racing sail design

[tshino]

recently I have noticed that many of the most current racing sails have a horizontal edge at the very top of the sail, supposedly increasing the sail area. a little like the topsail on a gaff rig. can anyone clue me in on the enginering philosphy here?

[gggGuest]

Look at the profile of a typical modern aeroplane wing and read up on aspect ratio and profile drag.

Another way of looking at is that the top of a triangular sail does naff all so why bother having it there...

[themanshed]

I spoke with Bill Robert designer of the Super Cats he explained that the wind shears off of the sail with a flat head sail. The old pointy headed sail the wind makes a swirl apex down the face of the sail

[tshino]

Thats what I was looking for, thanks manshed. I suppose this discovery/innovation could be used for many types of rigs...not just racing boats. what do you think?

[BWD]

^^Full moon poster.^^
could be? obviously, more like has been,
for what, 20 years? 200 year?
Don't know, lose count.

[themanshed]

Originally Posted by tshino

Thats what I was looking for, thanks manshed. I suppose this discovery/innovation could be used for many types of rigs...not just racing boats. what do you think?

Yes it would apply to all sails. By the way Bill is an aerospace engineer. Because you need to have a battened sail and an upright batten angling out to the flat head most cruising boats do not apply this technology. The batten is more visible in my white monofilament sail.

[ancient kayaker]

Another way of looking at it, is it lowers the center of pressure for a given sail area. The Una rig does much the same. Early airplane wings used semicircular and elliptical tip shapes but a clipped wing tip is almost as efficient and easier and cheaper to build.

[Guest20100203]

This is a discussion that has played out repeatedly in recent years. First lets dispel misconceptions, square topped sails aren't as efficient as elliptical topped versions of the same area, in regard to induced tip drag, which is what Bill Robert was referring to (I can't believe I'm about to defend the elliptical plan form). Tip induced drag is well understood by few and here it's described as a vortice traveling "down the face of the sail". This isn't correct. It's a vortice the trails the foil tip and generate parasitic drag that must be over come with lift from the foil.

Much has been written about the WWII Spitfire wing shape and it's elliptical plan form wing as the reason behind it's maneuverability. Well, it did have a pretty wing, but it was so because it needed room for more fuel tanks, not to intentionally offer the elliptical form. This wing was good, but the Spitfire being very small, light and well powered had much more to do with it's maneuverability then the shape of it's wings, particularly the tip.

What was discovered in further testing and used on all subsequent propeller driven aircraft was, that the elliptical tip had much lower induced drag, but it also had reduced area, so less lift. If the tip was squared off, you'd pick up more induced lift, but because there was a substantial area increase, the additional lift more then offset the induced drag loses.

The reason we're seeing square topped sails now and didn't for a long time (without an additional spar) is materials and technology. Reliable bat car systems introduced a new level of sail shape control in the 70's and sail fabric design in the late 80's and 90's have permitted bigger roaches and wider headboards. The new materials are much lighter and stronger so you don't have a huge weight aloft penalty or wash out (twist off) effect at the sail top detracting from your additional area.

I'm sorry Tshino had to wait 2 years for an answer on this thread.

[Omeron]

Also, for the same amount of sail area, you need a shorter mast. Weight saving where it counts most.

[Munter]

Keep in mind that plenty of the square tops you see around are more fashion than function. It takes pretty diligent sail trim to make the square top work effectively across all points of sail and many older boats don't have sufficiently powerful boom vangs to properly trim the square tops when not sailing to windward.
That said - they are pretty good looking...

[Mezaire]

Agree that they can be more fashion than function.

We are building a rig for a 45 ft fast cruising boat that will probably never be raced. Designer has a 1m long square top main with twin top mast backstays.
The problem with this, apart from the obvious of needing to trim the backstays everytime you tack, it also means the head of the main is a good 2m above the gooseneck due to the 45deg battern, making removal of the main halyard a total pain in the arse!!!

Tried to convinve the owner to go a standard setup but he likes 'the look' of the sqaure top!!

Mez

[ancient kayaker]

Does the the una mast pivot as a whole, or is the rotation of the curved gaff-like top controlled some other way?

[themanshed]

I'm not sure what everybody's background is, but I know that Bill is as a rocket scientist, successful boat designer, and builder. His designs are very fast and win regattas that speaks more then words in my book. He has proven his concept to me. He explained the wind as it leaves the sail but also spoke of the wind also going down the face of the sail, what do I know I've only launched Estes rockets as a youth....I may have over simplified it and perhaps it only stands for the fastest type of sailing - multihulls. I had a sail converted to test this concept and noticed a difference. In racing to win you have to be in constant max trim to perform well.

I must admit I live in a multihull performance world and may have a jaded view on other venues. My mono-o- maran experience is limited to racing on J-24’s.

But we do have a lively discussion now. I substitute of "fashion in design" for change I suggest "tradition" may be more of an influence to not change.

[Paul B]

Originally Posted by Mezaire

Agree that they can be more fashion than function.

We are building a rig for a 45 ft fast cruising boat that will probably never be raced. Designer has a 1m long square top main with twin top mast backstays.
The problem with this, apart from the obvious of needing to trim the backstays everytime you tack, it also means the head of the main is a good 2m above the gooseneck due to the 45deg battern, making removal of the main halyard a total pain in the arse!!!

Tried to convinve the owner to go a standard setup but he likes 'the look' of the sqaure top!!

Mez

I've spent many hours/miles sailing boats with this rig.

You are right about the attaching/removal of the main halyard, not to mention flaking the main under a cover. You need a ladder, or someone who isn't afraid to balance on the spinnaker pole fitting while fiddling with the halyard shackle. Then you'll need to remove at least one batten to get the sail under the cover.

Tacking can be a pain, but the real issue is gybing. You need someone with quick hands on the new backstay when gybing in a puff, and another quick pair of hands to make sure the boom doesn't hang up on the backstays.

This is not an optimal cruising setup and is not something for shorthanded sailing at all.

[Munter]

Originally Posted by themanshed

I'm not sure what everybody's background is, but I know that Bill is as a rocket scientist, successful boat designer, and builder. His designs are very fast and win regattas that speaks more then words in my book. He has proven his concept to me. He explained the wind as it leaves the sail but also spoke of the wind also going down the face of the sail, what do I know I've only launched Estes rockets as a youth

Apologies if I've offended. I meant no disrespect for you or your colleague. I merely wished to point out that while the square top has its merits it is not the appropriate solution in all instances and that where innapropriately used could result in a poorer outcome than a regular planform.

I'm no rocket scientist but I am an engineer and I have designed and built championship winning sails. We were using square top rigs back in 1998 in NS14s where the combination of very stiff alloy wing masts, limited sail area rules and the development class philosophy resulted in their early adoption (by some).