Birds and Fish. Assuming the predator can easily catch the prey (true) and that the predator does not migrate to follow the flock or school (true), the advantage of the group is that the predators will only eat a fixed amount. When the group is not present, the predator with switch to a different food source (work, laze about) until the next group comes along. Overall, the amount of predatation is less because the food source is not available to the predator all of the time. In other words, the pirates will still hit the herd, but only 1-2 boats and the rest will pass safely, reducing the pirate's over all opportunity.
Distraction. Interestingly, while some predators will charge into the flock, thrashing about in a target rich environment, most would rather pick off stragglers. It is theorized there are 2 reasons. First, it actually hard to track a single individual in the mele'. Perhaps this would be true in a convoy as well, if boats purposefully changed courses. Second, and more to the point, the straggler is perceived to be easier to catch. Moral: if you are in a group, don't be that straggler, be the guy with the fast boat and buddy-up with someone. And while it might seems loyal to wait for the straggler, World War II convoy strategy dictated that stragglers got left behind; loyalty only meant more time for the group in the danger zone.
Visibility. What is more stealthy, and individual or a herd? Unless the pirates are using good RADAR, a single boat is hard to spot. And for goodness sake, turn your RADAR off. Many rodents are nocturnal, preferring stealth to heard behavior. And they have been around a long time.
Defense. Some animals use a defensive strategy. In the case of bison, the males are impressive, not built for running away, and are a match for whatever they may meet, so long as they stick together. But when guns are involved, cruisers can win this.
The second type of defense involves sacrifice. Insects and even humans have the ability and even urge to taken on foes they cannot hope to best or drive off in defense of kin, even if only to buy time for escape. If you are by nature a protector, this is a danger. You will feel obliged to defend even when you have little to offer and may put yourself in harms way.
The best strategy, other than flying commercial air?
- Do the pirates use RADAR? If not, going outside of the shipping lane and running dark should help.
- A group may help as well. The pirates may be intimidated. Perhaps someone in the group is armed (they don't know and it is hard to control a group). I would think 4 boats staying tight is the minimum.