One more Squaretop vs pinhead question

Technical discussion of ARC products
Post Reply
DanBerger
Professional
Posts: 264
Joined: May 3rd, 2004, 12:29 pm
Boat Make/Model: SC 15, SC20
Location: Norfolk, VA

One more Squaretop vs pinhead question

Post by DanBerger » March 19th, 2018, 8:11 am

Bill,

I read the articles on the square tops--very informative! My question is about the sail shape difference on the leech. My pinhead has a huge roach in the middle of the sail and it tapers a little to the foot. That, to me, is like a bomber wing, which gives a lot of power in the middle of the sail where you can use it. I think that is why Supercat sails are cut better than Hobie sails because Hobie sails have the roach near the bottom. I laid a Hobie 16 sail on top of my SC15 sail and found the difference.

Now, when looking at a Squaretop sail--I'm using an ARC17 as a reference, it looks like there isn't a whole lot of roach (curvy leech). I'm thinking that it is still there but not as pronounced since the curve isn't tapering to a pin head. To me, the ARC17 squaretop looks more like a Spitfire wing.

The F18HTs have a HUGE roach and are squaretop. Something about them just didn't work, though. I think that curvy leech might have hooked too much?

I'm going out on a limb here, but I have this vision of the square top opening up when it spills off wind and then being able to see the curved leech I am used to??

Bill Roberts
Expert
Posts: 487
Joined: November 17th, 2003, 7:13 pm
Location: Stuart, Florida

Re: One more Squaretop vs pinhead question

Post by Bill Roberts » March 22nd, 2018, 3:28 pm

Dan, Dan, Dan, what a mess. You are talking about straight vs curved leech and tying it to square top. They are two different things. The square top affects the "SPAN EFFECTIVENESS" of the sail.
The curved vs straight leech affects the SAIL AREA to a small degree. Remember that when sailing to windward the leech of a sail and the aft few feet of sail width are running parallel to boat centerline. Therefore this sail area is generating a "side force" only. No forward thrust here. The sideways thrust makes the CB and Rudder generate more lift, side force, which makes more drag underwater.

The old original SC sails had large curved leeches to increase sail area. Increasing sail area on the leech is not a very good way to achieve more speed.
It can help on a reach when the main is trimmed out but it can hurt going to windward by causing more leech hookback when trimmed in. As far as curved vs straight leech by itself, sail area is the same for both leech shapes, , a straight leech is better aerodynamically. You will get more lift per square foot of sail area out of a sail that has a straight leech over a sail that has a curved leech.


(Dan's)I read the articles on the square tops--very informative! My question is about the sail shape difference on the leech. My pinhead has a huge roach in the middle of the sail and it tapers a little to the foot. That, to me, is like a bomber wing, which gives a lot of power in the middle of the sail where you can use it. I think that is why Supercat sails are cut better than Hobie sails because Hobie sails have the roach near the bottom. I laid a Hobie 16 sail on top of my SC15 sail and found the difference.

Dan, don't confuse sail area distribution with sail force distribution. They don't necessarily go together. The big punch/push from a sail is in the front 1/3rd not the back 1/3rd.

(Dan's) Now, when looking at a Squaretop sail--I'm using an ARC17 as a reference, it looks like there isn't a whole lot of roach (curvy leech). I'm thinking that it is still there but not as pronounced since the curve isn't tapering to a pin head. To me, the ARC17 squaretop looks more like a Spitfire wing.

The closest airplane wing to a sailboat sail is a glider wing. Spitfires fly too fast and are old aerobatic fighter aircraft, pull many Gs. Glider wings are very high aspect ratio with straight leading and trailing edges and squared off outer ends. Many millions of dollars have been spent developing glider wings. If we are going to follow the lead on the best airfloil shape, the glider wing is a good candidate.

(Dan's)The F18HTs have a HUGE roach and are squaretop. Something about them just didn't work, though. I think that curvy leech might have hooked too much?

I know nothing about F18 sails. It sounds like they are going for max area at the sacrifice of sail aspect ratio. This is a lousing game. Higher aspect ratio wings are less draggy, lower Cd, lower drag coefficient and they point higher.

(Dan's)I'm going out on a limb here, but I have this vision of the square top opening up when it spills off wind and then being able to see the curved leech I am used to??

Drean on, Dan.
Top
Last edited by Bill Roberts on March 31st, 2018, 1:21 pm, edited 1 time in total.

DanBerger
Professional
Posts: 264
Joined: May 3rd, 2004, 12:29 pm
Boat Make/Model: SC 15, SC20
Location: Norfolk, VA

Re: One more Squaretop vs pinhead question

Post by DanBerger » March 27th, 2018, 7:26 am

Again, Dang, Bill!!??

OK, so I get that glider wings are good, but we can't have THAT much of a high aspect ratio on our boats. My observations were from A cat and F18HT sailing. The A cats are very light and have high aspect ratio mains (like the Spitfire). It was my understanding that they didn't need the sail area (huge curved leech) because they didn't need the power that heavier boats needed. That is my bomber theory. If you look at the older bombers, they have huge low aspect wings with so that they could carry a lot of payload. They have since moved to longer, higher aspect wings, but nothing like glider wings. The F18HT (I believe) was an attempt at maximum sail area AND square top because they were double handed boats with spinnakers. They were like the big brothers of the A cats in the early 2000s.

The Shark is another example of a very curvy sail for a very heavy boat. But, again, very old technology.

So, when I look at older designs like the original Supercats, Hobies, Prindles, Nacras, etc. I see big, roachy sails. I was just wondering why sails were going higher aspect on older designed boats and if it really works or if it is trying to add new technology to an old boat.

So, while I'm at it, I noticed that your square tops have a very curved top panel where the A cats and most other square tops have a very... square top. Also, what id your opinion on square top jibs?

Bill Roberts
Expert
Posts: 487
Joined: November 17th, 2003, 7:13 pm
Location: Stuart, Florida

Re: One more Squaretop vs pinhead question

Post by Bill Roberts » March 30th, 2018, 1:30 pm

Ok Dan here we go again and this is going to be it, the end.
A Cats can't take any more sail area with today's aspect ratio because they are already very much righting moment limited and pitchpole limited even in medium winds, 10 to 12. If you want to make an A Cat faster, add righting moment, make the boat wider, and then add sail area. Also make the bows taller so the boat won't pitchpole so easily. The A Cats have high aspect ratio sail plans to get the most sail thrust and boat speed in normal sailing/racing winds out of 150 sq ft of sail area. An 18 sq meter boat would be faster than an A Cat built out of today's materials. A 20 sq meter would be faster than that. MAX RIGHTING MOMENT IS KING! Righting moment sets sail thrust and sail thrust sets boat speed. An A Cat with a low aspect ratio 150 sq ft sail would be slower than an A Cat with 150 sq ft high aspect ratio sail in winds up to 10 to 12. The high aspect ratio sail produces more sail thrust per sq ft of sail area than a low aspect ratio sail as long as the boat can be held down and sail trimmed perfect.

Glider vs Bomber:
The glider being a non powered aircraft needs the most efficient wing possible , most lift per sq ft. Total aircraft surface area sets aircraft drag. The wings are the biggest part of aircraft surface area. The glider needs the highest lift to drag ratio wing because the minimum wing area that will generate the required lift also results in minimum aircraft drag.
The Bomber:
A bomber with a high aspect ratio wing is a bomber pilot's dream. The aircraft would fly at cruise speed with less thrust and burn fuel more slowly and have greater range than a low aspect ratio wing. Something that comes with a high aspect ratio wing is also a thin wing, low thickness to chord ratio. High aspect ratio wings are thin, internal wing beams are slender. At max take off gross weight, the high aspect ratio glider style wings, glider wings scaled up in area to carry the bomber's weight, would break at the fuselage. The bending moment from the fuselage to the center of lift on the wing is so great it breaks the wing off attempting to carry bomber's weight. So, how do we reduce this wing breaking bending moment. Reduce the wing span. We still have to hold wing area so the chord of the wing must increase. The result is lower aspect ratio wing. The wing designer goes around this loop until he finds a wing span that won't break at max take off weight plus a few g's and has the correct wing area. This bomber wing plan form is much lower in aspect ratio than the glider's wing.
Summary:
The glider's wing is designed primarily for performance, max L/D.
The bomber's wing plan form is highly influenced by structural design requirements and has a lower L/D.
Meeting the severe structural requirements of the bombers wing, results in a much lower wing aspect ratio than the glider.

I have had square top jibs for years. I really can find no difference in boat performance.

My main sails.
When I order sails I specify the sail area, the foot length, the square top width and the luff length and leech length. The sailmaker determines the exact shape of the leech to meet the sail area requirement. A perfectly straight leech or slight curve makes no difference in sail performance.

Trinisailer
Novice
Posts: 14
Joined: August 11th, 2017, 7:31 pm
Boat Make/Model: SC 20
Location: Trinidad & Tobago

Re: One more Squaretop vs pinhead question

Post by Trinisailer » May 5th, 2018, 8:55 pm

I have a question , so if approx 2ft is removed on the top section of the pinhead mainsail, whats the difference in the foot length between
the pinhead vs squaretop

Bill Roberts
Expert
Posts: 487
Joined: November 17th, 2003, 7:13 pm
Location: Stuart, Florida

Re: One more Squaretop vs pinhead question

Post by Bill Roberts » May 8th, 2018, 2:57 am

I'm not sure I understand your question. I think you are talking about a pinhead SC20 mainsail. You are suggestion coming down the luff at the head 2 ft
and cutting the sail across to the leech to form a square top sail. Then you ask what is the difference in foot length between a pin head sail and a square top sail. The foot length did not change so this is where I get lost. Talk more.
Bill

Post Reply