(ii) Interest. The cost of debt shall be determined by applying each vessel owner's actual interest rates to the outstanding vessel indebtedness. MARAD shall assume that original vessel indebtedness is 75 percent of the owners' capitalized vessel costs, including capitalized improvements, and that annual principal payments are made in equal installments over the economic life of the vessels as determined in accordance with paragraph (b)(2)(i) of this section. Where an operator uses a variable interest rate, the operator's actual interest rate at the time of calculation of the average capital cost component shall be used. The ten-year Treasury bill (T-bill) rate plus one percent on the first business day of the year or the first business day on or after July 1 shall be used for operators without vessel debt and when the actual rate is unavailable.