Squaretop mains

Picked this quote from another thread… I think there's a lot of misconception about the superiority of square top mains. The only real world case I have seen square top vs. triangular main is the designing of the JPK 998. After a summer of sailing, the boatyard chose the regular main over the square top: Cheaper, easier to trim and handle, and always faster upwind. The only time the square top may have had a speed edge was down wind in light air.

 

Square top Mains

I believe thats my quote-you probably should have mentioned that if you're going to use it like this.
How do you explain the fact that almost every racing multihull and almost every ocean racing monohull(Open 60's, VOR) use square tops? And how do you explain the fact that almost every windsurfer, the entire Moth Class, and the Aussie 18's all use square top mains?
In my own experience(50 years of sailing, designing and building and racing sailboats), I have found the square top superior in automatic gust response which is a major advantage in my opinion.
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But the real question comes down to the open classes like the Open 60's, Orma 60's, VOR, Moths and Aussie 18's where they can use any kind of head they want-and they choose 100% to use square heads??

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The actual post that you partially quoted from the cardboard boat thread:

 

I have great respect for Mikko but I am also of the opinion that squaretop or big roach main sails have a performance advantage. There are costs for that performance, but I think that a new performance rig that is planning to use laminate sails is best with a square top main.

-square top has lower induced drag than triangle
-if you look at the crossection at the top 6 ft the squaretop is lifting and the triangle is dragging due to the relative size of the mast to the sail.

The only things a triangle offers are leach tension and a tiny reduction in weight.

 

The square top mainsail was probably first used on the Merlin Rocket dinghy in the late 1950's. It was a way of getting more sail area as the roach area wasn't measured.

I believe the ORMA trimarans have a 100ft mast length limit? So again a square top gives more area. I first used a square top on my liveaboard cruising catamaran launched in 1980. That was again because I wanted more sail area, but had bought a too short used mast. I made my own sails, but back then suitable sail hardware wasn't readily available so I had to make my own sail feeder, battens and batten end fititngs

You have to be careful not to be too pro square tops. You have to compare equal areas while production boatbuilders have to consider cost as well as average customer skill, not just outright speed

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Square top mains(and jibs)

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But you also have to look at all the evidence I mentioned earlier. Surely if there were an advantage to triangular sails at least one boat out of all those development classes would use it? The overwhelming evidence of the use of square tops on so many boats points to an advantage thats hard to discount.
With the apparent effectiveness of squaretops in the ayedeen Class now they're starting to use square top jibs. And most of the AC 45's and some of the AC 72's(with square top mains/wings) are starting to use square top jibs.
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I'd like to see a translation of the article Mikko referenced but that is poor evidence against square tops given their overwhelming use on modern fast boats and in Open Classes.

pictures: AC 72's, unknown, my boat-

 

They will use a square top on all classes with a maximum mast height. The same as overlapping genoas are used on all classes where the overlap is not measured. On classes with maximum sail area limits, the sails are triangular and there is little or no overlap.

 

Square Top Mains

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The 18's don't have rig height limits do they? MOD 70's are one designs and all use square top mains-why would they opt for that when they could have any rig at all? And then there are all the oneoffs-the majority with squaretops-do they know something that Marchaj knew-or is it just fashion?

 

The closer a lifting surface that you can make it like an ellipse, the better the lift to drag ratio. A square top main is similar to a tapered wing or lifting surface, which closest approximates the ideal ellipse on a fabric surface.

this I think is the main reason for the flat top main on performance sails. Even a lifting surface that has no mast that comes to a point is very inefficient, such as on the early super sonic aircraft. You will never see a modern aircraft with pointed wing tips, the early hang gliders had them but those are all gone, all of the modern hang gliders now use swept rectangular sail plans, and the high performance ones have high aspect ratio wings.

A main sail that comes to a point only offers a lot of drag for the upper 10 percent or more of the mast height. In many boats with a back stay, there is not much option, so that is why you will still occupationally see peaked mainsails, to gets some control of the mast bend with a back stay.

There is no way to make a pointed lifting surface efficient, it is just the physics of the flow conditions around the wing tip.

 

ORMA 60s had a mast height limit (30.3m above water).
Orma 60s have a mast height limit (27m).
Aussie 18s have a mast height limit.
Volvo 70s have/had a mast height limit (31.6m)
Formula Windsurfers have a mast height limit (6.25m)
Raceboard Windsurfers had (probably still do) a mast height limit.

Most of these classes can fit more SA on their limited masts by going to a squaretop. Nor are they particularly limited in righting moment, unlike most other boats.

Moths have a luff length limit. They also tried (and abandoned) squaretops with little "gaffs" in the '60s (it's an old idea) but gave them up because they did not work.

As Mark Drela and Tom Speer have pointed out for us here, a sail that has a smaller head than that created by an elliptical loading can be MORE efficient if you are righting moment limited. If I may quote Tom at http://www.boatdesign.net/forums/bo...orms-models-sail-design-21409.html#post189452

"Induced drag for a given amount of lift is inversely proportional to span squared. A pointed planform has more induced drag than an elliptical planform of the same span, but it can have a much greater span within the same strength constraints. The increased span more than makes up for the loss of efficiency of the planform shape itself";

and at http://www.boatdesign.net/forums/sailboats/square-top-mains-9668-3.html#post98561,he says " Actually, the optimum planform is not triangular, rectangular, or elliptical. It's egg-shaped, like a sailboard rig, with the maximum chord at roughly 40% of the luff length and a sizeable amount of roach in the head. If you allow the sail to twist off more than the optimum twist, you need to have more area in the head to make up for it. If you camber the head more than the foot, you need less roach in the head.
.....And even the spanwise lift distribution will change according to what you define as "optimum". There's always a limit to the heeling moment. You want to make the rig as tall as the heeling moment can stand. That's why 18' skiffs have tall, medium and short rigs. Once you've set the heeling moment, you can decrease the induced drag by making the spanwise lift distribution more tapered, but over a longer mast so as to maintain the same heeling moment.

A triangular sail is not a very good planform for its height. A square-top main is a much better fit to the optimum planform for the same size mast. A pin-head main might give the fat-head main a run for its money if it's fit to a mast that is taller so the two rigs have the same height of the center of area."

So squaretops are not inherently faster, although they certainly can be if mast height is restricted and you have lots of RM and you can get free area. Yes, a squaretop outline that is flat and twisted to create an egg-shaped loading may be faster, but they can also come with significant issues (extra cost for extra area, extra weight for extra area, extra battens, extra mainsheet load, yada yada yada).

It's interesting to note that the list of classes that allow squaretops includes none of the most popular classes (although F18s may come close, as do NS14s in Oz). The benefits don't come for free and arguably only really come in boats where there is a high righting moment and low-drag hulls, which allow you to have flat sails where the leach closing is not such a problem. As the JPK shows, such factors often don't apply.

The marginal nature of the improvement can be shown in Tornadoes. They added a square top (more SA) AND a spinnaker AND another trap and still only went about 4% quicker. At our cat club that equated to about 70 seconds per race (at the cost of thousands of dollars plus a significant amount of rigging time)...... On some boats I sail, the time you save in a race with a squarehead is way less than the time you lose in rigging and working to buy the gear, and most of us would rather be sailing than messing around removing highly-loaded battens so we can store our gear away.

With respect to Richard (whose other remarks I certainly agree with) and Merlins, I'm not sure their rig is a squaretop by modern standards. AFAIK the earliest thing we'd call a modern squaretop was Alan Goodall's world title winning A Class sail made by his brother Greg about '87. There's info about it in an old Sailing World. Significantly, Greg's own designs stayed with smaller heads for years......

 

I'm sorry, Doug, I didn't want to pin you so I thought it would be polite to post the quote anonymous. You are not alone in your opinion, I think most of us in the forum think that square tops are superior, after all many "modern" boats by well known designers now sport square top mains. And airplane wings are squar(ish) - but the sailboat is not an airplane. That's why I wanted to raise the topic.

Most often the reason behind square tops is a rating rule (like on most features we see on boats, no matter racing or cruising, and so it has been for 100+ years). Many rules like Open 60, VOR, Volvo, AC, ORMA trimarans, 18footers limit the mast height - no wonder you end up with square tops.

CT 249 above summarises my thoughts so well (better than I could have), that I don't have much to add.

 

Its took years and years before the 18's got them right and what you cant know is if the sail and boat development was also changed the old sail might be better but who will try it if the sail maker wants to only build square heads.
The thing that also changed for square heads was the mast to suit them
do ice boats use them?

 

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This was not a world title winning sail but was surely one of the first squaretops in the US-designed 1975 built 1977 by Johnson sails:

 

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Interesting. I used square tops "with little gaffs" on several models and they worked well in competition. The Tantra EXP-1 sail had a sort of gaff made of foam and a little glass that worked real well.
It appears to me that there is a lot more science-starting with Marchaj-saying that the rectangular planform is superior to a triangular planform and a lot more to square top advantages than what has been posted here.
New designs appear often using square tops when they have no other limits -mast or otherwise.
Mikko, I appreciate your interest and expertise but I think there is a lot more to this story. And I don't think I have a misconception about the superiority of the squaretop main and jib.
Back in the 90's before I started producing a line of radio controlled sailboats a friend and I exhaustively tested square tops against triangular rigs with the same area. The square tops won most of the time.
Do you think the automatic gust response of a squaretop is a significant advantage over a pin head main?


PICTURES-L-R, 1) F3-worlds first production RC sailing hydrofoil, Wing Tip Rig(rotating carbon "bent" gaff), 2) America One-square head main supported by small gaff(first producton spinnaker equipped RC model along with the S50),3) MicroMoth same rig as 1, 4) Aeroskiff model-squarehead main and jib -supported by small "gaffs",5) Super Scow, same as #2, 3) Randy Smyth, square head jib on A Cat.
Note A: most of these models used the Wing Tip Rig with either a rotating, bent gaff or a "normal" gaff but all had a unique feature: there was an upper outhaul that could be adjusted between races and seemed to work exceptionally well.
Note B: The Wing Tip Rig system was particularly suitable to models because it required no full length battens.

 

I am not sure what you guys are arguing about. Square top mains are more efficient, AND they put more sail area on the same size mast. Under any definition of 'better' related to performance it would seem they are the clear winners.

Now if you increase the mast height of a pin head, and don't allow the square the same additional height, then the pin may win out... But boats with appreciably more sail area tend to do that regardless of the style of rig. Right up until the loss of RM starts to hurt controllability.

 

Square top mains are more efficient

What criteria are you using in assessing efficiency?

What are you keeping constant?
- Mast height
- Sail area?
- Heeling moment?
- Cost?