sail aerodynamics

My main issue about Bogataj's explanations is that he's not precise about what he is considering fixed when he brings the mainsail and jib close to each other. He does say that their "total lift increases", and also says later that the angle of the mainsail "must be increased to allow for the downwash of the jib" (I'm paraphrasing).

My argument all along has been that the total lift increases almost entirely from the mainsail leech return angle being increased, not from any favorable mainsail/jib interaction. Bogataj sort of conflates the two.

 

Please excuse me for being so dense,...maybe its age or whatever
Can you describe this 'leech return angle' in another manner?

Is this in anyway associated with hooking the leech (which we know is detrimental)?

Is the leech of these sails (main or genoa or jib) best configured with it allows the airflow that entered our rig (via the apparent wind) to exit smoothly in a direction parallel with our vessel's direction?

This is how I imagined the ideal camber and flow over the sails of my aft-mast rig would appear in a close hauled instance to a beam reaching instance. Are my leeches the best they might be?

 

Petros, I appreciate your effort to explain this to me. but if you read a bit more of that other subject thread you might find some interesting questions I brought up. For instance:

I repeat, sail cloth can NOT transmit compressive forces. So how are the sails, which are attached to forestays and mast, getting their forward power to the boat?....all thru their sheet lines??



A few other observations from that other discussion:

 

I don't like this term, but that's what perceived to be the jargon for sails. It's the angle between the rearmost part of the sail (say the last 15% of the chord), and the apparent-wind direction. An aero guy would call it "aft camber".

 

This doesn't seem to belong in this Sail Aerodynamics thread. How the sail forces get transmitted to the mast and hull is completely immaterial, and is just a distraction here. The aero force that the rig feels also the total force that is imparted by the rig on the hull. PERIOD. It absolutely doesn't matter whether 80% goes through the mast and 20% through the stays and sheet lines, or vice versa. The total is always 100%.

 

the sail transmits the forces from the wind to the mast and boom through tension, the low pressure side is the convex side, and high pressure side is the concave side (that is holds the shape). If you trim improperly you will get the sail luffing or backwinding, losing the shape. think of a trampoline, the force on the trampoline deck is transmitted to the perimeter frame through tension, just like the sail on a mast and boom. If you hold the trampoline at an angle, so it is not level, a person can still jump on it, and the forces are still transmitted by tension to the perimeter frame. Just like a sail.

There are a lot of complexities of how this happens, it is just a useful simplification to use the CE for design purposes. So you are correct that using the a single force at the centroid as the CE is not exactly correct, and perhaps is confusing if you think about it too much, but when all of the micro forces on the fibers in the sail are resolved, the overall force averaged to one force vector is close enough to be useful for design purposes, particularly are simple sailing dingys and similar craft.

1. air flows over the curved surface of the sail, created a low pressure and a high pressure side.
2. this low and high pressure is resisted by the fabric, pulling it tight, and these forces are transmitted to the perimeter of the sail surface through the fibers of the cloth by tension (BTW this is why low stretch sail cloth is desirable, and why the sail cloth is coated so it does not bleed pressure through the cloth).
3. The edge of the sail surface transmit these forces to the mast and boom, which in turn are resisted by the shrouds, stays, sheets and other rigging.

 

All that's relevant is the total forces (lift, drag), and the moments on the rig. There's no general rule that the various sheet angles must be such-and-such to get the best performance.

 

That's it, the explanation I needed. Thanks.

Contrary to how I defined V2,w and V2,l in the sketches, I had used the velocity B.L. instead of displacement B.L. in my successive considerations. And the free-streamline flow displacement boundary layer indeed becomes asymptotically parallel to the fluid flow downstream. Hence, V2 = V2,w = V2,l regardles of the wake.

Yet another proof that multi-task working is detrimental for the quality of thoughts and results.

Cheers.

 

Tell that to any sheet line trimmer (racing sailor), and see his/her reaction :!:

 

Is CE really the CE?

I introduced that subject here in response to Petros's effort to explain the sequence of how the sails transmit their aero-forces to the vessel.

The question originally came to me when I was trying to justify utilizing the almost universally accepted 'center of effort' method of how the force of the sails got transmitted to the boat?

For example I looked at a headsail's attachment to our boats; along its headstay, and halyard, its tack connection, and the sheet trimming line. In an upwind sailing condition I find that all but one of these connections has the sail cloth pulling BACK (and to the side) on the vessel. Only one connection, the trim sheet line acts to pull forward on the vessel.
And that sheet line pulls at its deck level attachment, NOT thru the our imaginary CE. The question that arose in my mind was, how might that effect the sailing trim of our vessels?

Keep in mind that the sail cloth material can NOT PUSH forward against the mast, nor the forestay, nor its head and tack connections.

 

Brian : You shouldn't have to be reminded that sails propel sailboats because of the aerodynamic forces that they generate. Forces & moments are the fundamentals. If you know all the aerodynamic & hydrodynamic forces in any particular case, you can calculate the performance.

Any sail-trimming "rules" that you come across are only means to achieve the optimal forces & moments.

 

So it would appear as though I have my sails trimmed (or designed) too tight to the centerline of the vessel in that diagram I posted This is how I imagined the ideal camber and flow over the sails of my aft-mast rig would appear in a close hauled instance to a beam reaching instance. Are my leeches the best they might be?[/QUOTE]"]HERE in #557

 

I don't really understand what you are saying.
One minute you are calling the forces and moments 'fundamentals' as though each vessel's rig design had these inherent values unto itself, and then you say that sail trimming can improve these by trimming to achieve optimal forces and moments.:?:

Sounds like some of the old text books I read long ago....
http://www.boatdesign.net/forums/sailboats/sail-loading-rig-rig-loading-vessel-2293-4.html#post43031

 

Tight Forestays

Vessel Substructure to Support Rigging Loads
The Boat ‘Is’ the Structure

Who would have ever dreamed that a lake sailing, ultra light-weight, racing catamaran would encounter, and be capable of sustaining rigging loads comparable to those of an America’s Cup boat??
http://www.boatdesign.net/forums/sailboats/sail-loading-rig-rig-loading-vessel-2293-4.html#post114487


...and a boat with NO hull, supporting a tight rig
http://www.boatdesign.net/forums/sailboats/sail-loading-rig-rig-loading-vessel-2293-7.html#post277099

 

Brian : All I'm saying is that the above quote from markdrela is so obviously true, that I couldn't believe it when I read your flip response to it.