The harpoon cannon does 3 points damage.
Harpoon cannons can come in handy when defeating Krakens durring a Kraken Attack when you want to harvest Crystals. Spread them around your base (4-6 should be enough for a small base) before collecting the Crystal. After you have mined the Crystal ore and are at your base, get ready with some iron to start repairing all the damage the Krakens do. You can leave the heavy work to the harpoon cannons as they automaticaly attack Krakens. Similarly, Droids will be of use as they often kill at least one of the Krakens that chase you when you break the Crystals back to your base, lightening the pressure on the harpoon cannons.