Controlling twist on Square Top sail

Technical discussion of ARC products
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pairajacks
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Posts: 2
Joined: July 14th, 2017, 3:19 pm
Boat Make/Model: Nacra 5.2

Controlling twist on Square Top sail

Post by pairajacks »

I'm loving the new square top sail on my Nacra 5.2 but hardly beating Prindles across the finish line. I don't want to even look at corrected time.

Especially on the downwind leg, I can't seem to induce enough twist to get the upper telltales to stream back. The main downhaul was pretty tight. What "levers" do people use to control twist? I imagine like other parts of sailing, you reduce the twist until the telltale luffs and then let it out a little to maximize speed.

Bill Roberts
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Joined: November 17th, 2003, 9:13 pm
Location: Stuart, Florida

Re: Controlling twist on Square Top sail

Post by Bill Roberts »

parajacks,
Sailing downwind the twist is controlled by traveler car position and main sheet tension. A more centrally position car and less main sheet tension will increase mainsail twist. More down haul tension will increase twist even more.
What wind conditions are we talking about? With a full cut sail and light wind, it may not be possible to get the top to open up, twist off, and get the leeward side telltales to fly. In this case, the sail is too full up near the top. The sail has too much luff round built into it for the way your mast bends near the top. There is a lot more to a properly built "Square top sail" than the square top plan form/shape at the top of the sail. Does your mast have "Prebend? Sail batten compression is another important part of a versatile sail, variable sail shape, variable twist.
The square top sail design is very sensitive to the luff curve of the sail and bend of the mast and stretch characteristic of the sailcloth. I find that slightly stretchy sail cloth works much better than a hard stiff low stretch cloth like Carbon or load path sail sails.

pairajacks
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Joined: July 14th, 2017, 3:19 pm
Boat Make/Model: Nacra 5.2

Re: Controlling twist on Square Top sail

Post by pairajacks »

It's a Nacra 5.2 with a Slosail radial mylar sail. It's a pretty straight cut, especially when compared with the original dacron sail. The original sail luff was very curved by comparison. The mast is not prebent. The Nacra 52 does have diamond wires, and from the look of the original sail, flattening the sail for close haul required alot of main sheet tension.

It's interesting that to get the twist downwind you're recommending a more center traveller location and light main sheet. I'm going to try that. On the downwind leg of our races ( a broad reach), I'm told most people let their travelers all the way out.

Bill Roberts
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Joined: November 17th, 2003, 9:13 pm
Location: Stuart, Florida

Re: Controlling twist on Square Top sail

Post by Bill Roberts »

Hi parajacks,
You want to be ahead of "most people" so don't do what they do. With adequate telltail coverage including leech telltails,
trim the mainsheet and traveller to the positions where the greatest number of telltails are pointing aft, especially the leech telltails.
Remember we all sail in a wind velocity gradient. The slowest wind we sail in is down at deck level and the strongest or fastest wind speeds are at the top of our sails. This wind speed gradient brings with it a wind direction gradient since we sail across the wind to some degree no matter what our point of sailing. Therefore the mainsail especially should have some twist in it to match the twist in the relative wind vector direction which is the wind we all sail on.

T Peterson
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Boat Make/Model: Supercat 17
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Re: Controlling twist on Square Top sail

Post by T Peterson »

I think I am going to frame the first line of Bill's post above or maybe just tattoo it on the inside of my eyelids!

Bill Roberts
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Posts: 515
Joined: November 17th, 2003, 9:13 pm
Location: Stuart, Florida

Re: Controlling twist on Square Top sail

Post by Bill Roberts »

Good to hear from you Peterson,
As I remember on the SC20, the best traveler car position for sailing downwind was one to two feet to leeward of center. Trim more off center in stronger winds and closer to center in light winds. Watch those telltales, they are telling you what to do. Keep'em happy.

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