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Appropriate amount of headstay sag?

May 17, 2004
3,373
Beneteau Oceanis 37 LE 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
5,950
Slickcraft 26 Greenport, NY
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.
 
May 17, 2004
3,373
Beneteau Oceanis 37 LE 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.
 
Jan 1, 2006
5,950
Slickcraft 26 Greenport, NY
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.
I agree.
 
Aug 2, 2010
433
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
3,373
Beneteau Oceanis 37 LE 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
182
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
3,373
Beneteau Oceanis 37 LE 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
11,385
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.
 
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May 17, 2004
3,373
Beneteau Oceanis 37 LE 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
4,961
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
3,373
Beneteau Oceanis 37 LE 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
5,950
Slickcraft 26 Greenport, NY
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.