Getting the sail finished was a bit frustrating but I got it done. The main cause of frustration was that I needed to have the boat ready for a family holiday in August and there wouldn’t be any time to test it unless I got it all done last week. However, it all got done in the end — phew!
I did have an oh hell I’ve got to redo it moment when I saw the angle of the jiblets didn’t match up at all with the angle of the mainlets. Eventually I realised this was due to the angle of incidence of the jiblets making them appear wider at the leach. Phew – happiness restored!
I’ve been continuing my experiements on the jiblets.
Leading edge angle
The first issue was the approach angle of the leading edge. The symmetrical aerofoil has a leading edge angle of 90º. This means air will not flow round the leading edge of a sail with this shape.
The larger the angle the more forward thrust we’ll get from the sail but the lower the angle we can go into wind (larger tacking angle). Conversely a smaller leading edge angle will improve the angle we can go into wind but will reduce thrust from the sail. Ideally we’d optimise the angle for Velocity Made Good (VMG) given the performance and drag of the boat, but we just don’t have enough information so a reasonable guess will have to do.
The hard-sheeted jib angle on my hull (Wanderer dinghy) is approx 12º. To allow air to get past the mast, plus allow for the extra drag of a junk rig I’m assuming that the jiblet angle of incidence will be around 15º. This means that the maximum leading edge angle should be around 30º (45º – 15º = 30º).
Playing on airfoiltools.com I came up with this shape that seems to look about right – and something like a sail.
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.