d1 said...

Chris 249, I just wrote a long and detailed technical response, including listing my qualifications and work experience, but decided to shorten it a bit. So here it is:

Sorry that you think that I'm attacking your honesty. I never meant to do that, and certainly never meant to insult anyone. I think I'm generally saying the same as what you are quoting. Keep in mind that increasing the span increases the aspect and this becomes a different airfoil altogether with different CE, etc... Let's fix the aspect and then reduce induced drag - I'd love to hear your ideas.

It wasn't so much my honesty, but that of the person I was quoting that I was concerned about. And forgive me for doubting that you had qualifications (which I certainly accept) but I've read a lot of people who haven't done much homework write a lot of things that go against aerodynamics as the experts normally describe it.

There are so many people bagging others out on the 'net that I tend to over-react when I perceive someone else doing it, so I do go ott at times. I pretty much assumed that you weren't a pro in this area, simply because they are fairly uncommon. My apologies.

I'm always fascinated by learning from experts in this sort of area, and the reason WHY massive twist (i.e the sort of loose leach we see in boards, which is much looser than that used in boats) works in some situations does interest me*. I've spoken to sailmakers who said that they were not sure why it worked but that they thought the floppy leach "smoothed out the wake" exiting from the leach, but somewhere I'd got the understanding that the energy used by doing so would counteract any such gains. I'd be fascinated to find out more about what DOES happen.

I'm certainly no expert, but one thing that interests me in boat design (and boards) is that many unqualified would-be experts seem to have the knack of ignoring real-life experience and tend to love telling tales about how most rigs are terribly inefficient.

In contrast, most of the theories as explained by those who are very well qualified in this area (guys like Tom Speer and Mark Drela) seem to tally very, very well with what we see on the water in real life. That is one reason I am very interested in what they say.

Re "I'd love to hear your ideas"; I'm just an interested gumbie in aerodynamics, and my ideas about design are linked very much to the fact that to increase the popularity of a sport, technology should be used to improve accessibility rather than to increase performance. Unfortunately (IMHO) that area is almost ignored, which I think is a key to the falling popularity of windsurfing.




* of course, in boards many things are different because we normally ignore so many of the things that boat sailors are normally into; keeping sail plans small, going upwind in light winds quickly, being able to sail high angles and very broad angles in light to moderate winds, having sails that can handle the abrasion of standing rigging, having limited (in some ways) righting moment, etc.

AFAIK it's such factors that can make floppy leaches impractical in boats, and IMHO by failing to take such factors into account, windsurfing has hurt itself. But that's another story.

CJW said...

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.

Agree about the need for high RM to go with CoG and all that. The fact that high AR rigs are therefore seen on intrinsically fast boats and low AR rigs are seen on intrinsically slower craft also muddies the waters.

I suppose it gets down to what you call "high aspect" - for example, the 18 big rigs are about 1 to 1.7 AR upwind (close to 1:1 downwind) and the small rigs considerably less.

Interestingly, one of the highest-aspect rigs around would be the rare singlehanded version of the NS14, which interestingly enough is surprisingly slow, only 2% quicker than the Laser. Of course, there are other reasons for that. The Tasar and NS are also interesting comparisons, with the Tasar having a higher-drag hull and bigger but lower AR sails and now going very well downwind against the beautifully-efficient high-aspect little NS rig and that super-slippery hull. The NZ R Class rigs were also extremely high aspect years ago, but they changed rules and went for larger, lower rigs and went considerably quicker.

It also depends how much quicker you expect a high AR boat to be. I agree that high AR is faster, but when sailing things like International Canoes or Taipans I've still been surprised by how well the low-aspect boats go downwind, even when sailing angles in moderate winds. They are slower, all else being equal, but not always that much slower.

So yep, high aspect rigs are quicker but I still wonder how big the advantage is -especially since boards go well with a pretty low aspect rig.

isn't one of the aims of design to increase stability which reduces drag. a twisty sail generally provides more stability as the wind is constantly changing.

this is where a lot of the increases of speed have come from in windsurfing. without making blanket statements the increase of stability of fins/boards/sails has allowed people to sail faster.

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.

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.

The wikipedia that Paddymac posted earlier has a section en.wikipedia.org/wiki/Forces_on_sails#Relationship_of_lift_coefficient_to_angle_of_incidence:_polar_diagram



This shows how AR ratio affects the Lift Coefficient at vary Angles of Incidence.

At 30 degrees angle of incidence sail with an AR of 1:1 produce nealy twice the Lift Cofficient of high AR sail 1:6, but nearly twice the drag.

Interesting from the graph it shows that AR of 1:3 (similar to a windsurf sail) while almost as good as the 1:6 in achieving the best L/D it conditions to produce a much higher lift coefficient at angles above 15 degrees before dropping off.

Here is is the same thing in french

well said daffy, I've come to belief exactly the same things.