On a B&R without a backstay, I figure it’s important that the luff curve match the mast bend because there are no other adjustments. I staked the sail out on the ground to measure the curve and install draft stripes. It appears there is no curve to speak of in the luff.The sail is a stock 188sqft Hunter Doyle main. Why no curve or am I missing something?
Mast bend-luff curve
- Thread starter Quoddy
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Just a guess but if you stretched the sail from head to tack, you'd probably have removed evidence of the shape of the luff. Stretch it from luff to leech and see what you get.
P.S. The curve of the luff/sail 'belly' won't be equal to the bend of the mast. Otherwise, the sail would be completely flat when hoisted.
P.S. The curve of the luff/sail 'belly' won't be equal to the bend of the mast. Otherwise, the sail would be completely flat when hoisted.
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On a mainsail, the luff is usually cut straight and a mast that is 'pre-bent' or purposely curved will cause the luff in the mid panels of a mainsail to 'move forward' - thus flattening the mainsail into to it's designed/intended (when sailing) shape. This is usually NO luff hollow for a mainsail.
There is ALWAYS a smooth curve at the luff of a headsail that a sailmaker cuts away from the luff - to **match** the EXPECTED sag in the forestay(headstay).
For luff hollows on HEADSAILS:
To SEE the headsail 'luff hollow'.... lay the sail of FLAT (clean) ground/floor and make an accordion fold about 2 ft. behind the luff. This fold will look like a flattened-down letter Z. Work out all the wrinkles along the luff to be sure that luff is FLAT on the ground. The 'curve' that you now see is called 'luff hollow'. You can measure it for future reference (for when you need 'ultimate precision' of sail shape when racing, etc.). Another way to 'visualize' the luff hollow is to (after getting the luff sections FLAT on the ground) is to take a string from the head and tack and then move the stretched string about 4-6" 'back' from the luff .... and then mark where the string is located back from the luff - you can then apply 1/4" wide adhesive 'draft stripe' material along this 'line' or simply sew a row of stitches of a dark color - called a 'luff stripe'.
When you want PERFECT shape from the jib/genoa, especially for pointing, simply walk forward to the bow (when pointing) and with your eyeball perpendicalar to the 'luff stripe' ... see if its straight. If not straight, then make your rigging tension adjustments accordingly (very hard to do with a BN&R rig when 'sailing'; but,) ... on a 'normal' rig just increase/decrease backstay tension until that 'luff stripe' - or your remembrance of what that luff hollow looked like... and the boat will now point-like-a-banshee (if all the rest of the sail trim/shape is correct).
A boat with the headstay too loose and not 'matching the sail's luff hollow' - will have problems 'pointing', will heel aggressively, will be SLOW going upwind, and the keel will begin to skid to leeward (and the helmsman will erroneously think that he somehow developed 'weather-helm').
There is ALWAYS a smooth curve at the luff of a headsail that a sailmaker cuts away from the luff - to **match** the EXPECTED sag in the forestay(headstay).
For luff hollows on HEADSAILS:
To SEE the headsail 'luff hollow'.... lay the sail of FLAT (clean) ground/floor and make an accordion fold about 2 ft. behind the luff. This fold will look like a flattened-down letter Z. Work out all the wrinkles along the luff to be sure that luff is FLAT on the ground. The 'curve' that you now see is called 'luff hollow'. You can measure it for future reference (for when you need 'ultimate precision' of sail shape when racing, etc.). Another way to 'visualize' the luff hollow is to (after getting the luff sections FLAT on the ground) is to take a string from the head and tack and then move the stretched string about 4-6" 'back' from the luff .... and then mark where the string is located back from the luff - you can then apply 1/4" wide adhesive 'draft stripe' material along this 'line' or simply sew a row of stitches of a dark color - called a 'luff stripe'.
When you want PERFECT shape from the jib/genoa, especially for pointing, simply walk forward to the bow (when pointing) and with your eyeball perpendicalar to the 'luff stripe' ... see if its straight. If not straight, then make your rigging tension adjustments accordingly (very hard to do with a BN&R rig when 'sailing'; but,) ... on a 'normal' rig just increase/decrease backstay tension until that 'luff stripe' - or your remembrance of what that luff hollow looked like... and the boat will now point-like-a-banshee (if all the rest of the sail trim/shape is correct).
A boat with the headstay too loose and not 'matching the sail's luff hollow' - will have problems 'pointing', will heel aggressively, will be SLOW going upwind, and the keel will begin to skid to leeward (and the helmsman will erroneously think that he somehow developed 'weather-helm').
Only the sailmaker would really know as on Boats Like My J24 i have older mains with curve X and newer ones that require a straight mast
The mainsail likely has a positive luff curve to match the prebend,
otherwise you would notice overbend wrinkles along the luff.
Typically what happens is that the boltrope shrinks and contracts
the luff. If you lay sail sail out flat, the luff curve may actually
appear to be hollow. If you were able to stake the sail out
with adequate luff tension to remove the contraction you would
likely see a little hump right at the luff tape, that is the positive
luffcurve trying to push shape into the sail.
otherwise you would notice overbend wrinkles along the luff.
Typically what happens is that the boltrope shrinks and contracts
the luff. If you lay sail sail out flat, the luff curve may actually
appear to be hollow. If you were able to stake the sail out
with adequate luff tension to remove the contraction you would
likely see a little hump right at the luff tape, that is the positive
luffcurve trying to push shape into the sail.
If that was the case you wouldn't be able to adjust the sail's draft at all. It is the difference between mast bend and luff curve that allows that adjustment. When you curve the luff, you flatten the sail. A straight luff will have a fuller shape. Rich and the other guys have given you pretty good explanations.
For a visual, try suspending the sail by it's corners, horizontally. The play with it. You'll see the draft depth and position change as you alter the tension on the various sides and corners.
Without a backstay, the other adjustments you use are the cunningham, outhaul and halyard... in addition you can set draft depth for the day's conditions by adjusting batten tension. You can set mast pre bend by adjusting the shrouds.
On racing beach cats, the primary mast bending control is the cunningham...a 16:1 purchase is not unusual for this control.
Thanks
I was looking for something that doesn’t exist, that could explain the no show. I guess mast prebend will have to be set by experimenting with different amounts. Maybe 2” would be a good starting point.
I was looking for something that doesn’t exist, that could explain the no show. I guess mast prebend will have to be set by experimenting with different amounts. Maybe 2” would be a good starting point.
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