Homemade 21-panel Asym Spinnaker

[lohwaikin]

With the help of a young couple serious about all things boats, I made a 300 sq-ft asym spinnaker out of easily-available materials.
I will upload as much info as possible in this topic.

[lohwaikin]

It all began when 3 of us completed last year's 2nd season of Twilight Series where keelboats are restricted to white-sails only. We wanted to participate in this year's Sunday Series which permits spinnaker sailing. So over a weekend of discussion we made a convenient wire model of an i550 with asym spinnaker in craft wires.

Scale-35-5-wire-model.JPG

[lohwaikin]

Then I manually translated the wire-frame model dimensions into 3D CAD software (PTC CREO), and tweaked the lines to make them lines look smooth.

i550_asym_wireframe_creo_screen.jpg

For folks who work with 3d CAD software, I have uploaded the *.PRT file here if you're interested.

The vital stats are:

LUFF = 30.5 FT
LEECH = 25.25 FT
FOOT = 16.5 FT
AREA = 320 SQFT

Our sprit-pole is 4 feet long.

[lohwaikin]

It seems there is trouble uploading the 3D CAD file of 515 kb.
So here's the ZIP file (disguised as a DOC file)....

[lohwaikin]

Since the freeware SailCut (which I used to generate panel templates for my jib and main) does not offer spinnaker design function, I will have to manually translate the 3D CAD wireframes into 21 sail panels in 2D AutoCAD format. The translation into AutoCAD planar layout is an approximation process. I did this by adding a few datum planes across the 3D CAD model, these datums intersect the wireframe curves to produce a bunch of points in 3D space, and with them I built up each panel as a cluster of triangles. Painstaking process. I hope you get what I mean. Anyway the AutoCAD file is attached here.

autocad_asym_screen_capture.jpg

[lohwaikin]

A little clarification on the above AutoCAD screen:
1) The 3 blue boxes contain all the 21 panels translated from 3D CAD. Segment #1 has the head, segment #2 has the tack, and segment #3 has the clew.
2) Notice I added 15mm wide seams to every applicable panels (denoted by purple lines).
3) The 3 purple boxes contain all the components of the 3 corner patches. Polygonal shapes will be in ripstop Nylon, pie shapes will be in 6oz Dacron.
4) The green boxes are the nesting of 21 panels + corner patches along A1-wide (841mm) paper roll. I use these nesting to print out the templates in 70gsm thick paper.
5) The red boxes are the final nesting on ripstop Nylon fabric of 155cm width. You can find this 1.5oz kite material on eBay (the seller was some Emmakite company).

[lohwaikin]

I can't really upload the above AutoCAD file since it is about 900kb.
But here's the paper template nesting file of about 380kb (disguised as DOC but is actually DWG extension).
You can see with AutoCAD, surface area can be easily extracted for every of the 21 panels.
It comes up to about 30.4 m^2. Well that's around 320 sqft.

[lohwaikin]

Lofting the sail panels onto ripstop Nylon is almost impossible without paper templates.
Ripstop Nylon fabric are quite stretchy, plus the panels are quite slender; there will be significant dimensional error if lofting by direct transfer.
The paper templates are good investments, although it took me 2 trips (and some hair-loss) to the large format printer to have 18 strips of A1 width papers plotted out.
The printing costs S$90 (S$5 per 840mm x 3000mm 70gsm B&W print).
Back at home, I stitched these papers up with double-sided sticky tapes, and cut out all paper templates of 21 panels and corner patches.

[lohwaikin]

Here are the 18 PDF files of 840mm x 3000mm paper templates:

[lohwaikin]

Some more....

[lohwaikin]

and some more...

[lohwaikin]

Finally all 18 sheets of templates.

[lohwaikin]

The next purchase will be the 1.5oz ripstop Nylon fabric. I spent S$250 (US$185) on 35 yards of 5 feet width.

1-5-oz_nylon_emmakites_35yards.jpg

At the end of lines transfer and cutting out the panels, I was left with about 4 yards.
Here's how I nested the shapes into the fabric:

[lohwaikin]

A mock-up of one of the 3 corner patches:

corner-patch-mock-up.jpg

[lohwaikin]

With the paper templates, lofting out the panels onto fabric became really efficient; 3 nights of about 3 hours each got everything lofted and cut.
I traced the panel shapes using tailor chalks and "peel-able" Saintograph crayons.
Remember to trace the seams; the paper templates do not include that.
That's when the fabric nesting map came in handy.

Tracing_Panel_Shapes_On_Nylon.jpg

[lohwaikin]

Cutting the Nylon fabric with normal scissors...
3 persons working, it took about 3 hours to cut everything out.

Cutting_out_panels.jpg

[lohwaikin]

Oh yes, it is very important during the lofting stage to include "strike-up" marks along the seams of panels, maybe one every 3 feet.
It helps during the basting stage where we joined adjacent panels together with 1/4" wide double-sided sticky tapes.
I followed Sail-Rite's assembly approach on Youtube; basting up the kite in 3 segments of 7 panels each.
Unlike Dacron, the basting tapes stuck on real well.

Then the next stage will be the tuning of the sewing machine to work with light fabric.
I used V69 Dabond polyester thread left over from the last sail-making project.
Most online sewing forums recommend V46 or lighter thread for sewing lightweight Nylon fabric.

tuning_sewing_thread_tension.jpg

[lohwaikin]

And there we went, with plenty of cloth-pegs to roll up panels into tidy bundle, we gave each seam 2 passes of 5mm wide zig-zag stitches.

stitching-panels.jpg

panels_3_4_strike_up_marks.jpg

[lohwaikin]

Not much photo here, it happened too fast in one night. We bought a spool (about 30 m) of 3mm thick textile-braided polyester line to use as "bolt-rope".
The 3 edges of the kite were folded and basted with sticky tapes (tabling in sewing lingo) by 20mm (3/4") to trap the bolt-rope underneath.
Followed by 1 pass of zig-zag stitch.
The corner patches went on.
All 3 corners terminated with 1.5" wide stainless D-rings.
The sewing machine did all the stitching because the ropes and fabric were so light, there's no need to hand-stitch with wax-twine.

[admin]

Outstanding Loh!!! Now I know why sails are so expensive.

Can't wait to see pix of the kite under way! Thanks for posting this, it is inspiring.

Cheers, --TF

[Warren Nethercote]

Loh,

An interesting approach! A friend of mine did something similar in principle (but a bit easier) when he used a ship shell plate development program to design a couple of genoa jibs. The worst part was the sewing! He did find that estimating luff sag, both fore and aft and transverse was critical to making a sail that had a comfortably wide 'groove'. In that regard I wonder if your draft is too far forward in the wireframe - it could lead to a sail which will curl prematurely - but this may be offset by making the sail out of 1 1/2 oz fabric, which will reduce stretch. On the other hand, the draft forward design should ensure an open leech and reduce blowback on the mainsail.

[lohwaikin]

You are right! It ain't working very well!
It seems to have so much luff that we can't fill it up deeper than 90 degrees.
As you can see from the photo below, we did a "in-race fix" with a knot which shortened the luff by 1+ feet.
From then on I would say we did went 135 degrees but no deeper as the chute will collapse (not sure if the main-sail played a part).
We also tried to go beam-reach with it and experienced violent shocks to the rigging as the luff curled and refilled repeatedly in a stiff breeze (8-10 knots forecast).
That's when we released all lines, dunked the chute, and struggled with the retrieval, in the process loosing 2 places in a 5-boat race.

2017_Tekong_Race_04.jpg

2017_Tekong_Race_06.jpg

2017_Tekong_Race_08.jpg

[Warren Nethercote]

Loh, one problem may be with the narrow shoulders. The same friend who made the genoas was given a very narrow-shouldered asymmetric spinnaker in exchange for some engineering work. We found the same problem: it could not be flown effectively, if at all, at deeper angles where it would be blanketed and collapse. The asymmetrics designed for deeper angles appear to have broad shoulders, apparently to expose the upper part of the sail and help keep it ahead of the rig.