spreader length.

VicS

VicS

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Would it be true to say that with a conventional rig of cap shrouds plus two pairs of lowers that the spreader length, tip to tip, will be the same as the horizontal separation of the shroud anchor points, making the bottom section of the cap shrouds ( spreader to deck ) vertical
 
I've seen some old boats with wooden masts with what appeared to be vertical cap shrouds but it doesn't seem usual. My HR 34 with double spreaders is nearly vertical but the cap shrouds have to be pretty beefy.
 
Its a nightmare calculation that is almost certainly never done, preferring instead, mathematically modelling a version and looking for improvements.
Just the loading on the mast is not a simple formula, combine that with spreader height and length, a purely mathematical solution is not a sensible approach.
 
VicS said:
Would it be true to say that with a conventional rig of cap shrouds plus two pairs of lowers that the spreader length, tip to tip, will be the same as the horizontal separation of the shroud anchor points, making the bottom section of the cap shrouds ( spreader to deck ) vertical
Click to expand...

Certainly not on my boat - Parker 31.
And, roughly, from memory, not on more than 50% of boats built since 1950.
 
charles_reed said:
Certainly not on my boat - Parker 31.
And, roughly, from memory, not on more than 50% of boats built since 1950.
Click to expand...

The Parker 31 appears to have cap shrouds lead over aft swept spreaders and only one pair of lowers attached to the same anchorage points.
However still relevant info.
I take it the spreaders are appreciably shorter than I am suggesting above
 
As suggested the length of the spreaders is a bit of a variable question. If the spreaders were so long that the bottom part of the cap shroud is vertical this would give the best side support to the top of the mast. But then perhaps this max support is not necessary so spreaders can and often are shorter. The rig you refer to uses the spreaders only to give a better angle of arrival at the mast top so not nearly as critical as aft swept spreaders that provide push forward to the centre of the mast.
olewill
 
Just had a look at the drawings for my rig which is as you describe. The spreaders are roughly 75% of the shroud base distance. They are square to the centreline and canted up by a few degrees.
 
Seems to me that the longer the spreaders are the more effectively they will act as struts but this will be limited by risk of buckling under compression on the windward side and risk of entanglement with the rigging of other boats when alongside. A maximum would be for spreader outer ends to be vertically above the toerails.
 
johnalison said:
I've seen some old boats with wooden masts with what appeared to be vertical cap shrouds but it doesn't seem usual. My HR 34 with double spreaders is nearly vertical but the cap shrouds have to be pretty beefy.
Click to expand...
There is no particular structural reason why spreaders should not be as wide as the beam, or indeed wider. That is the function of those struts that stick out on the sides of the IMOC 60s. However, wide spreaders interfere with the flat sheeting of overlapping jibs, so designers make them as narrow as they think they can get away with. As for the cap shrouds being beefy, the lower shrouds really need to be strong, because the spreaders transfer the cap shroud load to the lowers.

Dougal Tolan said:
Its a nightmare calculation that is almost certainly never done, preferring instead, mathematically modelling a version and looking for improvements.
Just the loading on the mast is not a simple formula, combine that with spreader height and length, a purely mathematical solution is not a sensible approach.
Click to expand...

Designers certainly use mathematics to calculate solutions to rigging loads. They are always guided by the experience with all the designs that have gone before. All complicatedbut it leads to simple choices. Will we use 6mm, 8mm, or 10mm diameter rigging? Or larger, depending on the size of the ship.
 
BobPrell said:
There is no particular structural reason why spreaders should not be as wide as the beam, or indeed wider. That is the function of those struts that stick out on the sides of the IMOC 60s. However, wide spreaders interfere with the flat sheeting of overlapping jibs, so designers make them as narrow as they think they can get away with. As for the cap shrouds being beefy, the lower shrouds really need to be strong, because the spreaders transfer the cap shroud load to the lowers.



Designers certainly use mathematics to calculate solutions to rigging loads. They are always guided by the experience with all the designs that have gone before. All complicatedbut it leads to simple choices. Will we use 6mm, 8mm, or 10mm diameter rigging? Or larger, depending on the size of the ship.
Click to expand...

I dont doubt that maths is used, but not to directly output a best solution, more to test a proposed one (and determine required rigging size for that solution)

What hasnt been mentioned yet is that it may require a structurally lesser solution to reduce snagging a genoa.
 
VicS said:
The Parker 31 appears to have cap shrouds lead over aft swept spreaders and only one pair of lowers attached to the same anchorage points.
However still relevant info.
I take it the spreaders are appreciably shorter than I am suggesting above
Click to expand...

Both caps are led over 2 pairs of spreaders, the top are 650mm long and the lower 1100mm. The inters only over the lower spreaders. Both sets of spreaders are swept back by 22.5°. Distance from mast base CL to shroud fixing points is 1250mm.
The lowers are entirely separate.
I will point out that the Parker is big dinghy and sailed as such.
Most Bavs have aft-swept spreaders, not I think the 22.5° which make the backstay entirely for mast bend. As most Bavs have in mast reefing and are not tunable rigs their fractional set-up is, possibly, pretension.
 
charles_reed said:
Both caps are led over 2 pairs of spreaders, the top are 650mm long and the lower 1100mm. The inters only over the lower spreaders. Both sets of spreaders are swept back by 22.5°. Distance from mast base CL to shroud fixing points is 1250mm.
The lowers are entirely separate.
I will point out that the Parker is big dinghy and sailed as such.
Most Bavs have aft-swept spreaders, not I think the 22.5° which make the backstay entirely for mast bend. As most Bavs have in mast reefing and are not tunable rigs their fractional set-up is, possibly, pretension.
Click to expand...

Sorry but I was specifically asking about a more traditional ( mast head ) rig which has caps and two pairs of lowers and in which the spreaders are not swept aft
 
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