Small Standing Lug Sail Design

I am looking at a boat plan that calls for a small standing lug sail approx 33.6 sqft or 3.12 sqm
Sort of similar to this: 36sf RSS Canoe Sail https://duckworks.com/rss-canoe-sail/
I have the basic flat dims (red dashed lines below) but these do not include any rounding, hollowing (purple) nor basic camber and depth (green and brown) throughout the sail.
If I could get ideas for these numbers, I could attempt to finish the 3D sail design in SailcutCAD.
From there I would plot and cut and sew, similar to a Sailrite kit.
I have access to plotting and cutting (work) sewing machines (home and work) and materials (home), I just don't have the 'chops' to design a good sail.
I would rather not loft and broad seam at this point, but might be able to translate such information to inform the panel design. The current panel design does not have any rounding and only minimal camber/depth.
Thanks!

 

Many things to consider, how many panels and spar bend should also be taken into account.

 

Thanks @skaraborgcraft for the picture and thoughts. I had not thought about either panel count or spar bend, other than I like the look of the vertical seams. I see that some small sails have many seams and some only a couple panels.

 

The sail is small enough that even a flat panel sail with a dart would be effective like many polytarp are done.

 

Thanks @Skip Johnson
I did just that a few years ago for a 59sqft 'leg-o-mutton' sprits'l . One dart at the tack and some rounding luff and foot, boltrope, carpet and tarp tape, stitching, and grommets.


Since then I bought a lot of leftover sailcloth and a small walking foot sewing machine and have yet to make a sail.
Then I went and took a job where soft goods are made with larger sewing machines and CNC marking/cutting so I am getting a 'fever' to try one that way.

 

Hi Tops, I posted a thread here a while ago titled: DIY sailmaking, and posted many photos in other of my posts. It can be uncomplicated for a small boat, and almost inexpensive. I made 2 balanced lug sails from a drapery fabric, off the roll bulk sheet, like bed sheet but stronger, 50/50% cotton polyester. After 2 years of thrashing them in up to 25 Kn winds, they are still good - not blown out, little wear, and I learnt a lot. Save your proper sail material for after you have made cheap mistakes, and are sorted to do a professional looking job. Sailcloth isn't cheap.
Get a packet of spare needles for your machine, don't fold lots making thick needle breaking sections, and find the best thread tension for the polyester thread you will probably use. It's very satisfying using a home made sail, costing a hundred bucks or less, (unless you pay someone to sew). Make a smaller pattern on paper, including all angles, lengths, folds, cut outs, darts, reefing lines, and eyelet positions, check it, then mark it up full size on the fabric sheet, marking everything both sides, having a critical eye all the time. Warp is vertical, weft horizontal, and that matters; most initial stretch will be horizontal. Thoroughly check before the first cut. Lay mast and spars on it to check, allow for stretch. Maximum zigzag used, reversing can break needles. Pre-crease the folds if possible, and thin the corners. Gussets tacked in first, and luff/leech reefing points. I straight stitched first, 1 round of all edges to lock it all down flat easily, then zigzag straddling over, almost hiding it, 2 edges of the hems, and down the middle if they are wide. Radiating and hems over gussets. Going straight/curved on long runs is difficult and needs practice. Magnifying glasses to thread the needle. Loft space is the real challenge, to lay it out for markup, and observation, (keeping it all clean is another challenge).

 

Thanks @seasquirt , I appreciated the detailed tips!

I found the thread: DIY sailmaking https://www.boatdesign.net/threads/diy-sailmaking.69761/

 

You’re on the right track. For a small standing lug ~33.6 ft² (3.12 m²) you can get a sweet-handling sail with simple panel shaping and sensible edge curves—no fancy lofting needed. Below is a “use-these-numbers” recipe you can drop into SailcutCAD and then plot/cut.

Quick design targets (works well on canoes/dinghies)
Overall draft (depth): 6.5–7.5 % at 38–42 % aft from the luff (I use 7% @ 40%).
Twist / vertical draft distribution:

• At head (top 15–20%): ~4–5%
  
  
• Mid: 7%
  
  
• At foot (bottom 15–20%): 6%
  

This gives easy acceleration, a soft leech, and tacks nicely without flogging.

Edge shaping (round/hollow)
Use percent of edge length so you can scale to your actual red-line dimensions. Lay the curvature smoothly (broad, fair arcs), peaking near mid-span unless noted.

Edge Curve Amount Notes
Head (yard) Round +0.8–1.2 % Compensates yard sag; concentrate the bump at 50–60% of head. Example: 2.0 m head → +16–24 mm.
Luff (mast) Round +0.4–0.8 % Helps build body once downhaul is on. If you lace to the mast, stay nearer +0.6–0.8%.
Leech Hollow −0.7–1.0 % Kills flutter and keeps a tidy exit. Example: 2.6 m leech → −18–26 mm.
Foot (with boom or sprit-boom) Round +0.5–0.8 % Adds power low and improves tacks. Loose-footed: use +0.8%; shelf-foot not needed.
If your mast is notably stiff, stay toward the low end of luff round. Very bendy spars: use the high end of head round.

Panels and seam shaping (minimal, easy to cut)
You said “rather not loft & broadseam,” but SailcutCAD’s camber model still wants some seam shape. Keep it simple:

• Panel layout: Cross-cut, panels roughly parallel to the foot.
  
  
• # of panels: 4 panels (so 3 seams).
  
  
• Broadseam (max seam overlap) at mid-height:
  
  • Center seam: 10–12 mm
    
    
  • Upper & lower seams: 6–8 mm
    
    
  • Taper each seam’s broadseam to 0 mm within 150–200 mm of both head and foot.
    

This modest broadseam, plus the edge curves above, will deliver the 7% body without drama.

SailcutCAD inputs (a working recipe)

• Sail type: “Lug (standing)”
  
  
• Target area: 3.12 m² (use your exact red-line edges)
  
  
• Draft: 7% at 40% (vertical distribution: head 4.5%, mid 7%, foot 6%)
  
  
• Seams: 3 (4 panels), cross-cut
  
  
• Broadseam distribution: center seam 12 mm peak, upper/lower 8 mm peak; linear taper to 0 over 0.18 m at ends
  
  
• Edge curves: input as above (percent, converted to mm in Sailcut)
  
  
• Luff attachment: “Laced” (adds a touch more luff round) or “Parrels” (reduce luff round by ~0.1–0.2%)
  
  
• Roach control: none; rely on leech hollow
  
  
• Seam allowance: 12–16 mm (be consistent with your machine/foot)
  

Hardware & construction (keeps it bomb-proof)

• Cloth: 3.8–4.4 oz Dacron (130–150 g/m²) crosscut dacron; low stretch.
  
  
• Corner patches: 3-layer radial patches:
  
  • Tack & peak: 3 layers (approx 180 mm, 120 mm, 60 mm radii)
    
    
  • Throat & clew: 2–3 layers (slightly smaller than tack)
    
• Reinforcement bands: 50 mm along leech and foot; 30–40 mm along luff/head.
  
  
• Boltropes/webbing: Not required; 25–30 mm webbing at corners with stainless rings or soft shackles is fine.
  
  
• Reef (optional): One reef at ~80% of luff height; put −6 to −8 mm/m extra leech hollow above the reef to keep it quiet when reefed.
  
  
• Batten: None needed on this size/shape.
  

Rigging notes (makes the numbers work on the water)

• Downhaul: at least 4:1 (6:1 nicer) led low and forward to lock in draft position.
  
  
• Outhaul: snug but not bar-tight; ease in light air to let foot round “switch on.”
  
  
• Yard tie points: two light lashings keep the yard from sliding; don’t crush the head round.
  
  
• Mast partners/parrels: keep the luff within 10–15 mm of the mast to avoid slot leakage.
  

Example (how to turn % into mm)
Say your head (yard) is 2.05 m; 1.0% round ⇒ +20.5 mm at mid-head, faired to 0 at corners.
Your leech is 2.60 m; −0.8% hollow ⇒ −20.8 mm at mid-leech, fair to 0 near head/clew.

Do that for each edge, then apply the seam peaks (12/8/8 mm) in Sailcut, and you’ll be very close on the first hoist.

Materials & cut list (one-trip checklist)

• 4 oz Dacron, ~4.0–4.5 m² including patches and wastage
  
  
• 25–30 mm polyester webbing (corners), ~2 m
  
  
• 50 mm polyester tape (leech/foot), ~6 m; 30–40 mm tape (luff/head), ~5 m
  
  
• Two stainless rings (tack/clew) or Dyneema soft-shackle loops
  
  
• UV-resistant thread (V-92/Tex-92), size 18 needle
  
  
• Seamstick basting tape, 6–8 mm
  

If you truly want zero broadseam
You can cheat a usable shape using only edge curves by increasing the luff and head round ~+0.2%, and the foot round ~+0.2%, keeping the leech hollow at the same value. It won’t be as clean as the small broadseams above, but it will fly well enough for a first build.

Want me to sanity-check your exact red-line dimensions and spit out a ready-to-paste SailcutCAD parameter file (plus a 1:1 PDF plot)? Share: head, luff, leech, foot, and whether you’re boomed or loose-footed. I’ll run the numbers to your exact plan. www.marinersbase.com

 

Thanks @wallabycreek for the very detailed write-up.
I will dig deeper into all this over the weekend. I hope to learn SailcutCAD better through this whole exercise.
Crosscut is going to work better with the material and plotter/cutter but I may try to get vertical to work.

 

@wallabycreek since you offered...thanks for running these:


Luff Length 1350 mm
Gaff Length 1890 mm
Foot Length 1740 mm
Leech Length 2970 mm
Luff Rake -180 mm
Gaff Angle 28 degrees
Cloth Width 800 mm
Seams 13 mm
Hems 30 mm

Edit: boomed

So, to check my understanding of your last reply, you feel that even with the draft and rounding as specified, there is not enough shape in the paneled development straight from the program?

 

A friend used sailcut to design me a one off 15m2 balanced lug, with different cuts, vertical, cross-cut and even radial. I only got as far as making them out of printed paper spat out by the program, but would have no hesitation building the real sail with the information given. I did have to calculate spar bend for my yard for accuracy. Like anything computer driven, garbage in=garbage out.

 

Here is the design so far, with a paper doll and nested at my on-hand cloth width as vertical and cross cut to fit the bed of the plotter/cutter: