Luff tension?

[Quoddy]

I just read this in a lengthy discussion of sail handling and it didn’t seem right.
“As you increase the tension on the luff in higher winds, the sail becomes flatter and the difference between luff and stall becomes less and makes for difficult steering.”
I thought the opposite was true. Luff tension brings the draft forward, which makes for a less fine entry, which makes it easier to steer but harder to point.

 

[Alan]

Quoddy, tightening the cunningham (tensioning the luff) does move the draft forward but does nothing to change the draft depth. The sail will have the same amount of draft (it wont be any flatter) but the draft does move forward. Increasing outhaul will decrease the draft and typically this is what you would do. If you're bending the mast the luff tension will need to be increased to maintain the proper draft position.

 

[Sam Salter]

You're right Quoddy!

Reason for tightening luff is to bring the max draft position back to about 45% to 50% as the increasing wind blows it back along the sail.
You don't always want to flatten the sail, depending on how much wind there is.

Bringing the draft more forward than 45% or so will indeed give you more latitude with attack angle as far as maintaining sail trim is concerned. But here we're talking about difficulty in holding a course and maintaining sail trim - say in heavy seas.

RichH - you can jump in anytime now with the definitive answer

 

[RichH]

Hey! Both you and Alan have it right !!!!!

• Luff tension moves the Point of Max. draft fore or aft .... on dacron and other similar 'stretchy' sails. Less luff tension promotes a flatter 'entry' shape; more promotes a 'rounded' entry shape.
• The amount of draft is controlled by the outhaul; or in the case of a jib, by sheet tension. Additionally, on a 'bendy mast' rig the amount of mainsail draft can also be controlled by the amount of mast bend.

In the specific case of a jib, the position of max. draft is greatly influenced by the forestay tension as well as luff tension.


;-)

 

[Sam Salter]

Don't want to hijack this thread, but...
Rich or anyone else: what role (if any) does luff tension play with a high tech sail?
Can you move the draft on one of these sails? I suspect not!

sam

 

[RichH]

On the ultra non-elastic sails ... the shape you build is the shape you get. Only 'serious' control is the outhaul / jib sheets or fairlead cars which affects the 'amount' of draft.

Luff tension on a super non-stretch sail will have very little effect on draft, etc. position.

Take a cloth handkerchief, fold it into a simple triangle and pull between any two corners .... thats how a dacron sail is 'shaped'.
Then take a triangular piece of paper and pull tension between any two corners .... nada.

 

[Alan]

......and that's exactly 'why' high quality high tech sail cloth is soooo good. It holds its shape in any breeze. Dacron sails simply can't match this shape holding ability and would need constant tweaking to begin to approach the same kind of shaping. Same goes for the sheets and halyards. Double braid dacron stretches and gives up power and performance that high tech lines do not.

 

[Bill Roosa]

High tech/low tech

But you have to have a different high tech sail for every wind speed and pointing situation. So better performance but more sail handling.
Low tech allows one sail to cover a range of wind speeds and pointing situations. and cost less!!
What all that means is more refreshing beverages in the cooler and less sail handling...... happiness does not have to cost more.:dance:

 

[Alan]

Re: High tech/low tech

Hey Bill, I'll go along with the beer in the cooler but not the sail inventory. A sail that holds its shape means a wider operating range and fewer sails in inventory not the other way round.:dance:

 

[Barnacle Bill]

C'mon. Tightening the luff brings the draft forward and loosening it brings it aft. Tightening the outhaul (on a mainsail) brings the draft down and loosening brings the draft up. On a headsail, loosening the halyard will bring the draft aft, but it also will flatten the leading edge of the sail which does aid in pointing higher. As the wind pipes up you really want the draft forward for more power and as the wind decreases, you want to bring the draft aft. When in doubt, let it out. Many people have too much tension on their sail control lines which adversely affects performance. If you are sailing in very heavy winds, then flattening the sails is best. You do this with more halyard/cunningham tension, more outhaul (on the main), a lot more boom vang and more backstay tension too. The traveler is eased to leeward more and as wind decreases the traveler is pulled to windward.
On a headsail, you want the sheet lead to be in the proper position on the track so that when you slowly point into the wind (luffing the boat), with the sheet in tight, the entire luff of the headsail breaks or luffs at the same time. If it breaks down low first, then you have to move your lead aft. If it breaks up high first, then you have to move your lead forward. In a very heavy air situation you can move your genoa car lead aft to intentionally let the upper portion of the sail luff first to help depower the rig. This is for sailing into the wind. Off the wind is different. The lead stays the same, but you have to ease the vang to open up the leech of the mainsail to match the twist of the genoa to create a good slot between the two.

 

[Bill Roosa]

High tech and sail inventory

Well Alan I'd have to disagree. The high tech have well defigned shapes and that means draft position, entry angles, amount of draft, etc. Those cannot be changed because the material does not stretch (much). so as the wind speed or point of sail changes you would want to put on a different sail for that condition.
My dacron sail does stretch so I can morph it as needed to accomidate the changes.

 

[RichH]

If you are concerned and aware of the 'stretch' .... a high-end, high modulus dacron sail, especially used at the max. of its windrange design will begin to become permanently distorted after a 'few hundred' hours.

I used to crew on an open-checkbook race boat that used only the very best in laminate sails ... and after about 100+ hours (sometimes sooner) of sailing/racing time, those sails were sent back to the loft to be either 'shape adjusted' or scrapped. Friends and acquaintances of mine who crewed on AC boats used to tell me that many of the lam-sails were only used for 2 or 3 races !!!!

 

[Bill Roosa]

High tech sails and race car engines

Kinda like using a race car engine for the whole season. It just does not happen due to "wear and tear"
My commercial engine however will go 200,000 miles +++ with just normal maintenance. Same as my dacron sails, The baggness can be stretched out and that is acceptable.