Hi, I am defining a sail geometry using the SailModelfile.js and the sail topology parameters. Just to be sure and precise, are the length data measurement corresponding to straight lines measures or curvilinear measures?

In other words, is LFoot the straight line dimension from tack to clew, or the actual developed (curvilinear) length of the foot edge of the sail? Same question for LLuff.

Also, how is defined the position where to measure r25 r50 r100? Is it:

- r25 is the positioned as the line cutting through the points on 25% of the luff and leech?
- r25 is the positioned as the line cutting through 25% along either luff or leech and is parallel to the foot or head?
- PS: these definition are equivalent or close when having a triangle or close to triangle shape, but may results in some differences when considering a more squared shape (r100 >> 0) such as for a squared head mainsail.
- Once the position is defined, is the r25 ratio numerator (r25*LFoot) using the direct straight line measure from point on luff to point on leech at that stripe location, or the developed curvilinear dimension (developed) at that stripe location?

Thank you.

Hi Nicolas,

Thanks for your question.

First, please be aware that the definitions of the sail dimensions are written and available in the sail preset file in Runtime/SailModel/Presets and therefore vary on the selected sail preset (e.g. GenericSloopMainJib.js or TwoElementWing.js). The sail preset files use functions written in Runtime/SailModel/SST.js. You can write your own sail preset and the SST.js if you want it to function any differently.

Then, please find below the answers to your questions in the particular case of the GenericSloopMainJib.js preset.

- For the jib, ‘Jib.LLuff’ is the straight-line length between the headstay base and the headstay top, minus ‘LLuffFoot’.
- For the main, Main.LLuff + Main.LLuffFoot = Mast.height, and ‘Mast.height’ is the mast length before any rake is applied. Then the camber for the mast is applied afterwards as a fore-aft offset (for an unraked mast), so ‘Main.LLuff’, ‘Main.LLuffFoot’ and ‘Mast.height’ are all straight-line distances.
- ‘LFoot’, both for the jib and the main, is equal to the girth length of the foot edge of the sail, or in other words the actual developed (curvilinear) length of the foot edge of the sail.

‘LFoot’ is also used in sailSimpleSection’ of the SST.js class to define the sail section of the foot edge when v = 0 (where v is the curvilinear abscissa of the mast trailing edge reference line / headstay starting at the foot of the sail), along with other parameters:- the coordinates of the point verifying v = 0
- the camber of the sail at 40% draft (this is specified in the BOC/BIC files using ‘rCamberM’ and ‘rCamberJ’)
- the angle aTrim between the sail section leading edge tangent, projected in the (OXZ) plane and the x-axis, defined by aFoot
- the angle aChord between the sail section leading edge tangent, projected in the (OXY) plane and the x-axis, defined by aBoom and aTwist (defined at the BOC/BIC level).

- r25 (resp. r50, r75 and r100) corresponds to the girth length (or developed curvilinear dimension) of the sail section with leading edge positioned on the mast trailing edge reference line / headstay, with h = 0.25*Mast.LLuff (resp. h = 0.50*Mast.LLuff, etc.).
- for each section, the direction of the section, and therefore the position at which its trailing edge will be located on the leech, is defined by aTrim, where aTrim = 1/Mast.LLuff ( (1 – h) S.aFoot + h * S.aHead)