they under lots of pressure ?

Ok good to know my fears were valid. Now I think I am going to try to find 3/16 stainless screws and put them through the mast at the top ... or at least one to be 100% sure they are not coming off
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Old 25-04-2013, 18:04   #6
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Re: Mast spreaders, are they under lots of pressure ?

It's important to angle the spreaders up so they bisect the angle of the stays which they deflect. If there are more than one set of spreaders, you have to compromise because there will be several stays wrapping around each spreader tip.

As for the importance of spreaders to rig integrity:

An acquaintance of mine damaged a spreader when his boat was rolled 360 deg en route to South Georgia.
(longways rather than the more usual sideways ... if you can call a 360 roll 'usual')

I expressed surprise that the rig didn't break.

He shrugged (he's French).

Turned out the mast broke when the next wave hit.

Even though it was not 'even' a technical knock-down.
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Old 25-04-2013, 18:08   #7
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Re: Mast spreaders, are they under lots of pressure ?

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Originally Posted by s/v Jedi View Post
The spreaders are under lots of load but mostly in compression, so pushing against the mast. I think rivets are okay but not the aluminium ones. Switch them to stainless steel (One by one you can do it with mast standing) and use TefGel to isolate the different metals.
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Old 25-04-2013, 22:59   #8
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If the mast is bendy fore and aft and the spreaders are locked you can get some very high loads on the forward end of the spreader bases when the mast bends forward. This is the normal loading and most modern spreader bases wrap around the front of the mast to deal with these loads. The worst case is when the mast inverts or bends aft in the middle. This can commonly pop the aft end of the spreader bases of the mast. Or worse. Dont let the mast invert ever. Or bend forward to much.
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Old 26-04-2013, 02:20   #9
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Re: Mast spreaders, are they under lots of pressure ?

To add to the above, to prevent the mast inverting (bowing aft) and the excessive loads on the spreaders and their bases make sure the mast has a slight prebend with the middle bowing forward. And be careful when you are sailing with a heavily reefed main, as the leach loads can cause the mast to invert. The other common cause of inverting a mast is spinnaker pole loads when shy reaching. Some of the smaller fractional racing boats use the spinnaker pole topping lift as a babystay in strong winds to stop inverting when bashing to windward under a heavily reefed main. I have even heard that some need to have a dedicated removable babystay just for this reason.

Correct backstay tension is also important as is the tuning of the rig and the chocking of the base of the mast (the base should be slightly cut away at the front edge) and carefully chocked where it goes through the deck.

If the rig is a masthead with fore and aft lowers it should be pretty stable, just sight up the spreaders and check that the spreader ends are on a line between the chainplate and the mast tang, otherwise they can be putting lots of stress on the base fastenings. If they are flexible they should align themselves. The forward lowers often need to be slightly tighter than the aft lowers to keep the mast straight or prebent slightly under load.

Fractional and inline spreaders with inline lowers are much more finicky to get right and often have stronger bases to compensate for the extra loads the bendy rigs can impose. Same rules apply, but the racing boys can do some pretty weird things to get the rig bending just the way they want them to under load.

As has also been said make sure the angle between the spreader and the wire above and below the spreader are the same. I use an adjustable bevel to get it right (or some stiff cardboard with a pen), Then lock it onto the shroud with a wire seizing or a clamp arrangement.
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Old 26-04-2013, 02:31   #10
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Re: Mast spreaders, are they under lots of pressure ?

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Originally Posted by Andrew Troup View Post
It's important to angle the spreaders up so they bisect the angle of the stays which they deflect. If there are more than one set of spreaders, you have to compromise because there will be several stays wrapping around each spreader tip.

As for the importance of spreaders to rig integrity:

An acquaintance of mine damaged a spreader when his boat was rolled 360 deg en route to South Georgia.
(longways rather than the more usual sideways ... if you can call a 360 roll 'usual')

I expressed surprise that the rig didn't break.

He shrugged (he's French).

Turned out the mast broke when the next wave hit.

Even though it was not 'even' a technical knock-down.
A roll "longways" is a pitchpole, or half a roll longways. Never heard of boat coming back up longways as well. If that's what happened, then !
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Old 26-04-2013, 02:38   #11
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Re: Mast spreaders, are they under lots of pressure ?

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If there are more than one set of spreaders, you have to compromise because there will be several stays wrapping around each spreader tip.
I was always taught that the spreaders should be correct for the intermediate that runs over that spreader, you can pretty much ignore the cap shroud in most cases. But doing this correctly makes them look wrong so most people compromise. If the rigging is discontinuous it doesn't matter but if it is continuous you need to have a pretty solid clamping system to prevent the dreaded (and very dangerous) spreader droop syndrome. I did read something by rod Stephens about which wire to clamp. Seems there is an issue with differing stretch and movement. clamping both can cause issues if it's done to securely? I think he said to clamp the intermediate as it transferred the highest load to the spreader. Again discontinuous avoids all these problems in exchange for having to climb the damn rig every time to tune it.

Quote:
As for the importance of spreaders to rig integrity:

An acquaintance of mine damaged a spreader when his boat was rolled 360 deg en route to South Georgia.
(longways rather than the more usual sideways ... if you can call a 360 roll 'usual')

I expressed surprise that the rig didn't break.

He shrugged (he's French).

Turned out the mast broke when the next wave hit.
A pitchpole?

My own pet theory is that spreaders are a big weak link in a capsize. They are just not designed for the massive fore and aft loads that rigging being dragged through the water might impose. I think the more modern rigs might be much better in this regard due to the stronger bases and fore and aft moments of inertia for the spreaders. I also think a capsize would be quite likely to cause droopy spreader syndrome. A definite plus for discontinuous rigging.

The other weak point seems to be the gooseneck area. I like the hunter deck struts for this reason. The B&R rig looks to me like it might be a good concept for surviving a roll, especially if it had a backstay of some sort and a inner forestay with runners.
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Old 26-04-2013, 04:32   #12
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Re: Mast spreaders, are they under lots of pressure ?

DH and SnowP: I hesitated to refer to it as a pitchpole, because to me a pitchpole implies a 180.

It was a small boat with a short rig, but it's still pretty far out there, and the only case I'm aware of.

SnowP: And it's not something they made a big deal of (they didn't "pull a Tristan"); the gory details of the incident are not specified in the book one of them wrote, and it's not mentioned in the film they made with the camera Bernard Moitessier lent them, recently reissued on that DVD you got given at the French base - there is just a cryptic photo of the yacht under jury rig.

They gloss over the difficulties of clawing their way to Sth Georgia, upwind all the way, when they could barely sail above a beam reach - hard to tack on every tiny windshift with a jury rig...

The only reason I heard about it at all was I was getting into detail in a technical discussion on spreaders and failure modes, and asked J if he'd ever broken one. Shrugged, looked thoughtful for a bit, then came out with that story.

SnowP: Thanks for your thoughts on rigs. Pure gold.

I agree that spreaders present problems with capsize loads, and I've devoted some thought to that on the rig I'm designing at present. No doubt I'll pick your brains when I've got something roughed out. On a small offshore boat I was involved in with a generously strong single-spreader masthead rig (decent fore-and-aft dimension) we persuaded Yachtspars (with some difficulty) to design and fit the mast bases so the spreaders could swing fore and aft. I'm sure that eliminates a lot of the problems with stress on the roots (or, if the roots are sufficiently strong, transferred local stress to the tube) in a capsize. It also helps alleviate the chafe of the main when running: because the lee rigging is slack, that spreader can swing forwards under the pressure of the full length battens (and, incidentally, retension and restabilise the rig!)

The clamping issue is a difficult one, and I agree that discontinuous rigging solves it - provided the attention to detail is virtually beyond reproach.

As an engineer, I prefer the more determinate solution that discontinuous rigging represents, and the tidier stress paths, and the lack of deformation ... but against that, there are more failure points ....

I still feel squeamish about cold-heading stainless rods, and every head is one more which can pop off. I can see the attractions of Dynex Dux rigging - and particularly in combination with new layouts like the Millenium rig, which put a lot less load on the diagonals, and at the same time on the mast tube (by halving the effective length of each panel).
However I hate the notion that a single swipe from a sharp (or hot) knife could bring the rig down, so I think I'll remain a fan for metal, certainly for the lowers.

I think in an ideal world, rigging rods would be machined out of 2205 barstock, with the threads machined (and/or ground!) directly into the ends of each rod, then the remainder waisted down to the rod diameter in what's known as a Swiss ... (a lathe of special configuration for doing a good job of long, skinny workpieces), then externally honed to remove machining marks, bead blasted to induce a light compressive strain in the surface layer, then electropolished ...

Oops, must have fallen into a reverie there, where am I? Oh, bugger, it's reality. Oh well...
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Old 27-04-2013, 23:10   #13
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Re: Mast spreaders, are they under lots of pressure ?

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... to prevent the mast inverting (bowing aft) and the excessive loads on the spreaders and their bases make sure the mast has a slight prebend with the middle bowing forward. And be careful when you are sailing with a heavily reefed main, as the leach loads can cause the mast to invert. ......

The forward lowers often need to be slightly tighter than the aft lowers to keep the mast straight or prebent slightly under load.

.....
Excellent advice. One other point to keep the mast from inverting, which is often overlooked (it's a counsel of perfection, but worth thinking about when new sails are ordered)

Ideally the head of the sail in the deep reefed positions should fall at, or preferably a little above, the nearest attachment point of an inner forestay or babystay.
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Old 27-04-2013, 23:22   #14
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Ideally the head of the sail in the deep reefed positions should fall at, or preferably a little above, the nearest attachment point of an inner forestay or babystay.
Only issue then can be that the sail doesn't clear the runners (if fitted).
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Old 28-04-2013, 00:41   #15
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Re: Mast spreaders, are they under lots of pressure ?

Agreed, and in that case, the runners should probably be able to prevent inversion, provided the head of the reefed main is kept as close below the runners as reasonably possible, that both runners are set up hard, and provided they attach to the mast above the level of the hounds for the inner forestay.

They should do (but don't always), and even when they do, it's often a bare minimum, as the spreader generally prevents it. It's compounded if the boat designer elevates the height of the hounds higher, relative to the spreader, and hence the backstays, than the mast designer would prefer, which quite often is the case, as it makes for a bigger staysail.

You could also argue the designer has to consider the opposite scenario, when a storm staysail is carried on an otherwise bare pole. He won't want this creating excessive mast bend: it's usual for the staysail to be drastically oversheeted in such cases, as any flogging can be .... undesirable, to say the least.


If that creates an issue trying to follow my recommendation, particularly with some headboards which incline to be fat, it's inevitable that the reefed head will fall below the hounds, but I reckon it's still worth trying to keep that distance minimal, if that region is a sensible place for a reef.

If the head ends up not much more than, say, 1.5 x the fore & aft mast dimension below the hounds, I'm pretty sure the lever arm is not going to be sufficient to be a problem.


(It's another reason to shy away from fat headboards and lots of roach for such a rig, I reckon)
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