Headsail Sag

Is headsail luff sag a bad thing, per se? Obviously if the sail has been cut with a straight luff intended any sag will spoil the shape, but why does it need to be designed with a straight luff in the first place? A straight luff requires much higher rig tensions, which in turn necessitate stronger, heavier, hulls. Asymmetric spinnakers are essentially headsails with large amounts of sag and they seem to perform well enough, on the faster skiffs they even operate at apparent wind angles similar to conventional jibs heading upwind.
Am I missing something?

 

Am I missing something?

YES , a talk with your sailmaker.

If you let the sailmaker know your cruising style .

A headsail can easily be built for a sagging headstay.

FAST FRED

 

But why do racing boats go for a straight luff? Is there an advantage to a straight luff, or is just a matter of sticking with what we know? I would have thought a sagging luff might actually have aerodynamic advantages.

 

How, can genaker, spinoa or other be sailed upwind?
Persnaly I can't imagine headsails with large amounts of sag sailed upwind.
It would be too risky to use these sails, because wind is changing direction etc., I understand what you want to say. Personaly I'd prefer reacher or some kind of similar sail, then it can be used, fro my personal experience I know that they have a lot of power. But: it is very dificult to use them and as I said you dont have the freedom. All in all I like sails spinoa (abn boats uses them) very much.

 

All headsails have luff sag. It's physically impossible to avoid. But the amount can be controlled. Look a the tuning guides on sailmaker websites to see the range of settings. Etchells, for example, sail with a huge amount of sag imo.

 

Luff say is not bad per se. Just how it changes when a gust gcomes along. OK Say you are sailing along upwind in 10 knots of breeze. A puff comes and the wind is now 15knots. The forces in the sail increase ans this will increase the sag in the head stay. This will increase the camber in the sail. This is exactly what you do not want. You now have a deeper sail when you would prefer to have a flatter one. If you have a tigher stay to begin with the sag when the wind increases will not be as much as if the stay was less tight. This allows the sail to stay closer to the shape you want. If you didn't need to sail in changing winds then a looser tension would be acceptable.

Many boats change their tensions for the conditions on the day. When it is light they usually reduce the rig tension to allow the jib to be a bit fuller. When it is windy they run higher tension to keep the jib flat. One clsss that does this is the 49er where they only have one jib for all the wind range. The tension is adjusted depending on the gustiness of the day too. There is some writing about the NS14 that would be of interest.

Downwind the onslaught of the gust is less severe and you are "steering the boat under the rig" and so the need for sail to adjust to the gusting of the breeze is not as important.

Hope this explaniation is clear.

Nobody

 

Thanks for the explanation - very clear. You don't have a link to the NS14 article, do you?

 

I will have to dig it out of my boxes and scan it. I will try to have a look for it this weekend.

Nobody

 

That's very kind, but you don't ned to go to any trouble on my behalf.