Hello,
El Jinete ...
As you have mentioned, the Playmobil cannon released in the US and Canada (except for "Big Bertha") are "impotent" ... They have no springs and cannot shoot a projectile at a Playmobil ship or Klicky ...

The size of the springs for the ship's cannon and the field gun are slighty different. Sometimes you can find a bag with an assortment of small springs in Lowes or Home Depot. There are also some ball point pens that have exactly the correct size springs.
When I lived in Houston, I would go to Berings (which is probably the most marvellous hardware store in the world) and buy the springs that I wanted. If you can find a really good independent hardware store similar to Berings, you can even get different strength springs to make your cannon as powerful as you want them to be !!!
Here's a picture of my favourite Playmobil fieldpiece and the parts list ...


As you have no doubt noticed, there is no part number for the spring.
All the best,
Richard