As so many others, I discovered that on a broad reach or running, the performance of the sail improves almost dramatically when I pushed the sail out (with my arm or with the paddle). A much better solution of course is the so called "boombat" (http://www.hobiecat.com/community/viewtopic.php?t=7158&postdays=0&postorder=asc&start=0
So I also wanted to make my own boombat, but I couldn't find an apt "batten" (that is to say: I couldn't find it quick enough and I was impatient); and I didn't really like the fact that I had to make a small hole in the hem of the sail, although that is quite harmless. Maybe I like booms (for a mainsail) since I am used to it in sailing. Anyhow, I thought it could work, so I made a boom instead.
I wanted the boom to be lightweight (otherwise it will hammer the head rather unpleasantly or even dangerously in a unexpected gybe). Also it should still be possible to have a relatively flat shape of the sail when close hauled, and a more rounded shape for a broad reach or running.
Luckily, all these objectives were easy to achieve.
I took two pvc tubes, one 5/8 inch and one 3/4 inch. These fit in each other.
To stiffen it, I did a bamboo inside. I made the length of the boom a bit longer than the foot of the sail to be able to experiment with the trim. This makes for still rather flexible boom, but for the application it is strong and stiff enough, cheap and very lightweight.
I drilled holes in the ends. With a small rope I have it permanently attached to the sail at the mast-side of the sail. With a rvs karabiner it can be attached to the sheet eye (I hope that is the right term). I can experiment with the trim by adapting the length of the white line (see pic)
The sheet is directly connected to the sail, exactly as is the case without a boom. So the boom is attached to the sail as an extra. The effect is that the sail is sheeted as normal when close hauled. Only when "going broader than a reach" the boom starts acting. This setup works better than with the sheet connected to the boom and the boom to the sail. For then the sail would be too loose close hauled or too flat on a broad reach and the forces on the boom would be bigger, so it should be made stronger (means heavier).
This is how it looks like on my oasis:
By releasing the karabiner and pointing the boom upward along the mast, the sail can easily be furled around it. Furling the sail is even easyer than it was, since I have more grip now.
I find it very convenient that I always have it with me when I have the sail (nothing to forget or loose). Although it is a minor advantage, I like the fact that I can grab the boom easily to gybe in a controlled way. Also: the shape of the sail is not really dependent on the strength of the wind. However, the boom doesn't change the fact that the rear deck of the oasis is a bit short, so that the sheet is angled down a bit too much. But still I'm quite satisfied with the shape of my sail now (also because of the stays).
From the reports here I'm sure the boombat works also very well and it has quite a few other great advantages (even simpler, lighter, stops noisy sail-flapping even better). But I've come to believe that a boom can be a good alternative. In the right winds, my oasis really likes to run now, so I guess I'll stick to it!