Yak, looks fantastic -- what a brilliant idea! How does the helm feel with both sails? Is it possible to reef the main with the jib up, once set? I'm guessing not. Does it affect the attitude of the bow at all?
Already having the jib sail - it was a pretty cheap project!
If you loosen the uphaul line you can reef in the main sail and the uphaul line gets wrapped around the sail. Useful for a emergency way to reef the sail but cumbersome. If you don't loosen the uphaul enough you can't reef in all the way - too much and you have a sloppy/luffing jib.
Jib uphaul lines are designed to be deployed after main is all unrolled; and clipped back in place (jib removed) before proper furling.
The cure for that would be a small rotating plate fixed to the top of the mast - like a small lazy susan. The mast would then rotate under the plate - leaving the jib attched and the main to be rolled up.
Sailing upwind in heavy air caused the jib to be difficult in "picking a side". The bungie at the bottom might require doubleing up, shortening, or removing entirely (too much stretch). Perhaps also the angle of the line out to the ama handle was too great. Lighter air upwind was not so much of a problem.
Jib definitely added some additional "get up an go". Turning boat around was interesting, you could easily change sides of the jib, but the extra sail made the rudder feel "small". Probable just need more time getting used to making turns with the jib. Helm felt fine otherwise, once under sail.
With the wind behind me on a broad reach or running it was great! The jib's bungie allowed the sail to nicely cup infront of the boat in almost spinnaker fashion. Fastest being main on one side jib on the other.
Normally downwind with just the main sail the nose is a little low, in fact it will submarine at times. With the jib the nose seemed to be higher and at least on my test runs did not submarine at all.
My wind for tests were 5 - 12mph with maybe a gust or two to 15mph.
Because the main mast is flexible you could see the jib trim change slightly as you changed the main sheet trim. If you tightened the main sheet you would pull back on the jib's uphaul line (as the mast bent towards the stern). But I saw no adverse affect on main's sail shape or ability to trim properly. The fact that main sheet line pulls in the opposite direction to the jib uphaul should give jib support. On downwind sailing the main sheet line is very loose, allowing the jib uphaul to become looser, allowing it to cup nicely.
When first attaching and setting the jib uphaul I was careful not to apply too much pressure so as to bend the mast, but enough pressure to make the bungie and uphaul line taught.
I don't believe it wise to use the jib in anything over 15mph, but maybe a Hobie tester/engineer might want to calculate & experiment with how much stress the mast will take.
Hobie will continue to make it's small kayak sail, so they have a ready supply of jibs for the Island if they want to fabricate a proper Island jib kit. The small kayak sail comes in red, yellow, white or blue. Looks like it belongs on the Island
PS I edited my rope lengths above.