sailmaking

yeah, luff curve, my last exit to ruin an sail ;-)
diagonal wrinkles at a close hauled, a large vertical wrinkle at a beam reach! :blush:

I usually try a decent to no luff curve and hope that the mast is stiff enough while a gust hits the sail and the boom and kicking strap try to bend the mast.

I attach the sail to the mast with mast loops. With the sail on the boat, the boom at a close hauled and the boat on its side so that you can sight down the mast, I begin to connect the head and tack with the first mast loops to the mast. Then starting from head and tack I attach the left loops. While attaching the loops I usually see where it's too much tension and diagonal wrinkles occur. Yes, I may get a gap between the sail and the mast, but I'm not disappointed about this anymore.

Actually I'm with some sail-recycling, opening the seams of some old sails and rebuilding these with claudio's gadget to get a feeling how it works. Well, the luff curve is given by the old sails, but while canting the panels slightly you get an influence on the luff curve. The grommets are already attached at the old sail's head and tack so it's hard to lay the sail at a table without wrinkles and try a luff curve cut described by rodacarr:

'Elevate the clew of the sail until the sail swoops down toward the luff and becomes just tangent where the luff hits the table...'
http://www.theamya.org/sails/130_SailsPart1.pdf

At the next new set of sails, I'll give it a try.

 

This is how I do it. It's the amount and where to put it that is defeating me.

 

Have not been following to close but the talk has been luff curve.
Next to that MB might be mast bend???

 

Where did I write that? What post? That must have been a while ago

 

Hi Don,

As I mentioned, I was busy with 'sail-recycling'.
Well, next topic: antenna-recycling










Btw. This is the c-rig jib, so I'll need a larger table

 

As mentioned before I'm playing with an old jib and I've just build it up again with 6%draft.
Here is how I (me, the stupid user ;-) ) used claudio's gadget:





For the building and some measurements I used this:







For the wedges I measured the chord from the luff to the leech
(Luff: straight line between the head's and the tack's grommets)
Leech: straight line between the head's and the clew's grommets).

 

And here is what I got with the 6%draft jib
(the measurements are with different sag at the foot as mentioned, no twist, 20 g tension at clew and tack).
I measured the foot, the seams and the middle of the panels (grey rows).

These measurements were taken from both sides, SB and BB.
a is the location of the max draft.
b is the depth at the max draft (b(SB); b(BB)).
c is the 'chord' (luff conture to leech conture)





You'll notice an incline of draft with the upper seams above the 6%draft.
(You'll also notice the sail's chocolate-side)

 

with twist at the leech the upper seam's draft should be reduced.
same 6%draft jib, the measurements are with 20mm sag at the foot (6%draft), 12° twist and 16° twist, 20 g tension at head and tack.
(only the starboard side was measured)







jib with 16" twist



yes, the jib would need a better luff curve :blush:


After all these measurements, confusion didn't fade away.

BTW
Merry Christmas!

 

Well, it's not bad ;-)

Used the bandwidth of the draft/MultiplierFactor-curve.
Reassembled the lowest seam (one 0.13 mm plate more).
And learned how to trim the jib:







I measured only the favorite (SB) side. The BB-side should have 1 to 2 % less draft. So in a puff I should tack to SB ;-)

I wanted a 6 % Jib, so I think I'm pretty much close to it.

 

Probably not a new idea but a way to get the ‘right’ luff curve I thought of was to set up my boat with the mast rigged up, ie shrouds, backstay tensioned and (maybe) a little mast-ram, as if I’m tuning up. Place the boat against a wall and at the far end of the room a single light source so the mast (with curve) casts a shadow on the wall –works better at night. Tape the all but completed sail on the wall so the shadow of the mast is on the luff. Mark it with a pencil line and then cut along the line. Voila, perfect luff shape.
Tony

 

Hi Tony,
At least you'll get the mast curve. And with a stiff mast that's what would be needed.

With a more flexible rig there should be added some 'main luff allowence' which should compensate a bended mast in a gust (as mentioned by rodacarr in his amya article).

I would rig the boat as you described it (but without the main sail), set the main boom in the close hauled and lift the boom where the clew would be. At a more flexible rig there should be noticed some additional bended mast curve, including the main luff allowence.

So as you know, I allways cheated the luff curve with mast rings ;-)
So it may be only my silly theory

 

True.
Perhaps a slight overbend would help create that extra little bit of luff curve, then when rigged normally you'd get the desired 'bagginess'. And as you day a little extra luff can be gained by carefully staggering the mast rings.
Tony

 

Since building my Claudio gadget I've made 3 A-rigs and 2 B-rigs for my IOM. Each set getting better with practice. For the draft curvature I began with:

Main

• Top seam 10%
• Middle 8%
• Bottom 6%

Jib

• Top 8%
• Bottom 6%

with small variations. One set was: main 11, 9, 7, jib 9, 7.

What regimes have others used and what theories have influenced your choice? Eg: flatter for windier locations, etc.

 

Interesting, a little further research and plugging some numbers into the spreadsheet downloadable from here: http://www.stirling.saradioyachting.org.au/saildesign/seamcurvature.htm

...if I am reading it right, it seems I have been building a lot more curve into my sails that I expected. Those % numbers quoted above, when plugged into the spreadsheet give the following resulting %:

Main
Top seam 13.1%
Middle 12.4%
Bottom 9.8%

and

Jib
Top 10.3%
Bottom 9.1%

The reason: I am not taking into account the effect of one seam has on the one above, specifically, the curve in the foot of the sail and how that curve migrates up and affects those above. If I set a camber width of 25 mm at the foot that amounts to a 7% curve (25/355). Plugging that in to the spreadsheet and then adjusting till the % built into each seam is the % I thought I was getting, bumps up the actual results.

In other words, if I want 10, 8 and 6 on my main, I have to build in something like 7, 5 and 4 to acheive the actual curves, because of the extra curve supplied by me reducing the outhaul.

Mmm, the sails have performed ok, its hard to tell with so many other variables. So I guess I need to amend my original question to: what % are people building to acheive what actual draft curves?

 

I've emailed Ben Morris of STIRLING MARBLEHEAD YACHT RACING CLUB Inc the author of the (excellent and comprehensive ) spreadsheet and he confirmed my suspicions.
To get my IOM mainsail seams to have 8, 8.5 and 9% for the top seam, I need to only design 5.3, 4.9 & 6.9%. Given a 5% curvature in the main foot. This may not be news to many but it is to me.:knockout:
I knew the extra camber there meant you needed less lower down in the sail, but...

 

Hi Tony,
this was my problem, too.


with my 6% jib I used 6% draft in every seam and nearly no sag at the foot (2%) and about 16% twist at the leech.
2% sag at the foot, so no way to trim the jib flatter than those (blue) draft numbers!

so I tried to include the 'usual' sag at the foot and the number of seams and I did rebuild some 7% sets like this:




here the draft in the seams was 'calculated other the thumb' and I noticed that the center panels/center seam still had too much draft.
So Ben Morris' spreadsheet seems to pay attention to this. (thanks for the link, I didn't try the download until now)

so it's just getting better




BTW:
I'm working with 0,13 mm plates. Could (should) be thinner!

 

BTW:
I'm working with 0,13 mm plates. Could (should) be thinner!

Is that the same as panels? Ie 4 panels for the main?
If so I am using 50micron mylar which is 0.05mm. I tried 0.075 and I thought it was a bit too thick, especially for A-Rig. Maybe 0.075 is right for the B jib.

 

sail cloth is ikarex pc31 (31g/m²), a polyester rippstop material.

On the c-rig I use four panels, the panel at the foot is a bit shorter than the other panels. At b- and a-rig I use 5 panels.

For Claudio's wedges I use overhead projektor foil, which is 0.13 mm thick.



With my small seams (chord lenght) and the low draft in those sails I often get something like 'use 0.6 plates for the upper seam'.
So I should rearrange the panels so that the wedges/plates fit better to the desired draft.