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Forestay sag

Blue j

New Member
I have hull 1691 and I know the previous owner cut the mast down a bit. My forestay sags and my backstay adjusters are fully on. What can I do? The geometry suggests a shorter forestay but I don't know if that is legal. Putting on shorter backstay legs seems like it would spill too much air in light wind. Any ideas would be great.
 

VinceH

Member
What's involved here is the mast length, the forestay length, the position of the mast in the partners (J dimension), the butt position, the spreader dimensions and the shroud tension. That's all! :)

As the wind comes up, you should tighten both the uppers and the lowers so that the mast retains the prebend needed for the design of the main, but gets stiffer. If you tighten the uppers, the mast bows (bends). You then tighten the lowers, and that pulls the excess prebend out of the mast and it's back to the way it was. You've just stiffened the mast. Then when you pull on the backstay you'll be pulling the headstay tighter, rather then just bending the mast and making it shorter, slacking the headstay. Make sense? And when I say "As the wind comes up", I mean that if you're racing and you do a race where you're set up for 6-10kts of breeze, and afterward, preparing for a subsequent race, the wind is steadily over 10 kts and building, you may want to change and set up for 10-15 kts (or whatever your appropriate range-setting is).

You need to go through the class rules and the sailmaker tuning guides to understand how to set up your rig. I haven't examined the latest tuning guides, but they historically were nearly identical between different sailmakers, because sail designs had become pretty consistent across different makers, with only small differences. I think we're now down to North and Quantum as the primary suppliers in the US, along with Ullman and Doyle, and it would make sense to review carefully the information from them. I feel sure that the mast setup will be the same for both North and Quantum.
J/24 Tuning Guide | North Sails
https://www.quantumsails.com/getattachment/Sails/One-Design/Inshore/J-24/J24_TuningGuide_June2020_V3.pdf.aspx
https://doylesails.com/assets/cms/images/Doyle-J24-Tuning-Guide.pdf

Once you've gone through the effort, the only variables will likely be shroud tension, with changes in wind velocity. The mast butt, position at the partners, etc. stays the same.

The class rules will tell you what the critical dimensions are. See the latest rules at the International class website: http://www.j24class.org/wp-content/uploads/2010/11/J24-Class-Rules-2021.pdf See especially C9.4, which gives mast dimensions.

In some cases you may need to construct some jigs to make a measurement on your boat, like the dimension between the datum point on the mast and the height of the sheerline. You can do this, but think twice or more before cutting the butt of the mast.

J/24s inherently have very little weather helm, making it difficult to drive upwind. The primary goal of the mast setup then is to get the center of effort of the sail plan as far aft as possible within confines of the rules, in an effort to gain some feel on the helm. [At the same time, when fairing the keel, the point is to get the center of lateral resistance as far forward as possible within the rules].

So you want the mast shorter (at the bottom) so the headstay fixing point is lower, letting the mast rake as much as possible. Then you want the headstay as long as possible (within rules), to do the same. The J dimension (where the mast sits in the partners) combined with the position of the butt on the I-beam and the shroud tension will set the prebend of the mast to match the design of the mainsail.

Good luck, but if you go through this, there's not really much luck involved.

Vince
 

Blue j

New Member
What's involved here is the mast length, the forestay length, the position of the mast in the partners (J dimension), the butt position, the spreader dimensions and the shroud tension. That's all! :)

As the wind comes up, you should tighten both the uppers and the lowers so that the mast retains the prebend needed for the design of the main, but gets stiffer. If you tighten the uppers, the mast bows (bends). You then tighten the lowers, and that pulls the excess prebend out of the mast and it's back to the way it was. You've just stiffened the mast. Then when you pull on the backstay you'll be pulling the headstay tighter, rather then just bending the mast and making it shorter, slacking the headstay. Make sense? And when I say "As the wind comes up", I mean that if you're racing and you do a race where you're set up for 6-10kts of breeze, and afterward, preparing for a subsequent race, the wind is steadily over 10 kts and building, you may want to change and set up for 10-15 kts (or whatever your appropriate range-setting is).

You need to go through the class rules and the sailmaker tuning guides to understand how to set up your rig. I haven't examined the latest tuning guides, but they historically were nearly identical between different sailmakers, because sail designs had become pretty consistent across different makers, with only small differences. I think we're now down to North and Quantum as the primary suppliers in the US, along with Ullman and Doyle, and it would make sense to review carefully the information from them. I feel sure that the mast setup will be the same for both North and Quantum.
J/24 Tuning Guide | North Sails
https://www.quantumsails.com/getattachment/Sails/One-Design/Inshore/J-24/J24_TuningGuide_June2020_V3.pdf.aspx
https://doylesails.com/assets/cms/images/Doyle-J24-Tuning-Guide.pdf

Once you've gone through the effort, the only variables will likely be shroud tension, with changes in wind velocity. The mast butt, position at the partners, etc. stays the same.

The class rules will tell you what the critical dimensions are. See the latest rules at the International class website: http://www.j24class.org/wp-content/uploads/2010/11/J24-Class-Rules-2021.pdf See especially C9.4, which gives mast dimensions.

In some cases you may need to construct some jigs to make a measurement on your boat, like the dimension between the datum point on the mast and the height of the sheerline. You can do this, but think twice or more before cutting the butt of the mast.

J/24s inherently have very little weather helm, making it difficult to drive upwind. The primary goal of the mast setup then is to get the center of effort of the sail plan as far aft as possible within confines of the rules, in an effort to gain some feel on the helm. [At the same time, when fairing the keel, the point is to get the center of lateral resistance as far forward as possible within the rules].

So you want the mast shorter (at the bottom) so the headstay fixing point is lower, letting the mast rake as much as possible. Then you want the headstay as long as possible (within rules), to do the same. The J dimension (where the mast sits in the partners) combined with the position of the butt on the I-beam and the shroud tension will set the prebend of the mast to match the design of the mainsail.

Good luck, but if you go through this, there's not really much luck involved.

Vince
Thank you, this is the direction I was headed after reading the North tuning guide. Your help is spot on.
 

VinceH

Member
You're welcome. But while I'm over-splainin', I'll add that as shroud tension is adjusted higher and the mast gets stiffer, when you pull on the backstay you will increase the bend in the mast, depowering the main. But at the same time, with that stiffened mast you'll be able to transfer more of that force to the headstay, pulling some draft out of the jib and allowing more pointing.
 

Blue j

New Member
You're welcome. But while I'm over-splainin', I'll add that as shroud tension is adjusted higher and the mast gets stiffer, when you pull on the backstay you will increase the bend in the mast, depowering the main. But at the same time, with that stiffened mast you'll be able to transfer more of that force to the headstay, pulling some draft out of the jib and allowing more pointing.
Pointing is my goal! Thanks
 
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