Swept back spreaders

[Aquaboy]

Is there, how shall I say 'rule of thump' as to the angle spreaders should be swept back when you have no back stay?
It's not a performance boat and the rig tension is low so mast bending is not part of the equation.

 

[Laminar Flow]

Viewed in profile (laterally) the minimum angle of shroud to mast, especially without back stays, is 5 degrees.

 

[Wing Mark]

If you're not rying to bend the mast fore/aft, then the spreaders would not deflect the shrouds fore/aft, so they'd make the same angle as the shroud base makes with the mast base?
I'm used to dinghies, where I'd expect the spreaders to be angled back a little less, to induce some bend in the mast (top back, middle forwards).

Or are you asking where the shrouds should attach to the hull?
It's not the spreaders which 'do the backstay's job', it's the shrouds, making a fore/aft angle with the mast.

The less shroud tension you have, the further back the shrouds need to be.

If the mast is at all bendy, 5 degrees sounds way too little, compared with dinghy rigs.

Does that help at all?

 

[Laminar Flow]

If you're not rying to bend the mast fore/aft, then the spreaders would not deflect the shrouds fore/aft, so they'd make the same angle as the shroud base makes with the mast base?
I'm used to dinghies, where I'd expect the spreaders to be angled back a little less, to induce some bend in the mast (top back, middle forwards).

Or are you asking where the shrouds should attach to the hull?
It's not the spreaders which 'do the backstay's job', it's the shrouds, making a fore/aft angle with the mast.

The less shroud tension you have, the further back the shrouds need to be.

If the mast is at all bendy, 5 degrees sounds way too little, compared with dinghy rigs.

Does that help at all?

Perhaps I expressed myself imprecisely. The angle of 5 degrees refers to the angle from above the spreader and behind the mast, i.e. in the fore and aft orientation. This is the minimum; any less will impose significantly higher compression loads on the mast. More than 5 degrees is of course better as, conversely, it lowers the compression, but interferes with how far the main can be let out going down wind.

This figure also coincides with fore & aft angle of lower shrouds in general. The lateral angle of shrouds, rule of thumb (mast to chain plate and towards the rail) should never be below 9 degr.

 

[Wing Mark]

5 degrees seems very low for a rig with no backstay.

 

[savageseadog]

I've come across 22°.

 

[Laminar Flow]

5 degrees seems very low for a rig with no backstay.

Larsson & Eliasson, Elements of Yacht Design, Scharping, Konstruktion und Bau von Yachten, read all about it.

 

[Wing Mark]

Larsson & Eliasson, Elements of Yacht Design, Scharping, Konstruktion und Bau von Yachten, read all about it.

And take it out of context?

 

[anoccasionalyachtsman]

Perhaps I expressed myself imprecisely. The angle of 5 degrees refers to the angle from above the spreader and behind the mast, i.e. in the fore and aft orientation. This is the minimum; any less will impose significantly higher compression loads on the mast. More than 5 degrees is of course better as, conversely, it lowers the compression, but interferes with how far the main can be let out going down wind.

This figure also coincides with fore & aft angle of lower shrouds in general. The lateral angle of shrouds, rule of thumb (mast to chain plate and towards the rail) should never be below 9 degr.

Would you mind expanding on this compression?

 

[Aquaboy]

Thanks for all the response........tho' several have answered a different question!!
As I've moved the top of the mast forward with rig alterations I've ended up with the spreaders swept aft way to much which is interfering with how far out I can let the main go when down wind. This weeks project was moving the chain plates forward, well actually fitting 2 new anchors as the lowers still go to the originals. I like the sound of 22 degr. and I think I'm in that area. The other way to ask the question is the angle of the chain plates behind the Tabernacle.

 

[Laminar Flow]

Would you mind expanding on this compression?

The shorter the chain plate base, i.e. closer to the mast, and consequently the narrower the angle of the shroud to the mast, the higher the compression load.

If, for example, which of course would not be possible on a boat or for a shroud, the angle to the mast were 90 degr, the compression would be zero.

 

[Laminar Flow]

Thanks for all the response........tho' several have answered a different question!!
As I've moved the top of the mast forward with rig alterations I've ended up with the spreaders swept aft way to much which is interfering with how far out I can let the main go when down wind. This weeks project was moving the chain plates forward, well actually fitting 2 new anchors as the lowers still go to the originals. I like the sound of 22 degr. and I think I'm in that area. The other way to ask the question is the angle of the chain plates behind the Tabernacle.

Translation for top left: Achterstag wahlweise.... Backstay optional for boats under 1t.
Translation for centre: Minimal ... Minimum 5 degr without backstays.
Translation for bottom centre: Einfache Unterwant ... Single lower shroud with swept back spreader.


Hope this helps.

 

[anoccasionalyachtsman]

Thanks for all the response........tho' several have answered a different question!!
As I've moved the top of the mast forward with rig alterations I've ended up with the spreaders swept aft way to much which is interfering with how far out I can let the main go when down wind. This weeks project was moving the chain plates forward, well actually fitting 2 new anchors as the lowers still go to the originals. I like the sound of 22 degr. and I think I'm in that area. The other way to ask the question is the angle of the chain plates behind the Tabernacle.

Would you mind explaining why you've moved the mast so fat forward that this has all become a problem? Moving the lowers forward could be a very bad idea because it's their job to resist the forward thrust from the spreaders.

 

[anoccasionalyachtsman]

The shorter the chain plate base, i.e. closer to the mast, and consequently the narrower the angle of the shroud to the mast, the higher the compression load.

As I've moved the top of the mast forward with rig alterations I've ended up with the spreaders swept aft way to much which is interfering with how far out I can let the main go when down wind. This weeks project was moving the chain plates forward, well actually fitting 2 new anchors as the lowers still go to the originals. I like the sound of 22 degr. and I think I'm in that area. The other way to ask the question is the angle of the chain plates behind the Tabernacle.

If, for example, which of course would not be possible on a boat or for a shroud, the angle to the mast were 90 degr, the compression would be zero.

Would you like to edit that and expand a little more for the slow to comprehend?

 

[Laminar Flow]

Would you like to edit that and expand a little more for the slow to comprehend?

Sorry, please ignore the part where I re-quote the OP. Seems to have been a case of fat finger syndrome. I edited that now.
I had also written, which seems to have gone missing:

The compression load on a mast is calculated by dividing RM (righting moment) by the distance of mast to chainplate. Consequently, the greater the distance and wider the angle of shroud to mast, the lower the compression.
For practical purposes and to cover one's engineering a** the result of the equation (quotient) is garnished with a safety factor.

A wider shroud base, however, interferes with tight sheeting angles on overlapping head sails. Which in turn leads to multiple spreaders to keep relative shroud angles sufficiently wide.

High compression loads require very robust hulls and heavier mast sections, which goes some way to explain the preference for wide base, fractional rigs without overlapping head sails on lightly built hulls.

 

[Aquaboy]

Thanks Laminar Flow, I now see how the 5 degr. comes in. I couldn't visualise how that fitted. I was looking at the problem in a simplistic form purely from the spreaders and of cause its the shrouds and their angles which are more critical. And when I say problem none of its a problem really, just part of the fun in my mind of sailing a trying things and tweaking etc. All arouse because of a bigger main with much more roach which I was able to offset by moving the mast head forward. hence the issues with the spreaders when running.

 

[Wing Mark]

Sorry, please ignore the part where I re-quote the OP. Seems to have been a case of fat finger syndrome. I edited that now.
I had also written, which seems to have gone missing:

The compression load on a mast is calculated by dividing RM (righting moment) by the distance of mast to chainplate. Consequently, the greater the distance and wider the angle of shroud to mast, the lower the compression.
For practical purposes and to cover one's engineering a** the result of the equation (quotient) is garnished with a safety factor.

A wider shroud base, however, interferes with tight sheeting angles on overlapping head sails. Which in turn leads to multiple spreaders to keep relative shroud angles sufficiently wide.

High compression loads require very robust hulls and heavier mast sections, which goes some way to explain the preference for wide base, fractional rigs without overlapping head sails on lightly built hulls.

Compression load can also be from wind astern , consider the force on the sails resolved into tension on the shrouds on a dead run.
You don't want your shrouds working at a puny 5 degrees unless you've been through the complete design process to understand that it's all going to work.
I have some familiarity with boats where the mast can go over the bow if you let the main out too far!

 

[Daydream believer]

Have a look at the Hunter with a B&R rig. They have quite a bit more than 105 degrees. Plus the rig goes out to the hull.
120 degree spreader angle
They will have designed it rather than guestimated

 

[Wing Mark]

Have a look at the Hunter with a B&R rig. They have quite a bit more than 105 degrees. Plus the rig goes out to the hull.
120 degree spreader angle
They will have designed it rather than guestimated

A lot of those Hunters are roller reefing main, which implies an additional constraint of a straight mast.
Hence they end up with a lot of rigging.
Even so, it shows the consequences of not having a backstay are not trivial.

 

[Laser310]

I probably wouldn't go out of sight of land on a boat with 5 degrees of sweep on the spreaders and no backstay...

some small boats have mast sections that are much beefier than needed.., so you might see it on them.

here is the new Pogo 44 - looks a lot more than 5 deg.., and yes.., it can be a problem going downwind without a spinnaker - you basically need to sail hot angles, or chafe your mainsail.

With the spinnaker up, the high boatspeed puts the apparent wind forward and you can hold a deep course without chafing the main too badly.