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Spreaders.

Allan

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17 Mar 2004
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In Dartmouth recently we saw a beautiful small boat, about 20ish feet long. We noticed it had two sets of spreaders. That started a conversation about the gains of more spreaders, or indeed the negative effect. Our Starlight 35 has two sets, our previous boats, an Etap 22i, Moody 31 and Westerly 33 all had one set. We started to wonder what the deciding factor would be for the number of spreaders?
Allan
 

anoccasionalyachtsman

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Better control over the whole mast length vs drag and cost. It's Monday morning and I'd have to give the weight +/- more thought than I'm currently able.
 

flaming

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As I understand it, and at a simplistic level, the number of spreaders you need is a function of the width of the shroud base and the size (strength) of the mast section. So at one extreme you have multihulls and Imocas with massive deck spreaders that are able to spread the shroud base wide, and also use relatively beefy "wingmast" sections that don't have any spreaders.
And at the other extreme you have the rigs from the old IACC boats, which have a very narrow shroud base due to being very narrow boats, and have a very skinny mast. They have about 5 sets of spreaders.
 

dunedin

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As flaming says, depends a lot on mast section stiffness (as well as shroud base width). Lots of smallish older wooden masted boats with forestay at 3/4 height used “diamonds” to support the upper mast.
 

Laser_310

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As I understand it, and at a simplistic level, the number of spreaders you need is a function of the width of the shroud base and the size (strength) of the mast section. So at one extreme you have multihulls and Imocas with massive deck spreaders that are able to spread the shroud base wide, and also use relatively beefy "wingmast" sections that don't have any spreaders.
And at the other extreme you have the rigs from the old IACC boats, which have a very narrow shroud base due to being very narrow boats, and have a very skinny mast. They have about 5 sets of spreaders.
I think this is about right

IRC boats, for example, often have fewer spreaders than similar-sized boats built to older rules. IRC favours non-overlapping jibs, so the chainplates can be placed far outboard.
 

Laminar Flow

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Spreaders are used to reduce unsupported panel length and hence transverse moment of inertia. This allows for a smaller and lighter transverse mast cross section.

Multiple spreaders on their own do not provide for a smaller longitudinal section, however.

Re narrower shroud base: the minimum angle of shroud to mast should not be less than 10 degr., 9 degr. at the very least, less than this and the compression load on the mast increases exponentially.

So, in combination, multiple spreaders are employed to reduce compression loads by avoiding low shroud angles and reducing mast section by keeping panel length short.

Conversely, a wide shroud base reduces compression loading on a mast, which is why some contemporary designs forgo overlapping foresails in favour of a lighter mast section. Some even use outriggers to this effect.

Total compression load on a mast is calculated as RM divided by 1/2 distance of centre mast to chain plate x safety factor.
 

Ceirwan

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I seem to recall reading some time ago that keel stepped masts essentially get a set of spreaders 'for free' where they pass through the deck. Hence why you see that a keel step mast can be a smaller section than the equivalent deck stepped alternative.

I used to own an Eygthene 24 & that had a double spreader rig which always seemed overkill for a boat that size with a stumpy IOR style rig, however some variants were single spreader with the same shroud base & apparently no difference in mast section. Since the shrouds were essentially inline there wasn't any advantage to tuning the mast shape I could see.
 

Allan

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Spreaders are used to reduce unsupported panel length and hence transverse moment of inertia. This allows for a smaller and lighter transverse mast cross section.

Multiple spreaders on their own do not provide for a smaller longitudinal section, however.

Re narrower shroud base: the minimum angle of shroud to mast should not be less than 10 degr., 9 degr. at the very least, less than this and the compression load on the mast increases exponentially.

So, in combination, multiple spreaders are employed to reduce compression loads by avoiding low shroud angles and reducing mast section by keeping panel length short.

Conversely, a wide shroud base reduces compression loading on a mast, which is why some contemporary designs forgo overlapping foresails in favour of a lighter mast section. Some even use outriggers to this effect.

Total compression load on a mast is calculated as RM divided by 1/2 distance of centre mast to chain plate x safety factor.
Thank you for that information. It's food for thought! I don't fully understand the calculations but it seems to me the positives of having two sets of spreaders would be a slightly stiffer and slightly more robust rig. The negatives being more weight aloft and more things to go wrong. I suppose with more shrouds, having a breakage may be less catastrophic.
Allan
 

Laminar Flow

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I seem to recall reading some time ago that keel stepped masts essentially get a set of spreaders 'for free' where they pass through the deck. Hence why you see that a keel step mast can be a smaller section than the equivalent deck stepped alternative.
Stayed masts, as opposed to free standing masts, are calculated as columns under compression according to Euler's formula.

A keel stepped mast is considered a column with on fixed end, whereas a deck stepped mast is calculated as a column in a pin/pin situation where neither end is fixed. These two variants exhibit different behaviour under load with the one with the fixed end being considerably stiffer. As a result the constant for calculating the keel stepped mast is 50% higher; (for a column with two fixed ends, for example, the factor would be twice that of the pin/pin variant).

As a spreader does not provide any fore and aft support, unlike a deck collar, it really cannot be considered as a spreader. this is the reason that the fore and aft dimension of a mast is usually 1.5 times the transverse dimension. The only intermediate fore and aft support of a mast is provided by intermediary stays, such as a staysail stay, baby stays, running backstays etc. and, to some degree by the fore and aft splaying of the the lower shrouds.

Thank you for that information. It's food for thought! I don't fully understand the calculations but it seems to me the positives of having two sets of spreaders would be a slightly stiffer and slightly more robust rig. The negatives being more weight aloft and more things to go wrong. I suppose with more shrouds, having a breakage may be less catastrophic.
A single spreader rig, properly calculated, is every bit as robust as a multi spreader rig, in some ways more so, as there are fewer parts to go wrong and that need servicing. Multispreader rigs are actually more difficult to trim. The whole point of having more spreaders it to have a lighter rig. There is little doubt that as masts get ever thinner and more delicate and with multible spreaders, running back stays and cheek stays to increase the chance of operator error and points of failiure, the risks are increasing. A rig is a chain of individual parts, where the failure of any single link has the potential to bring down the entire lot.
 

dunedin

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As a spreader does not provide any fore and aft support, unlike a deck collar, it really cannot be considered as a spreader. this is the reason that the fore and aft dimension of a mast is usually 1.5 times the transverse dimension. The only intermediate fore and aft support of a mast is provided by intermediary stays, such as a staysail stay, baby stays, running backstays etc. and, to some degree by the fore and aft splaying of the the lower shrouds.
Are you sure about that? Many boats have aft swept spreaders, with fixed connection at the root, in order to control fore and aft movement of the mast. Often there are no baby stay or forward lower stays.
 

Laminar Flow

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Are you sure about that? Many boats have aft swept spreaders, with fixed connection at the root, in order to control fore and aft movement of the mast. Often there are no baby stay or forward lower stays.
Of course swept back spreaders, or rather the shrouds, provide forward support and yes, the Bergstrom rig, as used in some of the US Hunters and without any backstay, provides some form of a backstaying effect. This style of rig, as in said Hunters, also uses solid struts to mimic the effects of a keel stepped mast. In the scheme of things such rigs are rare(er) and the short staying base on these rigs, where the aft and upper shrouds have to take over the role of backstays, imparts enormous loads on the shroud/stays and quite a few of the Hunters have had problems with their chain plates as a result.
Off wind, swept back spreaders can interfere with the degree to which the main boom can be let out.
In 3/4 rigged boats, the swept back spreader also allows the mast to be bent more easily.

It is not the spreader, which is only designed to take compression loads that holds up the mast, but the shroud. With a wide enough chain plate distance, spreaders would actually not be needed. Some catamarans manage just fine without.

Swept back shrouds are hardly new either, as many gaff rigged boats employed them in combination with, at times, considerable mast rake.
 

pandos

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Without wanting to hijack a thread. I have a single set of inline spreaders on a deck stepped mast, with a mast head rig. 8mm wire rigging.

The shrouds pass through the spreader ends and are merely clamped in place.

Running from a few feet below the mast head are light 5mm wires which connect to the spreader ends and seem to be aimed at stopping the spreaders from dropping when under pressure.

Question. Why is there no provision to keep the spreaders from rising upwards (other than the clamps...) It seems to me that the Selden instructions put the spreaders slightly higher at the ends and thereby increasing the tendancy to slide upwards.

Have I got this entirely wrong?.

Now that the rig seems to have bedded in and stopped stretching should I go up and loosen the light wires and clamps sit on the spreaders and tighten them to the last?.

As a precaution I added a dyneema strop from the end of the starboard spreader to the chain plate, where the light wire seemed not to be in tension suggesting that the spreader was tending to lift upwards... Is this utterly unnecessary or should I add one on the other side...
 

reeac

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As has already been stated, a spreader works in compression. I have a head on photo of my old dayboat heeled at 30 degrees and showing clearly the deformation of the mast centred on the windward spreader. Having multiple spreaders will spread those spreader compressive loads more uniformly along the mast thus giving it an easier time.
 

anoccasionalyachtsman

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Without wanting to hijack a thread. I have a single set of inline spreaders on a deck stepped mast, with a mast head rig. 8mm wire rigging.

The shrouds pass through the spreader ends and are merely clamped in place.

Running from a few feet below the mast head are light 5mm wires which connect to the spreader ends and seem to be aimed at stopping the spreaders from dropping when under pressure.

Question. Why is there no provision to keep the spreaders from rising upwards (other than the clamps...) It seems to me that the Selden instructions put the spreaders slightly higher at the ends and thereby increasing the tendancy to slide upwards.

Have I got this entirely wrong?.

Now that the rig seems to have bedded in and stopped stretching should I go up and loosen the light wires and clamps sit on the spreaders and tighten them to the last?.

As a precaution I added a dyneema strop from the end of the starboard spreader to the chain plate, where the light wire seemed not to be in tension suggesting that the spreader was tending to lift upwards... Is this utterly unnecessary or should I add one on the other side...
That's two questions really.

The spreader should bisect the angle made by the shroud as it rounds the spreader, and in that position there's no force either up or down on the tip. The pinch bolt is there just in case really, but it also lets you set the spreader height before the mast goes up.

Second though - this thin wire sounds very odd - have you got a picture of your boat or a similar one?
 

Ceirwan

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The shrouds pass through the spreader ends and are merely clamped in place.

Running from a few feet below the mast head are light 5mm wires which connect to the spreader ends and seem to be aimed at stopping the spreaders from dropping when under pressure.

Question. Why is there no provision to keep the spreaders from rising upwards (other than the clamps...) It seems to me that the Selden instructions put the spreaders slightly higher at the ends and thereby increasing the tendancy to slide upwards.
As stated above me, the spreaders in theory at least should bisect the angle made by the shrouds.
As for the 5mm wire, its unusual, generally the clamping force of the spreader end is enough to prevent any movement. Do you have a picture?

Its possible that its a relic of some old spreader setup that didn't have any effective clamping.
 

pandos

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That's two questions really.

The spreader should bisect the angle made by the shroud as it rounds the spreader, and in that position there's no force either up or down on the tip. The pinch bolt is there just in case really, but it also lets you set the spreader height before the mast goes up.

Second though - this thin wire sounds very odd - have you got a picture of your boat or a similar one?
IMG_20200918_110523~3.jpgIMG_20200918_110523~3.jpg

I hope I have done this correctly...

The thin wire is actually attached about half ways from the spreaders to the mast head. I am pretty sure it is original as it all looks connsistent. I have only owned boat for 16 years whilst it is over 40 years old.
 

anoccasionalyachtsman

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View attachment 122543View attachment 122543

I hope I have done this correctly...

The thin wire is actually attached about half ways from the spreaders to the mast head. I am pretty sure it is original as it all looks connsistent. I have only owned boat for 16 years whilst it is over 40 years old.
And what boat is it?

It's not the best picture to give a definite answer from, but that looks to me that it's giving lateral support to the upper half of the mast (and definitely not to hold the spreader up). I'm pretty sure that you'll find a decent clamp inside the spreader as part of the tip fitting. The effect being that the lower half of the shroud is carrying the tension from its own upper half and also any tension in that mid panel support. It's not a common arrangement but I have seen it once before.
 

pandos

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And what boat is it?

It's not the best picture to give a definite answer from, but that looks to me that it's giving lateral support to the upper half of the mast (and definitely not to hold the spreader up). I'm pretty sure that you'll find a decent clamp inside the spreader as part of the tip fitting. The effect being that the lower half of the shroud is carrying the tension from its own upper half and also any tension in that mid panel support. It's not a common arrangement but I have seen it once before.
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?
 

Ceirwan

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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?
The spreaders should not be horizontal, the angle shown in the picture looks about right.

I see your point about the clamp taking extra load, but its managed 41 years so far, so I'd guess the designer got it right!
 
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