the Java compiler won't detect at game start if you mistyped a ship class, so I would first verify (and then verify again) that you typed correctly all your additions. Also, the log file generated in case of crash is very informative (enable dev mod, I don't know if it is as informative if dev mod is not on).
Then, if ships don't appear, check your probabilities, there is a random used for that for each convoy and most of the time, you have 60%+ of getting fighters, not ships.
If you want to modify the probability for a given ship to appear, you can duplicate or triplicate its entry also, like
private static String [] ships =
{
"sentinel_Hull",
"buffalob_Hull",
"manta_Hull",
"crow_Hull",
"crow_Hull",
"cobra_Hull",
"lance_Hull",
};