Many hours later
After a long day trying this and playing with that I have succeeded in making a projectile simulation. Would have been much quicker in interactive physics or Algodoo but using GeoGebra means you have to get into the mathematics much more. Now I know how to make sliders the sky is the limit (α = 90°, a = 0).
To run the animation click the "play" arrow bottom left. The animation will play every 3s until paused. To reset press the reset button.
One day later I've worked out how to add the image of the cannon and ship. I think it's a great way to learn about projectiles so my next mission is to write worksheets so that my students can build a simulation of their own.