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?
this is a very generalized statement but.
higher aspect (tall) is for speed and lower aspect is for race courses.
lower aspect usually has more get up and go and will point to windward better while high aspect has less drag off the wind and a little less bottom end but more top end potential.
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..
The reasoning should be the same as with the high and low aspect aircraft wings.
The higher aspect will have less induced drag for the same amount of lift. Induced drag is the turbulence which forms when the higher-pressure air under the wing tries to move towards the low-pressure side at the wingtip. The higher aspect wings are longer, but narrower for less induced drag. This type of drag is also the reason why wingtips and windsurfing sails generally taper at the end. Interestingly, in order to stop the equalization of pressure around the wingtips, most modern jets employ winglets. Perhaps this is something we'll see in racing sails pretty soon 
So yes, the top speed of the high-aspect sail should be higher (in theory), due to less induced drag. Coincidentally, the other component of the total "air" drag, the "parasitic drag" of the equipment and sailor, plays a smaller role.
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.
As a short light guy, aspect ratio is all about leverage to me.
High aspect is in theory more efficient, but the power is higher in the sail and has more leverage over my weight, so I can't hang on to it as well as a low aspect sail, where the power is lower down.
Most modern speed sails are going lower aspect, but it's not necessarily reflected in stated boom length, due to the clew cut outs of modern "reflex" sails.
So sailing underpowered high aspect is probably more efficient but overpowered I'd go for low aspect every time.
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.
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."
I don't really think the light wind example is that relevant though as we all know in boat terms it's a real leveler and in a downwind situation it's no longer an efficient wing per say...just a 'wind catcher'; should clarify but light wind I mean almost zero wind. As soon as you get a few knots boats with highly efficient high aspect rigs like A-class cats etc can start running angles to take advantage of the apparent wind they generate and it's goodbye.
Almost every high performance sailing boat has what I consider a high aspect rig; A-class, 18ft skiff, 49er, moth, tornado etc etc. You have to also note however that these boats all have a great amount of righting moment so they can afford to have these higher aspect sails and in turn a higher CoG; It's always a trade off of CoG height vs leverage, lift/drag ratios etc, a windsurfer is no different.
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.
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.
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.
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.
