sail aerodynamics

Tom,

Does that increase the effective AR of the jib?

Yoke.

 

I think you ought to consider the planform area of the main and jib together when talking about aspect ratio. They really act more like one multi-element airfoil than separate foils.

Vortex generators won't change the effective aspect ratio. They "just" manipulate the boundary layer.

 

Discontinuities with the Fractional Rig

Bullcrap. Masthead rigs were not "all to do with rating rules". There are other reasons that fractional rigs are chosen for vessels other than the aerodynamics of the situation.

Have a look at the 'discontinuites' between the interaction of fractional jibs and mainsails at this website http://www.wb-sails.fi/ and click on "The Quest for the Perfect Shape" and page down to "Think One".
...an excerpt..
"There are "discontinuities" in the sailplan at the junctions of the three areas: a discontinuity in the chord and the twist where the mainsail starts (I & II), and also discontinuities in the "quarter chord line" (gray dashed line). The quarter chord line is an important aerodynamic factor, as it determines the sweep within each area.

At the hounds, there is a discontinuity (dent) in the leading edge : increasing rake and bending the topmast back smoothes out this discontinuity. Sails, looked as a wing, are complex creature: Very highly cambered, much twisted, with discontinuities, a sharp leading edge and a slot in the middle - much more complicated than a 747 wing with all its flaps and ailerons"

There are quite a few other real interesting wind tunnel illustrations on this site as well. A few in particular look at some of the negative interaction of the flow off the head of a fractional jib onto the main. And the neccessity to modify the top portions of mainsails that act in a uni-rigged fashion in a fractional setup.

Many multihulls have not utilized masthead rigs as they don't lend themselves to rotating mast so well. Thats not to deminish the masthead rig as unsuitable in itself. And how about those CODE Zero sails utilized very successfully on the RACE cats and the Volvo boats....and for going up-wing in light airs. I think they were masthead sails

There are some number of other interesting discussions at this website including a stress mapper for genoas, and as affected by sail cloth materials.
"StressMapper has proved to be an excellent help for the designer. Demonstrating how the cut of the sail affects the usable wind range of the sail is easy. The stretch is directly reflected in the sail shape and the customer can see how important correct the sailcloth and cut is."

"It is obvious how much more the traditionally cross-cut Dacron sail stretches. The Dyneema (Spectra) sail also weighs considerably less."
No wonder the older sail materials and cuts didn't lend themselves as readily to big headsails, nor roller-furling ones as well.

 

The code zeros were effective because they were a rating free way to get a lot more sail on a rule limited mast height. Rather my point.

 

Other points of sail?

Is one of the advantages of a fractional rig (more accurately a rig that carries a large portion of it's area in the main) that the sailplan works better at points of sail other than beating to windward?

It seems to me that unboomed sails (Genoas) loose their shape quickly as you bear off the wind. The sheet lead needs to go outboard, so the AOA limit of a Genoa is limited by available beam at the clew.

Club footed jibs and Hoyt's (IIRC) wishbone jib allow proper trim at wider AWAs. Such sails cannot overlap the main however, so those sailplans may not (do not?) generate as high a CL as the same area with an overlapping headsail.

In light breeze (6knts) a typical masthead sloop flies it's Genoa up to BAW up to about 45 degrees, in 20 knts the Genoa or jib is up until BAW 70+ degrees.

It makes sense to me to have the mainsail larger than the headsail if the boat is to perform well on headings other than hard on the wind.

I haven't been able to find any hard numbers that compare distribution of total sail area and amount of headsail overlap. I have observed that in development classes where total sail area is limited, the designs end up being fractional rigs with large mains and small jibs.

I started the Area and Overlap thread in hopes that someone with more knowledge than I could shed some light on the subject.

 

Essentially, the jib benefits from the mainsails upwash. When the main is large in proportion to the jib, the jib gets lots of upwash. When the main is small in proportion to the jib, the jib get less upwash.

Thus, large main/small jib= jib has more drive per unit area but less area, small main/large jib= jib has less drive per unit area but more area.

Pick your poison.

Yoke.

P.S. A look at a few development classes without sailplan restrictions might be a good place to start.

 

Wow… a bout of insomnia at 2:30AM… I thought I’d start this “Sail Aerodynamics” thread to knock me out again. It’s now 7:00AM and I am just now finishing reading it non stopped (no caffine). Very interesting material! I still have http://www.wb-sails.fi/ and Avrel Gentry’s web site to research. I will hit those tomorrow. I had some questions concerning sails (searched the threads this time) and thought it would be better to add them to this thread than creating new ones. I also need to add them here before I forget them tomorrow.

I am working on a rig design that is a little different. At this time, I don’t really want to show it because I’d rather shoot at it more before I embarrassing myself by letting someone else shoot it down with their first post.

1. The design necessitates a masthead rig. I noted the debate in this thread on this topic. Could someone explain or point me to a site that would compare and contrast this one aspect.
   
2. Hopefully I’m not oversimplifying this, but I see a balancing issue with sails. On one hand, the gods of high aspect ratio want a tall skinny sail – more efficient design, but higher moment arm. However healing moment wants a low squat sail. What method should be used to optimize this?
   
3. I’m looking at several boats for reference. Most sailboats of my limited knowledge run the forestay (and jib) as far forward as possible some out on past the hull. However, I see on several (Tornado and some Hobies) and I wonder why they don't run the forestay all the way forward and increase the size of the jib. It does not appear to be a structural decision. Is this a class rule thing or is there some efficiency aspect going on here?
   
4. I note most all jibs seem to slightly overlap the mast. If I go to a self-tacking jib (with its own boom), it could obviously not overlap the mast. Would I be destroying some vital slot/gap geometry?
   
5. Center of lift on chord – Looking at “Theory of Wing Sections”, it appear most all foils here would have a center of lift well beyond the 25% mentioned above. Is that a real-world number for a “typical” sail?
   
6. So that I keep the same boat balance… would it seem reasonable to determine the center of lift of a current design (say Torando) using the 25% chord and making sure my new design would have its center of lift placed in the same location.

 

Catamarans like to keep their riggs as far aft as possible to:

1.) Prevent weather helm and
2.) Prevent pitch poling.

This is not theory but the result of hard won experience.

Also, Light multis often don't have the torsional rigidity to hold the luff of a large jib. The tension needed to hold a luff staight can be quite enormous.

Bob

 

I always knew that catamarans where backwards and defy the laws of physics.

How far aft should the Rigg be?

 

Thank you for your reply Bob.

Prevent pitch poling – Yeah, I’ve met that one up-close and personal like… cracked a couple of ribs. I’m working on a spreadsheet analysis for my rig’s healing (roll) and helming (yaw) moments. Looks like I need to add pitch. That’ll let me evaluate that.

Prevent weather helm - did you mean lee helm? My rudimentary sailing book indicates that some weather helm is a good thing and moving the center of effort further forward causes more lee helm. Since I’m doing a 20’, I running the comparison relative to a Tornado. I was running some trade studies with the spreadsheet and with the forward stay coming off the nose, I was having a great deal of trouble getting the center of action far enough back to get weather helm. So I was coming to your #1 in theory also.

New question: Assuming the pitch poling issue is solved by something else. If I move the dagger board forward (say even with the mast), it:

• would get a little weather helm.
• would be structurally easier and stronger. This is where the beam structure is already.
• appears it would better balance the boat – eg there would be less change in lee/weather helming with dagger board up or down… just magnitude of slippage.

Am I looking at this wrong or is there some other negative ramifications of moving the dagger board forward?

 

Elliptical sails

Is there a method for computing the CE for an elliptical sail? All of my sources use the traditional method for computing CE with no account for added roach of an elliptical sail.

I have a source that states that a fully battened sail will produce 15% more power that an unbattened sail. If that is the case, why don't we see more fully battened sails?

 

This spreadsheet may help.

It's just tradition and class rules. I've never seen a mulithull with anything else but full battens.

 

planform

OK, I'm going to reveal my ignorance.

In my mind, if a planform has a 15% increase in power for the same area, there would be no doubt as to the planform that I would use. What are the negatives with the eliptical form. Why don't we cut 15% off the top of a Bermudian rig and throw it on the roach of a shorter sail and increase our power output by 15%. Is the CE raised so that the heeling arm is increased beyond acceptable limits. Would the reduction in A/R reduce the efficiency of the sail? Again, in my own mind, since a certain percentage of the (main) sail is blanketed by the mast, why does the top portion of a Bermudian rig even exist. (Note: I've never seen a good explanation of a bermudian rig. My assumption is that it is just the main of a typical sloop rig. My discussion here revolves around an elliptical vs. a typical main in a sloop rig.)

This might be a stretch, but a gaff rig with a topsail (forgive me if my terminology is incorrect) starts to aproximate an eliptical sail. Think of it as your computer diplay on low resolution. Chapelle was getting close with this rig.

http://www.svensons.com/boat/?f=SailBoats/South/southwind.jpg

Catamarans, for the most part, carry elipticals. Is that because they have the form stability to carry more heeling moment? If that is not the sole reason, then can a monohull make efficient use of an elliptical planform.

If a person were to design a lightly ballasted, 30-40%, coastal cruiser, could there be advantage to multiple(2-3) elipticals on a shorter planform as opposed to a more traditional, taller sloop/bermudian rig. Granted, with three sails, you have three masts, but could three 20' masts have a lower lever arm(mass and CE) and inertial energy than a single 40'er? OK, I could have done the calculations, but I didn't, I'm lazy. I'll get back to you on that one.

Lots of questions. Just curious if anyone has some gut feeling/experience on the subject.

 

Here's my take on answering these very questions. Let's say that you designed the planform to produce the minimum induced drag for a given hoist, taking into account the effect of the surface, but only considering an isolated sail rig (no hull). For reference, take the semi-elliptical sail planform sealed to the surface, and compare to that. Assume the wind is uniform.

This figure shows what the design tradeoffs are as you make the rig taller or raise it up and down, increasing the gap between the foot and the surface:

Each point on the grid represents a different design, optimized for that condition.

Now say that you wanted to reduce the center of effort to control the heeling moment, so you made the planform more tapered, but didn't go all the way to a triangular head. These planforms look a lot like sailboard rigs, with the clew about 30% up:

These rigs are also have the leech twisted off somewhat so that each section has the same lift coefficient (the rig is evenly loaded so the spanload looks like the planform shape)

The design tradeoffs for this approach look like this:


You can reduce the drag by making the rig higher, but that raises the center of effort. The tapered rig allows you to either lower the center of effort for the same drag, or to reduce the drag for the same center of effort by making the rig taller.

These charts represent the very best you can do, at least as predicted by simple lifting line theory. Your mileage may vary.

 

Along about 1920, a guy named Max Munk at the NACA showed that if you have multiple lifting surfaces lined up in the streamwise direction, the drag due to lift is the same as a single surface of the same span with its lift distributed the same as the total of all the multiple surfaces. It didn't matter how far they were separated in the streamwise direction or how the lift was distributed between them.

So your multiple elliptical rigs act like one elliptical rig of the same height. The effective aspect ratio is the longest luff length squared divided by the total area. If your goal is to lower the center of effort, there's no reason to divide up the sail area - you'll get much the same performance with a single mast of the same height. They might be easier to handle, though.

For going to weather in smooth water, the taller rig is probably going to be the better bet, even if the sail area is limited by the boat's stability. But for acceleration and reaching, a bigger sail on a shorter mast may have better performance.