Sail twisting

Hi everyone,
I’m new to Grasshopper and trying to simply twist and open a sail.
Has anyone done this before ? Or could give me a hint !
Many thanks

Hi Juliette,
Do you mean like “Furl” and “Unfurl” a sail ?

I’ve used GH to draw sails before but you’ll have to give me a better hint about what you want?

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Sorry, yes i mean furl and unfurl the sail!

Ha! I was think twist in the sail design sense.
Here’s a shot showing of LOTS of twist. Blowing hard at the 2019 505 Worlds in Fremantle.
I made these sails using Grasshopper.


This is a first draft of a furling system where the angled furler rotates and grows in diameter. A cylinder represents the combination of furling hardware (‘Min_R (inches)’ = 0.75) plus the sail thickness as it wraps, determined by ‘Max_R (inches)’ = 2 and ‘Max_Turns’ = 40. All of these values are wild guesses. The ‘Furled (feet)’ panel shows the length of a string (or LP dimension of a jib) wrapped around the growing cylinder. (21.1 KB)

Sorry I thought I confused twist and furl but I was meaning TWIST the sail.

More precisely, I have a sail, and I just want to have different deformations of it, so I would like to write a script to deform the sail (for every opening angle, have different twisting angles) !
Hope thats clearer !!

By twist you maybe mean: mimic some sort of variable deformation most related with the upper part of the sail due to a loose leech ? (a classic “feature” in windsurf race sails - where downhaul controls how much the sail depowers (i.e. top “opens”) in a gust).


A problem well stated is a problem half-solved.
Charles Kettering
Head of research at General Motors from 1920 to 1947

Sounds to me like you are still a long way from stating the problem clearly.

Well sorry english is not my first language.
Is my second message not clear ?

Hi Juliette,

Pictures can transcend language, so a sketch or two might help, particularly if annotated with ‘opening angle’ and ‘twist’. You can upload images by dragging them into your post.

In the meantime try a forum search for ‘kangaroo sail’. It will bring up several hits, including this: Sail for a boat using Kangaroo - Grasshopper / Kangaroo - McNeel Forum


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I don´t think so.
If we are talking about a real sail design there are a lot of factors that influence twist.
Depending a lot to the particular design of the boat.
For the above shown 505 mainsail that could be:

wind force
mast stiffness fore-aft and sideways and taper of top section
sail layout ( cross cut, vertical) sail material and stretch in different directions
sail battens - layout, length and pretension
boom vang tension
cunningham tension
shroud tension
spreader angle and length
mast ram position

and some more…

what opening angle would have to do with twist is not clear to me.

If we are talking about windsurf rigs or catamarans the parameters are different.
It would be good if you could tell us a bit more about your problem…


None of your three posts describing the issue are very clear to me, and you have posted no geometry or images. The sails I posted above are only artistic approximations, not precise models as @JoergH described. Even so, they are somewhat complex, depending in part on fixed geometry (the headstay) and in part on sliders that define the clew position and shape of foot and leech edges.

Here is a second draft, perhaps more accurate? Same cylinder but a spiral of points is created, eight points per rotation, whose length determines ‘Furled (feet)’. (18.4 KB)

Sorry if this wasn’t clear then… I’ll try again.

I want to work on a simple sail.

By opening the sail : I want to create the angle of the sail created by the traveller track (in orange on the picture). I want to be able to choose this angle.
For every “opening” angle (so every position of the sail on the traveller track), I want to twist the sail : I want to recreate the color shape on the picture.


Your image shows a mainsail with battens. The boom appears to be roughly the same angle from centerline for all three traveler positions? Where is the wind coming from? (True Wind Angle relative to the boat?) I see in your notes that you expect batten angles will correlate to sail twist but I don’t know what the percentage numbers mean? I wouldn’t expect the boom to remain level in all three positions and am not sure your traveler positions make sense?

A mainsail will twist without battens or traveler (with mainsheet fixed on the centerline).

Get a (very) simple guideline then (I do hope that you know what K2 is - the most important GH add-on by a million miles).

  1. Forget battens (for simplicity) - but wait: a sail without them is kinda riding a Harley Davidson (i.e. pointless).

  2. Create your Surface parametrically (meaning boom var angle etc etc) and then do a “controlled” Mesh where the diagonals (shown below) may serve as guides for downhaul forces: i.e. for a given segment (Edge) the Unary force (downhaul) is the unitized (end-start) Vector * factor * hightFactor. This means that Forces vary (per Edge) and the sail is more “stiff” in the lower area. That said the new race hydros that run always at 1000% … well … they start to exploit leech depower options because there’s no reaction time in a gust (but if electronics take control … blah, blah).

  1. Separate the inner (clothed) Edges from the outer (naked) ones, create four (or three) different “classes” for the latter and apply Spring Forces accordingly. That said you can try to mimic Outhaul as well (but that’s a bit complex). Remember: K2 is an “optimizer” so if you input bananas … bananas is what you’ll get.

  2. Apply variable (rotated) Unary Forces for the wind. Then try to relax the Mesh.

But what all the above have to do with real-life and real-life sail design … is rather unclear to me.