If you are playing ERAS, the 30 gun frigate was technically "removed" because it is a Napoleonic era ship and too late for the time frame. Since ERAS was derived from GOF and that ship did exist in that older mod, ERAS ship list does have it, but was marked as "no encounter". However, it does technically still exist, but only very rarely will be encountered, because MK did want some of the removed ships to sometimes become available at sea for those that might want them from the older mod. However, there is no easy way to force an encounter to include one. The removed ships are also never available for sale in the shipyard. If you want to edit the Program\scripts\ShipsUtilites.c file, modify the SetShipyardStore function to add SHIP_30GunFrigate. The easiest way might be to simply replace SHIP_KETCH, or SHIP_WAR_TARTANE with SHIP_30GunFrigate in the file, then wait for one to appear in the shipyard.
Viewing post in White sails
You won't see the ship in your shipyards, because the code you edited is incorrect. From your earlier post:
iTest_ship = rand(17);
switch(iTest_ship) {
case 1: bAdd = true; nToAdd = SHIP_WAR_TARTANE; break;
...
case 17: bAdd = true; nToAdd = SHIP_BarcoCostero; break;
case 18: bAdd = true; nToAdd = SHIP_30GunFrigate; break;
}
iTest_ship = rand(21);
switch(iTest_ship) {
case 1: bAdd = true; nToAdd = SHIP_LUGGERVML; break;
...
case 22: bAdd = true; nToAdd = SHIP_30GunFrigate; break;
}
In the assignment to iTest_ship above, it will receive a randomly chosen value between 0 to 17, or 0 to 21. But then it investigates for the value in the following switch statements for a case of 18 and a case of 22, which are supposed to be true if the value for iTest_ship is either 18 or 22 in your additional lines. That will never happen.
You either have to change the rand(17) and rand(21) to rand(18) and rand(22), or simply replace one of the existing case lines with the ship type you want. Example: change SHIP_WAR_TARTANE and SHIP_LUGGERVML with SHIP_30GunFrigate. Then you will sometimes see the ship you want in the shipyard. The more case lines you replace, the higher the chance of seeing your desired ship.