Not all ships are available for sale in shipyards; some ships can only be obtained through encounters at sea.
To make all ships have just one sail, edit the function ref procGetSailTextureData(), in file Program\battle_interface\BattleInterface.c. Set it to the texture you want for all ships (probably silk based on your description), geraldTex blank:
BI_objRetValue.normalTex = "ships\parus_silk.tga.tx";
BI_objRetValue.geraldTex = "";
BI_objRetValue.sailscolor = argb(255,255,255,255);
return &BI_objRetValue;