When I use mine I follow simple rules.
1. Must be either in line of sight of its target or have a ship which is that can see the target and signal the fall of shot.
If in sight line may fire every three cards, if acting on signals every four.
2. Must range in its shot. throw a dice 6 is a hit, 5,4 over and 3 or two is under. One is a miss fire and the Ketch takes an A chit for premature detonation.
Once ranged in, a hit is scored each round unless the Ketch is moved in any way. If so the ranging process must begin again.
3. Hits scored result in two A and two B chits damage. Any special damage also results in a fire.

4. Max range is two rulers. May not fire at targets under half a ruler distant.

5. The Ketch may be moved on a spring to change its direction of fire. If so rule two applies. To move on a spring use the same rule as for Deft Captain.

Feel free to use this as the basis for your own rules as you see fit.

Rob.