Downhaul

For which sail? I presume since you use halyard tension, you're referring to a cunningham - I could be wrong though. Attaches to the luff, just above the tack?
 
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The main advantage is at the bottom mark. It'd be a sod of a job to wind the halyard on, whereas a decent cunningham purchase makes it easier. And, as you say, the black (or white if you're cool) band at the top of the stick.
 
Normally you would hoist the main to the black band at the top of the mast and adjust luff tension with the cunningham. A racing sail will be designed so that at full hoist there is still some slack in the luff that can be taken up with the cunningham.

It is also typical to use a light line and a purchase led to a cam cleat, rather than a heavier line led to a clutch.

A huge advantage of using the cunningham, particularly on a purchase and cam cleat, is that it is very easy to adjust it easily. The easier it is to adjust, the more likely you are to adjust it.
 
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Its an underused control on a yacht,

So we adjust halyard tension between upwind and down,
the cunningham pulls the draft fwd in the main and IN COMBINATION with other controls can help to make the main flatter and induce twist.

it is key however to remember to release the Cunningham before you ease the back stay and before you go downwind otherwise you can stretch the luff of your sail
 
DFL1010 said:
The main advantage is at the bottom mark. It'd be a sod of a job to wind the halyard on, whereas a decent cunningham purchase makes it easier. And, as you say, the black (or white if you're cool) band at the top of the stick.
Click to expand...

This bit I dont get. The mechanical advantage on my halyard winches is 16: 1 and then I have a 12 inch winch handle to add convenience as opposed to pulling on a rope. To make a cunningham lighter you would have to have maybe a 4 pulley block system and even then you are missing out the ease of the winch handle
 
The cunningham is working against the internal tension in the sail and the friction of the luff rope or slides.
The halyard has to work against the leech tension, much more friction because of the forces on the headboard and slides etc and the stretch of the halyard.
The effect of the two can be grossly different as the luff sticks in the mast in the middle third of the sail.

Also the cunningham is easy to change while the sail is fully powered.
In some classes, you cannot get the same luff tension without taking the headboard over the black band.
In my dinghy, we tension the halyard so the head stays crammed into the top sheave, so the leech and luff tensions don't pull the head down at all. Even spectra halyards stretch a bit, and they effectively get longer as the mast bends...
There are further differences if the sail is not loose footed.
 
Actually lw395, you've missed the biggest difference. The halyard is working against the weight of the sail whilst all the cunningham is doing is stretching the luff. Obvious when you think about it :o
 
I think the reason Cunninhams are effective is because it acts on the bottom 2/3rds of the main where the greatest sail area is and there is less in the way of forces and friction acting on the luff. If you think about it, on a fractional rig the top part of the mast will have the most bend and the main the most roach. Friction will prevent the Cunningham doing much near the top of the main but a tightening of the halyard on a tack will take care of that if the extra Cunningham tension doesn't shake through.
 
birdseye said:
Actually lw395, you've missed the biggest difference. The halyard is working against the weight of the sail whilst all the cunningham is doing is stretching the luff. Obvious when you think about it :o
Click to expand...

Weight of the sail probably matters in a bigger boat.
Weight of the boom too, which comes into leach tension.

But that is probably quite small compared to the sheet tension.
 
Undoubtedly small compared to the sheet tension but not so for the halyard tension. In light winds you will only be wanting enough on the cunningham to take the horizontal creases out of the sail and that will be far less than the weight of the sail. As the wind builds so will the tension needed. I tried to find a calculator on line but the only thing |I did find was a harken one for sheet load not halyard.
 
As others have said, cunningham tension principally goes into the bottom 1/3 of the luff. But is quite capable of taking up the tension by itself if you forget (or run out of winches) to get the main halyard tension back on at the bottom mark.

In fact, many high end boats don't have a halyard to adjust, using halyard locks, and relying solely on the cunningham (which is then more properly called a downhaul) for luff tension. What they lose in terms of friction affecting luff tension in the top 1/3 of the luff they more than gain in halved compression on the mast.
 
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