Originally Posted by Maeda Toshiie, with Geezer57's edit
Rules of MTW naval combat:
1. Command stars make a difference.
2. The slowest ship in the stack dictates the speed of the entire stack.
3. The speed of the attacking stack must at least match the speed of the defending stack in order to have a 50% chance to initiate combat. If one of your fleets engages the enemy, all your other attacking fleets will initiate combat as well.
4. Random throw of dice makes a (big) difference in combat, but can be reduced by the effect of hugely overpowering stacks.