Hi,
One thing I observed very early after I finished my junk conversion on Ilvy, is the distorted shape of my sails luff. The webbing is about 50 mm wide, and its forward edge flaps over to lee. Have a look at these fotos:
Obviously, this distorted leading edge is not leading the incoming flow in an optimal way into the sail. The lee-bend webbing will definitely induce eddies, which increase drag. How much drag, I can only guess with no testing facilities at hand. However, it is drag and thus bad. Have a look at the following sketch:
I do not know how big the actual effect on sailing performance is, neither do I have the means at hand (by now) to test this with experiments. It might be substantial; it might be negligible. However, I think it’s worth understanding first, to then see if it could be improved by simple (!) means. Below, I try to find a simple solution to modify the luff edge.
The profile shape and form of the sail close to the luff is very important to the performance of the sail (L/D, α-tolerance, etc…), thus windward performance and ease of sheet handling. In contrast, the trailing edge (leech) is of far, far less importance to sail performance. Slieve did a fantastic job explaining these aerodynamic fundamentals in his detailed “Some thoughts” document in an easily understandable way.
Luff making for Ilvy
For Ilvy, I mainly used Arne’s fantastic tutorials on how to fabricate a junk rig with barrel cut camber. One minor thing I changed was the way the webbing is attached to the sails edge. As I read Arne’s documents, he simply hems the canvas and sews the webbing on top. Fair enough, simple and straight. Now for Ilvy, the webbing color looked nice on the display of my laptop when I ordered it, but when I unpacked it I was close to throwing up. It was neon-yellow! Not good. It would totally destroy the fantastic yellow colour impression of Ilvy’s sail fabric. However, time was running and I wanted to start sailmaking instead of waiting for another order loop. Therefore, I enveloped the ugly looking webbing into the nice looking canvas. (Please don’t ask for the extra hours… ) Have a look at the following sketch.
To verify, whether this altered webbing method is reason to my distorted luff, it would be great to get some feedback, including fotos if possible, on the shape of original “Arne’s luffs”. Is it also distorted like on my foto above?
Optimizing the leading edge
Let’s continue, assuming that my type of enveloping the luff webbing is not the reason for the distorted luff. What might be solutions for optimizing the leading edge shape?
In the other thread, named “The mast balance of the JR”, Slieve described that his SJR is sewn with only thin boltrope at the luff (he listed different diameters from 3-6 mm lines). It is easy to imagine that a luff with such a reinforcement (sewn-in boltrope) would naturally produce a more optimal leading edge than the webbing type. Even more so, if the boltrope is not sewn “tightly” into the luff (like in bermudan sails) but loosely enveloped. This would create something close to the typical nose of airfoil profiles. To explain what I mean, have a look at the following:
Looks good so far, but of course this “nose” would be way to small in comparison to the chord length to be of any effect. However, this loosely sewn-in boltrope is able to replace the luff webbing without any disadvantages: simple and fast to make, easy to mount, nothing new (has been used a lot in junks before), sufficient to take the loads. The load on the luff edge is minimal anyway – different to the leech edge!
Hence, I derive my conclusion: when building Arne-sails, build the luff with loosely sewn-in boltrope instead of webbing! Keep the webbing type for the remaining three edges. This makes for aerodynamically nicer luffs without complicating the build in any ways.
Outlook to further development
That loosely sewn-in boltrope made me think… To increase the α-tolerance (widen the angle of attack of the sail profile, at which it does not stall but draws nicely), we would need a thick nose instead of the slim knife-like edge of a typical sail. The concept of the loosely sewn-in boltrope gives that thick nose, but only in a small scale. Too small in relation to chord length to be of any significant effect….
But how about using a thicker boltrope? Or put something thick around the boltrope line, inside the enveloping canvas. Pipe insulation comes to my mind…
It is made of flexible foam, thus wouldn’t mind bending when the panels are folded down into the lazys. One can buy it in different diameters, longitudinally slotted or unslotted, and for next to nothing. Put these, with the boltrope inside, into the vertical, loosely sewn luff pockets and you have a good nose shape with attached flow, upwash and the like.
Worth a try?