I'm considering a particular sail and trying to decide between a 6.7 and a 7.0, the 7.0 has a significantly longer luff and actually a slightly smaller boom.
It was said that the 6.7 has the largest roach of all the sizes.
So it appears the 7.0 is a tall aspect sail.
What are the applications of a tall vs. wide aspect on a sail?

Agreed - but I think that 7.0 is much older than the 6.7 if it has a much longer luff and shorter boom?

So the age of the sail may play a part in how it handles..

Well we're talking about a Hot Sails Superfreak and it is claimed that the 7.0 was an extension of a 6.3 which was to be the largest size until some personal requests were made. The 6.3 and the 7.0 share the same boom and it is said they just added more head to get to 7.0. So it seems that perhaps the 7.0 came before the 6.7
But what I'm getting at is the characteristics of the different aspects in general.

Sorry about that - not very good at expressing myself - English not my first language.

Generally, and as Gestalt said it: the low aspect ratio will have more fullness and grunt at the lower end. The high aspect ratio will give you the extra speed for a few reasons - one of them is less drag (for the aerodynamic reasons I attempted to explain), and another one is more wind (the wind higher from the ground gets progressively faster due to less friction).

I'm not sure about the speed of gybing and rotating - it might not be a major factor. The CE (centre of effort) is likely to be higher with a taller sail, yes, so the higher aspect may be more difficult to control. However, the CE is also less likely to shift between the back and front hands as the wind speeds change.

I've seen the sail in link, and my first impression was that the lack of taper of this sail offsets the efficiency advantage of the higher-aspect airfoil.

d1 said...

>>>>>>
I've seen the sail in link, and my first impression was that the lack of taper of this sail offsets the efficiency advantage of the higher-aspect airfoil.

I suspect, when you have enough twist, taper is irrelevant.
The head of the sail generates very little lift, but it does act as a damn between the low and high pressure sides of the sail, making induced drag a non issue.

I see to remember from dinghy sailing days that a high aspect was more efficient upwind, it could sail higher and faster, but a lower aspect was better off the wind.

Interesting comments from answers.yahoo.com/question/index?qid=20100216064252AAonvOr
A 'high aspect' cut sail will be tall and skinny. It is designed for faster sailing, but gets less of the energy out of the wind. It expects a lot more wind to flow over it and it plans to move quickly through the air. A low aspect sail, relatively wider than a high aspect will capture more of the power out of a given breeze, but it is not the fastest sail.

Beaglebuddy said...

>>>>>>>>>>>>

Well I'm 197 CM tall and the boom is always at the top of the cutout so perhaps the center of effort issue isn't as important for me? However I'm really still just a beginner so anything that's easier to use is beneficial.
I have googled this and some people have said that taller sails are for waves and wider is for racing, also taller is for lower wind but there is also other statements and sail specs that seem to contradict this so I'm really quite puzzled.

Yes not such an issue if you're tall, but low aspect may be more user friendly for a beginner.

some 80s wave sails had all their power up high, in an effort to get above the wind shadow caused by waves. but they were horrible to sail with!! The slightest gust would have you over the front.

I'm not surprised you're confused, there's been so many different ideas over the years, some in direct contradiction. With marketing hype, a fair amount of pseudoscience and urban myth thrown in.

paddymac said...
>>>>>>>
My interpretation:
If going upwind - maximise lift/drag ratio. Put your draft forward and use a high aspect (low drag). Downwind this ratio is not as important as power.

Given windsurfing speeds off the wind and the consequential reduction in angle of attack I imagine the effects are not as pronounced as a keel boat sailing very broad.

I still reckon with twisty sails, induced drag is taken care of by the twist, aspect ratio is much less a factor

According to aerodynamicists (such as a Boeing and America's Cup wing designer) and aerodynamic theory, winglets are not particularly efficient. It's much better to just make the wing/sail taller or longer. The problem is that is not always possible; for example aircraft have to fit on runways and within hangars.

As a post from Tom Speer (Boeing aerodynamicist who was also involved with the wing-sailed trimaran that won the last America's Cup) says;

"A simple span extension will do the same thing as a winglet, with less wetted area. You also get a reduction in stall speed from the increased wing area. So from a purely aerodynamic point of view, this is the way to go.

But winglets are useful when there are other constraints. For example, if the span is extended, it may not be possible to get into the same gates at the airport. And it is possible to achieve the same increase in effective span with less wing-root bending moment (and therefore less structural beef-up) with a winglet than a span extension."

As a naval architect notes, the joint between the winglet and the sail or wing can create massive drag;

"According to Fluid Dynamic Drag by Hoerner, the junction of two foils (such as a horizontal winglet atop a squre head sail) creates an interference drag that is typically equal in magnitude to the drag created from a foil 10 times longer than the length of the connection. In other words, if the square head was 1m in chord and an endplate was placed on top to prevent tip loss, the interferece drag between the sail head and the endplate would be equal to the drag one would expect from a 10m wing. So whilst you will reduce tip loss drag, you will gain an interefence drag. It will depend on the particular geometry of the design as to whether the gains outweigh the losses."

Gestalt said...

paddymac said...

I see to remember from dinghy sailing days that a high aspect was more efficient upwind, it could sail higher and faster, but a lower aspect was better off the wind.

i'm not certain with dinghys but i'm thinking it's the opposite to that.

With dinghies, cats etc, the higher aspect rigs are definitely at their best upwind. Low aspect rigs create higher maximum lift than high aspect rigs at broad angles of attack, according to Marchaj etc.

This can be quite embarrassing when you are sailing a "efficient high aspect" boat against those old-fashioned low aspect boats. You can get to the top mark ahead and then find them creeping along at the same speed (or better) downwind in the light stuff.

A classic example is the International Canoe, which has a high aspect rig (8m mainsail with a luff about 20' on a boom about 5') and knocks something like a Laser silly upwind or around a course most of the time. But in light winds downwind, when both are sailing with stalled flow (or close to it) the Laser rig (7m, with a boom about 8' and a mast about 17') actually seems to produce more power; it certainly produces about the same speed.

Chris 249 said...

Gestalt said...

paddymac said...

I see to remember from dinghy sailing days that a high aspect was more efficient upwind, it could sail higher and faster, but a lower aspect was better off the wind.

i'm not certain with dinghys but i'm thinking it's the opposite to that.

With dinghies, cats etc, the higher aspect rigs are definitely at their best upwind. Low aspect rigs create higher maximum lift than high aspect rigs at broad angles of attack, according to Marchaj etc.

According to that same Marchaj...

Triangular planform [this is a good summary of what he has to say www.onemetre.net/Design/Circulat/circulat.htm]

The second feature is that the upwash along a foil with a triangular planform increases quite dramatically towards the pointed tip. An ideal "elliptical" planform does not show this feature, and neither does a moderately tapered planform. This upwash means that the tip of a triangular foil is very prone to stalling, unless it is set with some wash-out -- that is, twist.

Increasing upwash

According to Marchaj, the need for twist in a sail has much less to do with the wind gradient, and much more to do with the upwash along a triangular planform.

Apparently sail twist is needed so that the AoA remains constant along a tapered foil like a sail, if there is too little twist the AoA at the tip would be greater than at the foot of the sail, causing premature stalling and increased induced drag.

These other two links are interesting as well if you like that kind of stuff... :-)
www.onemetre.net/Design/Downwash/Downwash.htm
www.onemetre.net/Design/Downwash/LiftLine/Liftline.htm

yeah i agree with cjw,

this topic has gone so far off track because windsurfers, which are not 50 foot catamarans, have different requirements.

if you look at the AR of a windsurfing sail. it's not high aspect in sailing terms. in windsurfing terms high aspect is about 2.5 and low aspect about 2

to go upwind yeah you could go for a low drag high aspect sail but to point high you need power and a high aspect sail puts the power in a difficult to manage spot. if you keep the power low in the sail it is more controllable. by making the boom longer you can increase the power and keep it in the controllable zone. increasing the power provides more windward ability at lower speeds, more acceleration, better early planing outcomes etc.

you then reduce the drag by designing twist into the sail and by reducing the head size and dealing with tip drag.

having a longer boom also stops you oversheeting which upwind is a killer.

off the wind for a yacht, the drag a low aspect sail has improves the downwind speed. a windsurfer going off the wind though is travelling faster than the wind so reducing the drag as much as possible would be favoured.

Gestalt said...

what this should say is upwind put your draft as far forward as is efficient, which in most cases is not as far forward as when sailing off the wind, ie. when sailing upwind you actually run your draft further back then when sailing off the wind. the reason for this is that if your draft is too far forward when travelling upwind the flow detaches and your sail stalls. one reason why adjustable outhauls get tightened when sailing upwind.

off the wind for a yacht, the drag a low aspect sail has improves the downwind speed. a windsurfer going off the wind though is travelling faster than the wind so reducing the drag as much as possible is favoured.

also as the speeds are higher the apparent wind moves forward. running a deep draft forward helps maintain flow attachment.

paddymac said...

Gestalt said...

paddymac said...

I see to remember from dinghy sailing days that a high aspect was more efficient upwind, it could sail higher and faster, but a lower aspect was better off the wind.

i'm not certain with dinghys but i'm thinking it's the opposite to that.

Struggling to find a source that states this simply and definitively but this one seems ok:

http://www.wb-sails.fi/news/SailPowerCalc/SailPowerCalc.htm

Aspect ratio

The rig aspect ratio affects the pointing ability of the boat. A taller rig produces more drive at a given wind angle and for the same sail area than a lower one. Try a very high aspect ratio P= 17.35 m E= 5.00 m, and compare with the default rig (Reset). There is a catch with high aspect ratio - heeling moment goes up - this puts a practical limit to the tallness of the rigs we use on contemporary boats. Low aspect ratio also performs better off wind.

If you wanna get really serious have a look at en.wikipedia.org/wiki/Forces_on_sails#Influence_of_Aspect_ratio_and_Sail_Planform_on_Induced_Drag

My interpretation:
If going upwind - maximise lift/drag ratio. Put your draft forward and use a high aspect (low drag). Downwind this ratio is not as important as power.

Given windsurfing speeds off the wind and the consequential reduction in angle of attack I imagine the effects are not as pronounced as a keel boat sailing very broad.

No expert ... but

Draft position and draft amount are two separate variables. Draft position, according to the sail theory I have read, is (relatively) forward for upwind and back for downwind. Draft amount is less upwind and more downwind. Windsurfers don't have cunninghams, backstays etc to adjust the draft position, so it's a bit of a moot point. Outhaul primarily adjust draft amount.

Why do you want the draft forward upwind? Because angle of lift is closer to the direction of the craft. You use less draft amount so the flow does not detach. Pretty sure the wikipedia article supports this argument.

As to the specifics on the windsurfer - I think we agree on the point I made earlier "Given windsurfing speeds off the wind and the consequential reduction in angle of attack I imagine the effects are not as pronounced as a keel boat sailing very broad."

Standard sail trim changes on a dinghy at the start of an upwind leg - cunningham on, outhaul on. Downwind - cunningham off, outhaul off, adjust vang for twist.

racerX said...


According to that same Marchaj...

Triangular planform [this is a good summary of what he has to say www.onemetre.net/Design/Circulat/circulat.htm]

The second feature is that the upwash along a foil with a triangular planform increases quite dramatically towards the pointed tip. An ideal "elliptical" planform does not show this feature, and neither does a moderately tapered planform. This upwash means that the tip of a triangular foil is very prone to stalling, unless it is set with some wash-out -- that is, twist.

Increasing upwash

According to Marchaj, the need for twist in a sail has much less to do with the wind gradient, and much more to do with the upwash along a triangular planform.

Apparently sail twist is needed so that the AoA remains constant along a tapered foil like a sail, if there is too little twist the AoA at the tip would be greater than at the foot of the sail, causing premature stalling and increased induced drag.

These other two links are interesting as well if you like that kind of stuff... :-)
www.onemetre.net/Design/Downwash/Downwash.htm
www.onemetre.net/Design/Downwash/LiftLine/Liftline.htm

I'll check them out tonight, thanks.

It seems to me that in boats we are working very hard to ensure that the AoA at the tip is similar to the AoA at the foot (although of course gradient and varying sail depths come into it as well). In most classes, we're working the mainsail leach so that it's right at the edge of stalling.

But FW and speed sails (and even longboard sails at the top end of the range) seem to move into a completely different dimension of looseness, with the extreme leach in the head far floppier than in any type of boat.

Obviously it works, just as tighter leaches work for other types of craft. I'm not saying anyone is wrong, I'm just interested in the way the different approaches work in different situations.

Beaglebuddy said...

Getting back to the original question.... would you fellas consider this 7.0M a high aspect sail? www.hotsailsmaui.com/sail.php?uid=2#sail_feature
Comparing it to the closest size, the 6.7, the 7.0 seems very different.

they are much of a muchness. there is probably some other defining difference like draft and coe position.

in very general terms. a shorter boom makes the rig feel lighter but can be more twitchy. the longer boom can make the rig feed the power on smoother and provide better acceleration. it's a balancing act.

the 7m at guess would probably feel more twitchy when compared directly with 6.7 but which one is more powerfull depends on knowing more about the 2 sails.

both those sails are heading towards high aspect in windsurfing terms.

only way to really know is to sail them.

Please don't attack people's honesty by claiming that they are taking things out of context or defending the indefensible. Let's have factual discussions rather than insults.

The Speer post I quoted comes from a thread titled "Winglets on Sails" which is surely pretty much on this context. See the post at

www.boatdesign.net/threads/winglets-on-sails.22521/page-2

On the same website, Mark Drela says "And by far the best orientation for a winglet, from a purely induced-drag view, is horizontal. i.e. as a span extension. Structural considerations, specifically minimizing wing bending moments, is what favors the vertical winglet.

So unless there's a draft rule in effect here, mounting one of these anti-vortex panels vertically like a keel or daggerboard would be far more effective than mounting them horizontally. It will produce more heeling moment, but on a cat this should have a very small effect on the overall moment balance.

BTW, whatever the mounting orientation, it would be beneficial to put a proper leading edge on the panel. LE vortex separation is effective, but in terms of L/D an attached LE flow is better."

See www.boatdesign.net/forums/multihulls/anti-vortex-panels-22688-3.html#post206091

In a model plane forum, Drela states;

"You will get the least induced drag by flattening out the curled tips -- going out spanwise is always better than going up.
More specifically, if a 1" high winglet reduces the induced drag by X percent, then a 1" semispan extension will reduce the induced drag by 2X percent. These percentages assume the best possible situation, that the winglet and the span extension are well designed.
One other disadvantage of a curled-up tip is that it doesn't allow running the aileron all the way to the tip. Likewise for the swept-back "crescent" tips which were the rage some time ago.

When yo"u also add manufacturing simplicity, a simple flat tip looks like a win-win-win.


See www.rcgroups.com/forums/showthread.php?t=913362&page=4

So we have Drela, a world-recognised authority on aero- and hydrodynamics, and Tom Speer both saying that winglets are no better aerodynamically (worse, actually) than adding to the span.

The book "Assessment of wingtip modifications to increase the fuel efficiency of Air ...By National Research Council (U.S.). Committee on Assessment of Aircraft Winglets for Large Aircraft Fuel Efficiency, National Research Council (U.S.). Air Force Studies Board" says
"Several approaches to wing retrofits, which increase the effective aerodynamic span, are possible, The addition of winglets is perhaps the most obvious approach....simple wingtip extensions are also viable alternatives for reducing fuel consumption. Rather than adding winglets with a height of 10ft, one could add 5-ft horizontal span extensions to each wingtip and achieve similar drag savings....

In www.kuleuven.be/optec/files/Kroo2005.pdf , it was noted that "Figure 3 (from
Jones) shows that with fixed integrated bending moment (a rough indicator of wing
weight) winglets produce about as much drag savings as planar tip extensions."

This information comes from recognised experts in the field - what are your qualifications?

I did see once again that the guy who quoted Horner was looking at foils that are thicker than sails and therefore his claim of drag is wrong.