Sail cut

Jackdaw · Jun 25, 2014

Its very unlikely that they make the luff STRAIGHT.

All sailmakers will know that the luff on a fractional rigged boat will have some pre-bend to factor in. Often the sailmaker will ask; I've never had one not. Sometimes that will 'know' how much based on the boat type.

Assuming that they put too little, the lower-middle of the sail will be much flatter in its 'power-up' configuration; the default setting for 8-12 knots of breeze. Look at the draft stripes. In 8-12 you would expect a decent depth to the sail, like the picture below. If the luff was not cut correctly, there stripes will be much flatter. This would have the boat underpowered in light-medium airs.

Chief RA · Jun 25, 2014

I would think that in order to properly shape the sail you would have to take the bend out of your mast as part of your tuning procedure or at least adjust it some. I am pretty sure you already know this and just want feedback. Do you use a cunningham? Chief

Jackdaw · Jun 25, 2014

Chief RA said:
I would think that in order to properly shape the sail you would have to take the bend out of your mast as part of your tuning procedure or at least adjust it some. I am pretty sure you already know this and just want feedback. Do you use a cunningham? Chief

Chief,

B&R rigs are backstay-less fractional rigs with a fixed amount of pre-bend. Its not adjustable in the traditional sense. Owners can effect sail shape (as you note) with cunningham, halyard tension, and outhaul.

Chief RA · Jun 25, 2014

Jackdaw: I didn't associate B&R rigs with no backstay. Never sailed one of those! Had Neilpryde cut me a 175% drifter once and they said"you don't want that". I said "make it", and it was a great sail!
Thanks, Chief.

kloudie1 · Jun 25, 2014

On the 34, the B&R rig is mast-head and there is a backstay. The prebend is set before the mast goes up; backstay tension affects the forestay sag like a traditional masthead rig. Jack is right on with adjustments available. Ya might get the sailmaker to ride along and give tuning suggestions.

Jackdaw · Jun 25, 2014

kloudie1 said:
On the 34, the B&R rig is mast-head and there is a backstay. The prebend is set before the mast goes up; backstay tension affects the forestay sag like a traditional masthead rig. Jack is right on with adjustments available. Ya might get the sailmaker to ride along and give tuning suggestions.

Claude,

Thanks a ton for clarifying that. I totally forgot that Hunter used the B&R with a masthead rig on the 34.

kloudie1 · Jun 26, 2014

Yeah the sister 31 of that era has a backstay too; before Hunter had the cajones to give up the backstay .. The stay and B&R config makes for a pretty strong rig, especially after correcting the soft spot in the cabin top under the mast step. After this era is when they started messing with the struts and stuff, then the backstay disappeared.

Rick486 · Jun 26, 2014

ranchu56 said:
Seeking input from the community. We're trying to improve our racing results and got to thinking on the way home. If a sailmaker doesn't take into consideration the 5-8" pre-bend set in the B&R and made the luff straight, what affect would that have on trimming the main and or sailing performance?

Thank you,
Randy

Having raced one designs and cruising class for many years on the bay, I can suggest that getting a good start, learning and playing the local wind shifts, knowing and using the tidal currents to your advantage will improve your results more and faster than over focusing on sail trim nuances.

RichH · Jun 26, 2014

If your sailmaker forgot to add a convex shape to the luff (eg. straight luff), the sail would set VERY flat especially in the forward panel areas of the sail between the luff and 'midcord', plus the amount of draft would be 'minimal'; also, the leech would be pointing towards the leeside and not 'straight back' when properly raised and with 'moderate to light' mainsheet tension.

How to determine the amount of this curved luff leading edge shape in a sail: (see Note below) spread the sail on CLEAN and FLAT ground/floor. Make an accordion fold with and parallel to and the the luff section 'under' and with the accordion fold about 2-3 ft back from the leading edge. This will allow the luff edge to be laid almost totally FLAT on the floor, etc. Use your hands to get all the wrinkles and 'high spots' near the luff edge to lay FLAT and readjust the sail to get the LUFF nearly absolutely FLAT against the floor, etc. Then take a string, etc. and stretch it between the very front of the luff at the head of the sail AND at the tack of the sail, pull tight. The area that is 'in front' of the pulled tight string is the 'roach-like' shape along the luff ... and it should be matching closely to your mast bend. For a 'straight mast' the 'added' material will only be ~3/4 to 1-1/2" of added 'curve'. A sail cut for a B&R rig will have a LOT MORE convex material at the luff. Measure/mark the where maximum amount of 'extra cloth' makes its maximum 'convexity' - how much and how far above the tack, raise the sail and see/measure if the maximum 'convex-ity' closely matches with the point of maximum forward bending deflection in the set up and properly tensioned B&R mast. If there is a large discrepancy between the measurements, go to http://www.riggingandsails.com/pdf/selden-tuning.pdf and 'retune' and correctly 'prebend' your B&R increasing or decreasing the amount of bend to better 'match' your current average sailing venue conditions.

Note: To see how to make the 'accordion fold, go to http://www.ftp.tognews.com/GoogleFiles/Matching Luff Hollow.pdf and see the bottom (triangular-like / fig. 3) illustration -- which shows how to measure the 'luff hollow' on a jib, which is concave in shape .... or the exact reverse/opposite to the added convex curve to the luff of a mainsail. The line A—A in the drawing is the pulled tight 'string'.

hope this helps.

rardiH36 · Jun 28, 2014

RichH:

Thanks for your great input. As usual, I am reading and digesting.

This current thread has relevance for me since:

1) In the past I have removed the luff tape from a couple of head sails that still have good fabric life left, but who's shape has "blown-out" over time. Re-cut the sail fabric somewhat straighter along the luff. Then re-attachecd the luff tape. You inputted to my queries about these projects. The outcome has been good. The luff tell-tales in both cases stream correctly again.

2) I am currently in the process (but taking my time) of cutting down a too big but very good condition full battened mainsail so that it will fit my mast and boom dimensions. The luff on this sail after doing the accordion fold is quite straight. Although maybe it has a slight amount of convex. But close enough to my only slightly pre-bended mast. And I have already taken care of the shrunken bolt rope syndrome.