[SanderO]

My boat is a 7/8 fractional rig.... tightening the back stay is used to curve the mast and luff of the main and flatten it. Tightening has little to no effect on the head stay tension... shrouds on swept back spreaders do that.

 

[SanderO]

I believe the shrouds on the swept back spreaders would have to tension when the mast "moves" forward at the point of their attachment which is the location of my forestay. This is my conception of what happens:

The forestay essentially has the same tension / same length (of course) and doesn't "move". The mast BOWS and with it the main changes shape.

 

[SanderO]

You are not getting this right at all. Your analysis would only be partially right if you were not flying the jib. Once you fly the jib, the forces on the jib cause the forestay to sag. If you added the sag in the forestay and analyzed the load paths correctly (Please see image below) you would find that the loads on the forestay increased dramatically with backstay tension and that would reduce the sag in the forestay allowing the hounds (attachment point for the forestay) to move aft.


Backstay Loads on the Rig by Jeff Halpern,
In the illustration above, the blue lines represent the rig before the backstay is tensioned and the red lines are after being tensioned. The arrows show the forces acting on the mast. Assuming a 7/8 fractional rig, per above, if the horizontal load aftward from the backstay is 7 units, then the base of the mast would need to exert an aftward force of 1 unit to resist rotation. To prevent the mast from falling over the stern, the forestay would need to exert a forward force equal to the force from the backstay plus the force at the deck that would therefore be equal to 1+7 or 8 units.

But that is not all the forces at play.. As the mast bends and the side loads try to move the mast sideways, the geometry of the upper and lower shrouds further add aftward forces to the rig. Those aftward forces are not subtracted from but added to the loads imposed on the forestay so that the actual horizontal load on the forestay at the hounds equals the combined 1 unit at the base of the mast + 7 units at the top of the mast, + the aft forces at Uppers and the aft forces at Lowers

Those large forces take the sag out of the forestay (effectively lengthening the distance from the stem to the hounds) and allows the hounds to move aft as shown.

I hope that clears that up for you.
Jeff

NO... the location of the attachment of the forestay does NOT move and sees no change in forces.
The problem with you "geometry" is that how is it possible for a mast to bend its whole length?
The arc of the mast passed through the fore stay attachment... which is held "in place" but the swept back stays and the forestay... Pulling the top of the mast aft at the top bends it... mine mast is tapered at the top as well... and causes the entire mast to bow... with 2 fixed points... the step and the fore stay attachment tang.
The length BELOW the forestay tang pulls the lower portion or mainsail (luff) - FORWARD
and
the upper portion of the mainsail luff - AFT
obviously in varying amounts in a smooth curve
This FLATTENS the main, removing draft

According to Selden who produced my rig. "forestay tension is adjusted using the runners"

It's in their publication titled HINTS AND ADVICE - On Rigging and Tuning your Selden Mast

 

[SanderO]

You need to account for the fact that when the foresail is set it is pulling sideways and back. the forestay curves and that brings the mast forward at the forestay attach point. and increases the load on the forestay

That has nothing to do with bending my mast with the back stay.
How much do you think the additional force at the forestay tang is when you sail in 15kts true?
Do you not recognize that the swept back spreader and shrouds "balance" the forestay?
(If the didn't... the mast would fall forward)

 

[SanderO]

I will take that to mean that you have never had to engineer a rig, or work with a sailmaker to buy a sail intended for a wide wind range. But down to question at hand, since the mast does not fall down the forces on the mast need to be in equilibrium. That means that any new load imposed anywhere on the rig needs to be offset by an equal and opposite force somewhere else on the rig. If you think that the forestay "sees no change in force" with increased backstay tension, then you need to work through a load diagram that explains where that increased force is resisted. With that in mind, I suggest that you look at the load diagram that is in post 21 and the caption below that diagram. If you think that diagram has an error, please let me know where you think that the error is so we can talk through this. I think that it will be helpful to anyone reading this thread.

Similarly, if you think that "the location of the attachment of the fore stay does NOT move" you clearly have never paid attention to forestay sag. Forestay sag is important to understanding why the position of the hounds moves fore and aft relative to the stem. The effective length of the forestay changes with sag. When there is a lot of sag, the distance from the stem to the hounds is shortened, and when there is minimal forestay sag, the distance is increased. That increase allows the hounds to move aft relative to the stem of the boat. The pictures below show the amount of headstay sag with very little tension on the backstay. (The magenta line is straight line in each picture.) that is roughly 8-10 inches of sag in the top picture. Increasing backstay tension can cut that in half. (I probably should have tensioned the backstay before this picture was taken so that I could stand the boat up a little)
View attachment 137575 View attachment 137576

If you want to see how forestay sag operates in real life, I suggest that you lie down on the bow of your boat and sight up the forestay while under sail. Then have someone tension and easy the backstay. You will be amazed at how much the sag increases and decreases with backstay tension.

Respectfully,
Jeff

I have never designed a rig. I have tensioned/tuned my rigging using the Selden manual.
I am not an engineer either. Your magenta line is appears to be aft of the tack location slightly
A taught line with sag under centenary forces... or I suppose when loaded by a sail. It will stretch under load as a floor deflects under load.
The forestay is pinned so under load and "sagging" it will bow and "pull" at the tack fitting and the mast which is where the force that moves the boat is applied.
So the loaded forestay will stretch a bit and pull the mast forward a bit. That force is "resisted" by the aft lead shrouds which is another location where forces is applied to move the hull forward.
The mainsails forces are transferred to the mast and the boom and to the mainsheet

So the geometry question is how does the mast change shape with an increase in back stay tension... no change in tension of the shrouds or forestay?

I looks to me... that if the forestay is sagging the mast has bowed forward somewhat at the location of the forestay tang.

Runners and check stays can pull at the mast bow flattening it... adding tension to the forestay... counter acting the backstay and sail caused tension. Obviously running backs provide needed mast support in strong wind forces.

If the wire is sagging and presumably not lengthening it has to pull at its ends..
My lee shrouds slacken especially with strong winds. The rig is being moved to lewward. The windward shrouds hold the mast tipping over and see increased forces of the wind.

My boat can "sail" with no back stay. It is help in place by the shrouds which are "opposing" the forestay forces.
Adding tension obvious pull the mast aft from the top. It is prevented from toppling by the forestay pulling forward and the shrouds pulling aft... and from falling athwartship by the shrouds.

Next summer I will see if I can observe what is going on in real world sailing.

I don't know what hounds are.

 

[SanderO]

Jeff,
I am still trying to understand the forces in the rigging.

forestay sag - sag can only be caused by:
catanary forces
movement of the hounds forward (moving aft would "remove" sag)
lengthening

mast at dockside is held vertical (guyed) by the shrouds and the forestay. The backstay is not supporting the mast. In fact when hauling the backstay is "removed" so the travel lift can get in position.

Under sail upwind
sail will exert a force along the luff (I am unsure how those forces are distributed. Intuitively I would guess that the forces approach 0 at the upper swivel fitting of the roller furler... and increase to max near the furler drum. Sheet angle is supposed to be "aligned" to the mid point of the luff. The "loaded" sail will cause the forestay to sag. The sag is somewhat analogous the the deflection of a loaded floor joist.
The lee shrouds slacken and the windward shrouds take additional load. The shrouds are tuned to the mast straight as the boat heels and the shroud loads shift to the windward side. You don't want the lee shrouds too loose or the swage connection will fatigue from movement of the shroud.
Tensioning the backstay (shortening) will pull the top of the mast aft causing it to bend... because it is "fixed" at the hounds... where the forestay and the upper shrouds are located. Compression forces (vertical from tensioning the backstay) will also contribute to the mast "bowing".
Mainsail flattens when mast bows... draft is reduced.

To remove sag from forestay, tighten windward runners and check stays which at attached above and below the "hounds" and pull the mast at this location aft.

So... the fuzzy thing in my mind is how much do the hounds move aft by tensioning the backstay. If the hounds to move aft...the shrouds will slacken a bit and the mast will lose aft support mid length. I am thinking why runners are important in more than moderate winds.

If the hounds don't move aft... the forestay lengthening is likely only from loading by the sail.
What is the effect of sag? Is or can the sail be designed to take the forestay sag into consideration.. ie it is designed such that a loaded luff is slightly bowed in shape.
The vector diagram... not to scale.... shows the fore and aft forces counteract and net force is 0... hounds is not moving forE or aft. To add tension... ir reduce forestay sag the hounds MUST move aft. How far aft must it move to reduce sag? Seems to me that moving aft an inch or two will have a very very little impact on forestay sag/tension. It's like rotating a rigid "column" a small fraction of a degree.

So... in a masthead rig... the forestay when loaded and sagged will pull at the masthead (forward). Adding backstay tension counteracts the sag by pulling the masthead aft. This is what runners do in a fractional.

I am not understanding your explanation... because I don't see how the hounds move enough to add tension to the forestay.

 

[SanderO]

SanderO, What type of boat do you have? I want to look up pics of it to help me understand.
I have little to no experience with fractional rigs. Am I understanding correctly the following correctly?
You have a backstay that attaches at the masthead.
You have running backstays that attach to the mast at the same height as the fractional headstay.
You also have check stays attached to the mast at a point lower than the headstay.

I have a Contest36s The running backs are not being used. I would use them for offshore and if I had to sail in heay weather upwind for a decent period of time. Rig is fine without up to maybe 20kts true.
My runner and check stay are attached to a 4:1 block and tackle which is clipped to the slotted toe rail. I can tension using the block and tackle... or even use a winch for more tension. One stay connects above the ht of the forestay... and the other below it. They are not on in this photo.

 

[SanderO]

Looking at the photo of SanderOs boat I am surprised at how "sporty" the rig looks. It appears to be a pretty bendy section, and a fairly extreme fractional rig.

It is. I can bend the top with a 6:1 block&tackle

Having so much un-stayed mast above the forestay must allow significant mast bend with backstay tension. The spreaders are swept, but not radically so. I would imagine that running backstays on that rig would contribute significantly to forestay tensioning.

Agreed

Certainly more so than on a 7/8 frac rig. It also looks like the rig would be prone to pumping in heavy air and seas, hence the need for check stays.

It is called a 7/8 rig. It's a Selden spar. The shrouds are 9 or10 mm Ø 1x19), I've never notice the mast pumping. The mast is deck stepped.

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[SanderO]

That is a 7/8 rig? It looks like more to me.

It is more like 3/4 by my measurement.

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Selden calls it 7/8... I don't care really the number... but I like the performance!

I drew this from "memory" and actually extended the mast... I will scale the down. But measuring from the photo it appears to be a 13/16 rig without respect to distortion.

 

[SanderO]

If you can bend your mast with with only a 6:1 tackle that sounds like a wildly bendy section . I have a pretty bendy mast but mine has a 24:1(double cascade and 6:1) and some of the sisterships to my boat have 48:1. If you can bend your mast with a 6:1 purchase then I understand your confusion. Unlike good practice for a cruising boat, your mast is not stiff enough to transfer much of the load to the forestay. and would need the runners and check stays to maintain headstay tension in a breeze. That is really weird since 'spaghetti masts' were typically only used on full blown race boats and only for a very short period of time.
Jeff

Here's the block&tackle.... So yea this is a bendy mast.... and the boat was sold as a racer cruiser.... And I didn't think bending it did much for increasing forestay tension per Jeff's explanation. it is tapered above the forestay. Perhaps Jeff is correct.

Boat and rig were manufactured in 1985.

Why would this be a spaghetti mast? OALsection dimensions? Section wall thickness? Seems to be about maybe 4mm (1/8") Both?

 

[SanderO]

Its hard to tell from the pic, but it looks like you have a cascade purchase? If so that would double the 6:1.

It looks like a bendy mast because the overall mast section looks quite narrow. It could just be an illusion.

Looks like a nice mast though.

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Sorry I could not find a better photo of the back stay... it is 6:1 which is shackled to a wire running over a block which is attached to the 1/4" wire back stay going back down to the pad eye you see on the coaming at the transom... so YES it is not a straight 6:1. (2 triple blocks) The actual back stay is isolated and used as an HF radio antenna.

 

[SanderO]

The wire running over a block and shackled to a 6:1 tackle is the description of a single cascade 12:1 backstay adjuster. That is pretty light purchase to bend the mast on a 36 footer.

To explain how a cascade works the single block is 2:1 and the 6:1 tackle is pulling on the 2:1 so that the 6:1 is doubled to 12:1.

The term 'Spaghetti mast' referred to a spar that were so bendy that it took runners and check stays to maintain headstay tension. It was a popular racing gimmick in the late 1970's and early 1980's, that quickly fell out of favor even for race boats, let alone cruising boats.

Jeff

The spar seems quite rigid.... but I have nothing to compare it to. It's deck stepped and just under 48' long/tall. I am guessing that the section is 10" x 6".