The 'trasher' as you call it (maybe grinder would be better), has no reload, can be used all the time, and adds a set amount of impact damage upon coming within a few meters range of an enemy. Think of it like a gatling gun with a 3 meter range. Pull the secondary trigger and it keeps going. However, since is it also the winch, you can't fire the harpoons while the grinder is on. The grinder is automatically on after harpoons are fired since it reels them back in.
Ramming a ship would cause the set amount of grinder damage (if it is on) plus the current inertial damage. Moonshining with a Goldfish into a harpooned enemy would cause some pretty serious damage. It is a pretty high-risk gun, though. Long to mid-range damage and ability is minimal.
Just lightly bumping a ship would cause just the grinder damage as long as it is activated.
Since the grinder is secondary, all ammos would effect the harpoons. In think it would just be a single shot weapon with five harpoons at the same time.