Jiblet design

No – not the guts of a chicken. The jiblets in a Split Junk Rig are the small jib sections in front of the mast. Since they are in clean air in front of the mast they should be more efficient than the main sections (mainlets?). hence they can provide a critical part of the drive of the sail. So their design is important.

I’ve been playing with some prototypes. The established thinking is that an angled shelf foot design is best, with the caveat that the actual sail shape will be different to the shape you think you’ll get. This tends to make me think ‘why?’ and try to understand how to get a predictable shape, when I should probably get on, make the thing and go sailing.

Ok – so first I tried a barrel cut jiblet. This was built into a panel with batten pockets and tapes to see how it all worked.

Barrel cut jiblet

This panel had the correct angle of incidence but very little camber. I couldn’t understand why – I checked the spreadsheet many times and added cross-checking but I couldn’t find any errors. So I made another panel with shelf-foot. This still used the same offsets from the spreadsheet, but should have a better shape due to the 3D effect of the panel joints.

Making shelf-foot panel

Sticking the panel to tubes with tape showed that this still had the same problem – poor shape at the luff / leading edge. Eventually it dawned on me why:

  1. The extra bit added in at the leading edge is small. Making this a shelf-foot doesn’t make any difference – there isn’t enough material or curvature at the edge of the sail to matter.
  2. The front edge holds the sail in. To get a full leading edge I need to add extra material on the leading edge at the centre of the sail.

A quick bit of spreadsheet work & I estimated that the extra needed in the leading edge of this panel was around 54mm. This should provide all the extra needed back to the point of maximum camber. I made up a panel to add this and sewed it on.

Curved edge of luff strip sewn to straight edge of panel

The jiblet now takes up its expected camber. The leading edge / luff is probably a bit too full and doesn’t need all that extra material so I’ll scale it back a bit. The sides can be barrel cut which will make the jiblet easier to construct.

Ideally the jiblet would have broadseam to provide the full 3D shape over the length of the panel, but I don’t think this is going to happen for this sail. Maybe the next one…

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