Dan,Lester
I’ve made a full set of blocks and while building it occured to me that shortening the radius or increasing the angle both cause more draft in the sail. My question is, do you end up with a different sail shape depending on which one you change? Also using blocks do you actually end up with a circular draft curve or is it eliptical? Neither my math skills or imagination can seem to be able to deal with these questions.
Thanks
Don
Lester,
A sail I just built had a seam length of 3.75 inches, and I built in 8% draft, I checked this while flying (about 1.5 knot of wind, just enough to give the sail shape.) with about 3 degrees of twist, and enough outhaul tensing to flatten the sail at the boom.
I approximated the spline in CAD by taking some measurements off the block, and the local radius at the center of the block was 14.6 inches according to Rhinocad.
Hi Dan
Great data, thanks! The 8% draft – is this “8% draft sewn in, and while flying it had some greater draft which isn’t known”, or “while flying it had 8% draft, though it isn’t known how much was sewn in”, or even “the sail had 8% draft sewn in, and when flying it showed 8% draft as well”? (How were you able to measure the flying draft? Digital photograph and the Accumeasure software?)
the local radius at the center of the block was 14.6 inches
OK! If we take a 3.75" chord along this block, ‘h’ is then roughly 0.125" or 1/8", and this represents a draft of about 3.2%. So if “all” of the block curvature was sewn directly into the sail, somehow ignoring any bevel, the most you could obtain would be a draft of 3.2% (according to theory!). Also according to theory, if your bevel angle was around 6 degrees (do you happen to know what it is in fact?) then this block would sew in a draft of around 0.7%…
I’m just guessing here, but maybe you have a sail that shows 8% draft while flying, as a result of sewing in around 1% of this draft on your block? That sounds quite right to me (!), the other 7% of the draft coming from a fair amount of leech tension (you mention very modest twist) and some light wind pressure.
I just finished building a jib for my 3R today, and was able to start photo analyzing it. Attached is an excel spreadsheet with my results so far. I set a baseline and then took 3 photos. I then analyzed the photos, and made a change based on the results. I was able to repeat the process 3 times before I headed into work. Across all 9 photos the 75% draft stripe had an average draft of 8.7%. I should note that the 75% draft stripe is above the upper most seam. The wind source was a large fan placed on the far side of the room that provided just enough air to fill the sails. To determine flying draft I analyze a digital image using Accumeasure, and then feed the Accumeasure results into your correcting spreadsheet. (http://www.onemetre.net/Download/Sailmeas/Sailmeas.htm))
The bevel angle of my blocks is 6 degrees.
On the way into work, I think I figured out a way to measure draft ?actually? built into the sail. During my dinner break, I will see if I can get it done quick, if not I will post my results tomorrow morning.
Attached is a quick paint render of my idea.
[ul]
[li]Lay the sail out on a flat surface, making sure the seam is puffed up.[/li][li]Place blocks on each side of the seam.[/li][li]Lay a strait edge over the blocks[/li][li]Keep lowering the height of the blocks until the straight edge just touches the sail.[/li][li]The height of the blocks is the draft built into the sail.[/ul][/li]It?s not perfect, but it should eliminate all the variables except gravity. Hopefully while wasting time here at work, I can think of a more elegant solution, but the basic idea will be the same.
OK I was able to get a quick test done during my lunch break. IN the following two shots you can see the set up, I know its kind of crude looking but it gave me back good numbers, not what I was expecting, but good numbers none the less. The two blocks are brass tubes with some masking tape rapped around them to increase their diameter, and the straightedge is a stainless steel rod.
Now for the good part.
As the seam sets in the photos above it has a chord of 3.875 inches. The average diameter of the two brass tubes is 0.196 (one was 0.192, and the other was 0.200) inches according to my calipers.
Thus the draft built into the seam is 0.196/3.875*100, or 5.05%
Knowing that the bevel angle is 6 degrees, and using “sin(bevel angle)”, working the problem backwards yields a radius of 1.938 inches. If I use “2 * SQRT(2*(1-cos(bevel)))” I get a radius of 2.473 inches.
Neither of these two radiuses is close to the 14.6 I got for the block earlier. Just for extra added info; below is a picture of the block, with a 6 inch ruler sitting on it. I know my block doesn’t have a circular cross section, but the difference in radiuses is just too great.
Hi Dan
It seems that the block is doing something that my theory simply does not accomodate! I think I know what this is now, your photos have helped, and will post again shortly. I hope that I’ll also be able to explain why some folks can get un-beveled blocks to work, even though “in theory” no draft gets sewn in…
OK, complete re-think of the theory. It now seems to me that the block bevel adds material to the seam in such a way as to “force” draft. Increasing the sail girth by just 0.6 mm on a seam of 200 mm is enough to generate nearly 4% draft!
http://www.onemetre.net/Download/Sailblok/Sailblok.htm
Comments welcome!
Sailblock draft line at 40%.
Alan Hayes (New Zeeland and sometimes contributor here) in 2000 shared with me his method/block for sailmaking. I have reproduced it and find it to work very well. Made from sheet metal, the sail panels are kept in place using magnetic tape. Eliminates masking tape, allows for slight panel movements or relocations, yet holds panels in place when taping the seams. The small wedge blocks (visible at seam of both metal plates) are pushed in and out to develop/reduce the amount of camber between the two panels, while the sail is aligned to the forward metal panel edge and adjusted toward the back or front to locate the maximum camber where desired.
One HUGE benefit of Alan’s methood is that it allows an easy way to change the amount of camber by the location of placement of the small wood wedges. Thus, for my multihull sails, I am able to tape up a fairly “flat” cambered sail, while for monohull sails I am able to push the wedges further in, making the camber a bit more. With a sail board, you are kind of stuck with the amount of camber built into the board.
The thumbnails also show a completed version of his sails on his Mini40 trimaran. (also home uilt) - The guy is GOOD !
Alan - if you read this … thanks so very much for making an unknown process so easy to understand - and do! My thanks once again !
Hi Dick
So the small wedges give you the bevel angle you need? If the wedge max height is, say 1/2", and the plate is 12" long, then you have a bevel angle whose sine is around 0.5/12 = 0.042, angle about 2.4 degrees?
Hi Millertime
What bevel angle does your block have?
Lester - that is correct. Alan’s suggestion seemed to work quite well, with the wedges pushed in until the panels were 3/8 of an inch high. This gave me about 3 degrees. ( 0.375/12=0.312)
I must admit that it sure would be easier making full size sail panels, as trying to cut and align the panel curve is very difficult at these small of numbers. One sneeze and you have to start all over! Of course, with full size sails, I would be complaining about how much floor space was needed. When I did the graphics on my main sail on the big catamaran I had to use the main corridor of our government center after hours. Was a super work surface - clean, buffed and granite ! 
Lester,
3 degrees, I got my kit from ebay.
Also check on ?Model Yachting Resource Center’s Article Library?
Read all of sail making by Dave Acree.
http://www.myrc.org/Library/Library.htm#SailMaking
Don
Lester.
You want a Parabola or a Bi - circular arc. With a circular arc it seams to me your draft line can only be at 50%. You want your sail to be like a airplane wing, I think.
I?m no expert I?m still learning.
Hi Don
Sure. Larry’s sail blocks have a straight run inserted aft of the circular arc, allowing draft at 40% or 45% if you move the panels aft a little during assembly. When bent and flying, draft can be brought forward some by Cunningham tension. Some sailors and sail makers think 50% draft sewn in is just fine…
It would be interesting to hear from anyone who has constructed a block that represents the difference between the “naturally” flying shape (close to a catenary) and the desired shape (as you say, maybe a NACA section). In theory, you want to sew in a shape that takes away the “undesired” natural flying shape of an unpanelled sail and forces the real shape you want.
That eBay block – do you know who was the original manufacturer? You show some instructions with it – does it say who is the author?
Dave Acree’s diagram showing a panel laid taped onto a block is excellent. It illustrates how the luff and leech edges of the seam make a slewed or twisted overlap with the other panel. The slew or twist adds extra length into the seam to cause the seam’s “bump” or sewn-in draft.
Hi Dick
Erm, 0.03125 is the sine of the bevel angle. The bevel angle itself therefore seems to be around 1.8 degrees… (smile)
What is the section shape of your curved metal block? Circular arc?
this is the latest Jib for my 3R. This is trial #3 and is good enough for now; i will refine the design more in the spring.
Lester -
apologies - I have the panels packed away at the moment and am in the process of packing personal items for a couple of weeks road trip to New Mexico. I will pull them out upon my return and get back to you. Actually, if I recall correctly, the panels may have a foil shape to them already, with maximum height (camber) pre-bent into them. This means my sail panels from front to rear would be lying on the panels and already have the front to rear shape (camber) and by inserting wedges it would develop the draft into the sail from top to bottom. On the other hand, they could well be circular, but it has been slightly over a year since their last use, so memory fades with age. :watching2 Sorry
When I return from trip, I will pull, check and advise.