Appropriate amount of headstay sag?

May 17, 2004
5,028
Beneteau Oceanis 37 Havre de Grace
One of the boats in our club thought that we had too much headstay sag in one of our races a few weeks ago. They were race committee for the night and took some pictures. Below is taken while close hauled in 10-11 knots true wind. I figure it's just under 2' of sag, with an I of 45.8 feet.

HeadstaySag201908.png


The red lines are what I used as visual references to measure the sag. How does this look to the group? If @bob_pattison is around, I wonder how this compares to Neil Pryde's design?

From the dealer the backstays were quite a bit tighter, but that was causing the upper shrouds to be very loose. Our yard eased the backstays last year and that seemed to balance things out better, but it did ease the headstay a bit. We have a furling main so we can't add any more backstay tension without bending the mast too much. We might be able to tighten the headstay if I can figure out how to do that under the ProFurl jib furler.
 
Jan 1, 2006
7,039
Slickcraft 26 Sailfish
I can't really see the mast head in this photo. It appears to be a fractional rig or maybe 7/8ths?
Batter up! The short easy answer is that if the curve of the luff matches the curve of the forestay you are in pretty good shape and the concern would be what happens when it breezes up. Search the archives for posts from RichH, I think, who made some excellent posts on this topic including instructions on how to measure the luff curve of the sail off the boat.
On a mast head boat with a stiff mast tightening the backstay will increase tension on the forestay. Tightening the backstay will not move the mast tip appreciably but will reduce sag as it breezes up.
On a fractional rig it is different. The mast is usually more flexible and tightening the backstay can bend the mast, and along with other effects, moving the upper middle mast forward reducing forestay tension. That's why fractional rigs can have running backstays, jumper struts or other means to support the upper mast. Modern fractional rigs have swept back spreaders, and double spreaders, with diagonals, as I think the boat in the picture does. The finer points of tuning them, I'll leave to someone else - I think it's significantly more complicated.
 
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May 17, 2004
5,028
Beneteau Oceanis 37 Havre de Grace
I can't really see the mast head in this photo. It appears to be a fractional rig or maybe 7/8ths?
It‘s fractional but a very high fraction, 9/10ths. The masthead is about 1 foot out of the frame. It’s a double backstay from the masthead.

Search the archives for posts from RichH, I think, who made some excellent posts on this topic including instructions on how to measure the luff curve of the sail off the boat.
Thanks for the pointer to RichH’s posts. I’ll try to measure the luff hollow over the winter and see how it compares.

On a fractional rig it is different. The mast is usually more flexible and tightening the backstay can bend the mast, and along with other effects, moving the upper middle mast forward reducing forestay tension.
Interesting point. On a 9/10ths fractional I don’t think the headstay attachment point moves forward much or at all with increasing backstay tension, but it’s something to think about.
 
Mar 13, 2011
175
Islander Freeport 41 Longmont
I would definitely say its too much sag. Heading close-hauled, you want the head-stay as tight as possible. As you've mentioned, there are issues with how much back-stay tension you can get or with how to tighten the head-stay. You may be able to do a bit of both to keep the mast in column for the furling main. And as you also mentioned, there is the entire rig balance which was improved by the earlier tuning. I think you are into the art stage of rig tuning where you will have fun trying to balance it out. Tighten the head-stay and you'll point higher but, get the mast out of column and you'll have issues with furling the main.

Fair winds and good luck.
 
Aug 2, 2010
502
J-Boat J/88 Cobourg
Just posted a question like this a few days ago in another forum and the opinions were strong that 6-8" was too much sag. I am not sure but I am looking forward to all the wise feedback.
 
May 17, 2004
5,028
Beneteau Oceanis 37 Havre de Grace
Just posted a question like this a few days ago in another forum and the opinions were strong that 6-8" was too much sag. I am not sure but I am looking forward to all the wise feedback.
Thanks. I’d certainly have some adjustment to do to get under 6” of sag.
 
Apr 21, 2014
184
Hunter 356 Middle River, MD
Do you get any pumping of the jib sail in higher winds? Does the jib halyard tension play any part in this form the sail perspective?

I wonder the same on our boat but we have a B&R rig and wonder what is the correct sag when the wind comes up as we don't have a back stay.

Boat looks good right by the Havre de Grace bridges probably right off of Tidewater marina...

Jeff
 
May 17, 2004
5,028
Beneteau Oceanis 37 Havre de Grace
Do you get any pumping of the jib sail in higher winds?
I haven’t noticed any pumping of the head stay. There is some leach flutter at some trim angles, but I could probably fix that with fairlead positioning or leach line. When I’m wing on wing if I let the sail collapse there is a fair bit of motion in the head stay when it re-fills.
Does the jib halyard tension play any part in this form the sail perspective?
I’m going to try to experiment with that. The halyard is pretty loose from slipping an inch or two through the clutch over the course of the summer. Our typical races are in 4-5 knot winds, not the 10-11 we had that night, so I haven’t bothered taking the halyards in at all, but it would be interesting to see if that helps or hurts

Boat looks good right by the Havre de Grace bridges probably right off of Tidewater marina
You are correct. This was one of the Thursday evening races held in the mouth of the river there this summer.
 
Nov 8, 2010
11,386
Beneteau First 36.7 & 260 Minneapolis MN & Bayfield WI
Its amazing to me that people think that can know what the answer to this question is without knowing the breeze.

Boats with non-overlapping headsails are VERY sensitive to rig tension (and the resulting headstay sag) Because the jib is so high aspect, it is vitally important that the sail has the correct entry and overall shape. Too flat (tight) and the sail never develops the correct shape, as it is too skinny to recover from the flat entry in the way that a genoa might.

In light airs (below say 8) that sail will benefit from being set with a soft rig with lots of sag. On a boat that size, over a foot of sag is not too much, In breeze, just the opposite. We adjust ours ever time we go out.

David reports that was 10-12, so decent breeze. Too much sag? Depends on how the boat was feeling.
 
May 17, 2004
5,028
Beneteau Oceanis 37 Havre de Grace
Its amazing to me that people think that can know what the answer to this question is without knowing the breeze.

Boats with non-overlapping headsails are VERY sensitive to rig tension (and the resulting headstay sag) Because the jib is so high aspect, it is vitally important that the sail has the correct entry and overall shape. Too flat (tight) and the sail never develops the correct shape, as it is too skinny to recover from the flat entry in the way that a genoa might.

In light airs (below say 8) that sail will benefit from being set with a soft rig with lots of sag. On a boat that size, over a foot of sag is not too much, In breeze, just the opposite. We adjust ours ever time we go out.

David reports that was 10-12, so decent breeze. Too much sag? Depends on how the boat was feeling.
Thanks Jackdaw. Glad to hear that it doesn’t look crazy out of whack anyway. Close hauled the boat usually feels ok below 14 knots true or so. Above that it starts to get a little squirrelly, but that might just be other things too like the halyard tension, trim, and generally being overpowered. I’ll take a look at the luff hollow and see what my headstay tension adjustment options might be over the winter just to see if experimenting with it would help or not.
 
Oct 26, 2008
6,044
Catalina 320 Barnegat, NJ
@Davidasailor26 Would the creases in the center of your luff indicate too much sag? I'm thinking there is too much given the wind conditions.

I was wondering about the same thing with my 110% genoa. These were taken during our race in about 18 to 22 knots wind. I had my backstay on as much as I could hand tight. I'm a little hesitant to go more (at 4:1). I don't know if you can really measure with the lengthy pendant up top (at least 3').
Jib Sag 1.jpg


I wonder how much more it would look if the picture were head-on. This a 30-year old sail, I think.
Jib Sag 2.jpg
 
May 17, 2004
5,028
Beneteau Oceanis 37 Havre de Grace
@Davidasailor26 Would the creases in the center of your luff indicate too much sag? I'm thinking there is too much given the wind conditions.

I was wondering about the same thing with my 110% genoa. These were taken during our race in about 18 to 22 knots wind. I had my backstay on as much as I could hand tight. I'm a little hesitant to go more (at 4:1). I don't know if you can really measure with the lengthy pendant up top (at least 3').View attachment 171222

I wonder how much more it would look if the picture were head-on. This a 30-year old sail, I think.
View attachment 171223
Interesting thoughts. I think the creases might be more from low halyard tension than the sag. It’s a possibility though. When I have the chance I’ll take the halyard in a little and see what happens to the wrinkles. For the angle of the picture - after I measured the sag with the lines on the photo I divided the measured sag by cos(40). I figure that should compensate for the offset angle of the picture, at least to an approximation.
 
Jan 1, 2006
7,039
Slickcraft 26 Sailfish
You are not going to get a no sag forestay in moderate wind. In fact with rod rigging you won't either. I've posted before that at a certain point the backstay adjuster is no longer adding tension to the forestay. It is bending the boat. We have a local sailer who lost two masts by over tightening the backstay. The second time the rigger refused to give him an adjuster. That was on a Pearson 32 so it wasn't a flimsy mast. To not have wrinkles in the jib as it breezes up I would consider a ultra low stretch halyard.
 
Jul 1, 2023
3
Cape Cod Shields Monterey
I'd say there was too much sag in the OP's photograph, because draft is very much 'knuckle forward' (look especially at the draft stripe) and because the leech is closed even though the foot is quite tight. In conjunction, those are obvious signs of too much sag for the sail and conditions. Neither of those are visible in the photographs from Post 12, so I'd say sag in those shots is not excessive.
 
May 17, 2004
5,028
Beneteau Oceanis 37 Havre de Grace
I'd say there was too much sag in the OP's photograph, because draft is very much 'knuckle forward' (look especially at the draft stripe) and because the leech is closed even though the foot is quite tight. In conjunction, those are obvious signs of too much sag for the sail and conditions. Neither of those are visible in the photographs from Post 12, so I'd say sag in those shots is not excessive.
Interesting point about the draft and the leech being closed. The jib has always had a lot of twist so I keep the cars pretty far forward to try to close the top a little. That probably contributes to the closed leech.

Last year I did some tuning and tightened the backstays a bit, along with the uppers just a little so they still don't get slack.The mast bend is still fine for the roller furling main, and it seems to have taken a little bit of the forestay sag out.

This was a picture from shortly after the adjustments; It's about the only one at a head-on angle I have since then. Again the red lines are added to show the sag. Measuring the lines this seems to be about 1' of sag, instead of the previous 2', still in about 11 knots of true wind. Nearly close hauled; maybe cracked a couple degrees off as we were 3 minutes pre-start.

HeadstaySagAfter2022Tuning.jpg
 
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Jul 1, 2023
3
Cape Cod Shields Monterey
This was a picture from shortly after the adjustments; It's about the only one at a head-on angle I have since then. Again the red lines are added to show the sag. Measuring the lines this seems to be about 1' of sag, instead of the previous 2', still in about 11 knots of true wind.
I don't know the Oceanis 37 and rarely use furling sails, but that still looks like too much sag for the sail and the breeze. Thus, lots of questions!

Is it feasible to tension the backstay yet more and use the lowers to sufficiently limit mast bend? What's sag like when it's blowing 20? Does the jib have a lot of 'return'? Does the main backwind early? Do you seem to point pretty high but with less than expected VMG?

Here is a quick sketch that shows (a bit) how the last three questions are relevant:

1688270752355.png



1688270752355.png
 
May 17, 2004
5,028
Beneteau Oceanis 37 Havre de Grace
I don't know the Oceanis 37 and rarely use furling sails, but that still looks like too much sag for the sail and the breeze. Thus, lots of questions!

Is it feasible to tension the backstay yet more and use the lowers to sufficiently limit mast bend? What's sag like when it's blowing 20? Does the jib have a lot of 'return'? Does the main backwind early? Do you seem to point pretty high but with less than expected VMG?

Here is a quick sketch that shows (a bit) how the last three questions are relevant:

View attachment 217439


View attachment 217439
Tightening the lowers and D2’s would probably be the key to being able to tighten the backstays more without bending the mast too much. Tightening the D2’s does take a trip up to the spreaders so it’s a little harder to experiment with that.

Since tightening the backstays I’m pretty happy with the boat’s performance. The boat doesn’t point as high as it did before the original adjustments were made loosening the backstays in the first place, but the sails are also older now and probably getting past their prime. The sail cloth is now 9 seasons old and has about 1500 hours on it. Here’s what it looks like from the foot up:
1688302499685.jpeg


When the sails were newer the jib seemed to let me get a few degrees higher than I can now, and the main would back wind unless the traveler was very high. Now the jib seems to break well before the main, around 34 degrees apparent wind.

I can’t say much for how any of this looks in 20 knot conditions. In summer on the Chesapeake we have a lot more days of 3-5 knots than 15+ knots. So I’m generally ok with running the rig a little softer to match those prevailing conditions.
 
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Jul 1, 2023
3
Cape Cod Shields Monterey
Since tightening the backstays I’m pretty happy with the boat’s performance. The boat doesn’t point as high as it did before the original adjustments were made loosening the backstays in the first place, but the sails are also older now and probably getting past their prime. The sail cloth is now 9 seasons old and has about 1500 hours on it. Here’s what it looks like from the foot up:
Your mention of the sail's age/use and three pictures showing the luff pretty eased brings to mind the possibility that 'easing the luff bolt rope' will result in better shape and control. 'Easing the luff bolt rope' is not the same as easing luff tension. In the last several months I've eased the luff bolt rope on 4 older mains and a jib, and the effect was substantial each time. Turns out bolt ropes shrink rapidly up to a point with age/use, such that (a) the rope tensions before full hoist, (b) tension on the halyard and/or Cunningham has less (possibly negligible) effect on draft position, and (c) the luff deveops myriad small wrinkles. Easing the luff bolt rope involves removing a little stitching at the tack and any reef points. It's a DIY thing but it seems most ask a sailmaker to do it.
 
Aug 2, 2010
502
J-Boat J/88 Cobourg
Tightening the lowers and D2’s would probably be the key to being able to tighten the backstays more without bending the mast too much. Tightening the D2’s does take a trip up to the spreaders so it’s a little harder to experiment with that.

Since tightening the backstays I’m pretty happy with the boat’s performance. The boat doesn’t point as high as it did before the original adjustments were made loosening the backstays in the first place, but the sails are also older now and probably getting past their prime. The sail cloth is now 9 seasons old and has about 1500 hours on it. Here’s what it looks like from the foot up:
View attachment 217440

When the sails were newer the jib seemed to let me get a few degrees higher than I can now, and the main would back wind unless the traveler was very high. Now the jib seems to break well before the main, around 34 degrees apparent wind.

I can’t say much for how any of this looks in 20 knot conditions. In summer on the Chesapeake we have a lot more days of 3-5 knots than 15+ knots. So I’m generally ok with running the rig a little softer to match those prevailing conditions.
It looks like you could move the jib cars forward to me. Too much draft at the head compared to the foot for my boat for sure.