sails; square top and roach

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J Drew
Professional
Posts: 104
Joined: September 9th, 2013, 12:39 am
Boat Make/Model: SC 20
Location: n. florida

sails; square top and roach

Post by J Drew »

After reading about square top sails, I was looking at the sail on a friends SC20. It has a full 40% top and the leech goes basically straight down to the foot without much roach.

My latest endeavor is a SC17. I wouldn't call it a pin head sail, but it does have an interesting roach.

So, aside from the obvious, what is the differences between the two sails and would a square top with less roach in the leech be class legal or more productive than the original design?
Bill Roberts
Expert
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Joined: November 17th, 2003, 9:13 pm
Location: Stuart, Florida

Re: sails; square top and roach

Post by Bill Roberts »

Drew, Check with Tom Haberman on class rules for any and all SCs.
I know there was an overall general objective to keep sail area constant between a roachey pinhead
and a square top straight leech sail for the same class.
Some boat owners have gone with ??? sailmakers and there is no telling what they made.
DanBerger
Professional
Posts: 280
Joined: May 3rd, 2004, 3:29 pm
Boat Make/Model: SC 15 w/ spin!, SC 19
Location: Norfolk, VA

Re: sails; square top and roach

Post by DanBerger »

So, The America's cup boats have square top jibs, or something to that effect. Smyth made a really nice self-tacking jib for a SC17 that had a small square top, too.

So, are the benefits the same as a square top main? I'm thinking it would open the slot a little better where (to me) it is pinched off at the head of the jib.

Is this something Aquarius might go to??
Bill Roberts
Expert
Posts: 515
Joined: November 17th, 2003, 9:13 pm
Location: Stuart, Florida

Re: sails; square top and roach

Post by Bill Roberts »

Hi Dan,
Welcome to the 21st century. Aquarius doesn't make sails as you know. Aquarius does make the class rules for sails. As long as a sail made by any sailmaker is made within the maximum sail measurements, lengths and areas, it is class legal.
In the case of a jib, if a sailmaker made a jib to the maximum luff and leech measurements and then put a 6 ins or so square top on it, it is not class legal. It is over class legal sail area limit. Also the leech is closer to the mast and mainsail at the head and early backwinding of the main will occur.
On the other hand if the sailmaker had studied the plans for the jib, he would see that he could square top the jib by sliding down the luff and leech with a horizontal line until the desired square top width measurement was achieved, 6 ins or so. This might be ~8 ins or so down from the head on the class legal plan for the jib. A class legal square top jib will be a few inches short of class legal max luff and leech measurements and a few square inches less than max legal jib sail area. This sail will be class legal and the head of the sail will have sufficient chord, width, that a desirable airfoil shape can be achieved immediately below the head of the sail.
On a pin head jib, at the head and immediately below the head, the sail is so stiff and thick and narrow that a desirable airfoil shape will not form until several inches below the head. Frequently this part of sail has straps on each side for support a ring at the head. This short length of the sail at the head makes drag, not thrust, but it is a necessary part of the sail for structural reasons. The square top cleans this up a little aerodynamically.

Dan, the leech of a jib is already vertical so there is no significant "spanwise flow". What goes on aerodynamically at the head of a jib with vertical leech and a pin head mainsail with large, wide, roachy leech are totally different .
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