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.
The blog has moved to WordPress.com. I was running WordPress on Apache on Debian Linux at Bytemark, but it was taking too much effort to keep everything up to date. It is cheaper and more secure to move it to WordPress.com.
It is certainly an easy migration – very smooth and automatic.
Now I should have more time for sorting out boats!
With the mast nearing completion it is time to finalise the sail plan. I’ve been using a scaled-down copy of the Poppy plan for the preliminary design. I’ve now followed through to make sure everything works ok.
As recommended I built a string and stick model of the sail. This is the first attempt – a copy of the Poppy rig. With old garden bamboo it looks pleasingly authentic!
I thought I’d describe my current junk rigged boat and show some of the design features. The boat is called Owl because it is green (as per The Owl and the Pussycat).
The boat is a 11′ GRP clinker-style boat originally built in the early 1980s for a gentleman to take his kids out in. I suspect it was adapted from a standard hull – probably designed for rowing or fishing. It had:
A flat centreboard that is a bit far aft;
A rudder that only just touched the water;
A standing lug rig that wouldn’t go upwind, with heavy yard and boom that reduced stability and threatened to hit heads.
However it did have a free-standing mast and, without the lug-rig up, was extremely stable. A perfect candidate for a junk rig!