def _add_vessel_class(df):
    """Creates 'Class' column based on vessel LOA ft."""
    df.loc[:, "Class"] = "Panamax"
    post_row = (df.loc[:, "LOA ft"] > 965)
    post_loc = df.loc[post_row, :].index
    post_pan = df.index.isin(post_loc)
    df.loc[post_pan, "Class"] = "Post-Panamax"
    return df