Our average number of operating vessels increased to 27 for the three months ended September 30, 2021, from 25.5 for the three months ended September 30, 2020.
We had four product tankers employed under time charters as at September 30, 2021 compared with none as at September 30, 2020. Revenue days derived from time charters were 362 for the three months ended September 30, 2021, as compared to none for the three months ended September 30, 2020. The increase in revenue days for time-chartered vessels resulted in an increase in revenue of $5.0 million.
We had 2,024 spot revenue days for the three months ended September 30, 2021, as compared to 2,334 for the three months ended September 30, 2020. We had 23 and 26 vessels employed directly in the spot market as at September 30, 2021 and 2020, respectively. The decrease in spot revenue days resulted in a decrease in revenue of $6.0 million, while changes in spot rates resulted in an increase in revenue of $2.8 million for the three months ended September 30, 2021 as compared to the three months ended September 30, 2020.
Voyage Expenses. Voyage expenses were $23.1 million for the three months ended September 30, 2021, an increase of $6.3 million from $16.8 million for the three months ended September 30, 2020. Voyage expenses increased primarily due to the increase in bunker prices resulting in an increase of $8.5 million, offset by a decrease in spot revenue days of $2.2 million for the three months ended September 30, 2021, as compared to the three months ended September 30, 2020.
TCE Rate. The average TCE rate for our fleet was $10,319 per day for the three months ended September 30, 2021, a decrease of $2,113 per day from $12,432 per day for the three months ended September 30, 2020. The decrease in average TCE rate was the result of lower spot rates for the three months ended September 30, 2021 as compared to the three months ended September 30, 2020. TCE rates represent net revenues (or revenue less voyage expenses) divided by revenue days, as further defined above.
Vessel Operating Expenses. Vessel operating expenses were $15.5 million for the three months ended September 30, 2021, a decrease of $0.6 million from $16.1 million for the three months ended September 30, 2020. This decrease is due to the timing of vessel operating expenses between quarters. Vessel operating expenses, by their nature, are prone to fluctuations between periods. Average fleet operating expenses per day, including technical management fees, were $6,373 per vessel for the three months ended September 30, 2021, as compared to $6,714 per vessel for the three months ended September 30, 2020. The timing of crew changes as a result of COVID-19 related delays in 2020 was a contributing factor in the difference for the three months ended September 30, 2021 when compared with September 30, 2020.
Charter Hire Costs. Charter hire costs were $2.3 million for the three months ended September 30, 2021. We chartered-in one vessel in September 2020 and another in June 2021.
Depreciation. Depreciation expense for the three months ended September 30, 2021, was $8.0 million, consistent with $8.1 million for the three months ended September 30, 2020.
Amortization of Deferred Drydock Expenditures. Amortization of deferred drydock expenditures for the three months ended September 30, 2021 was $1.1 million, a decrease of $0.6 million from $1.7 million for the three months ended September 30, 2020. The deferred costs of drydockings for a given vessel are amortized on a straight-line basis to the next scheduled drydocking of the vessel.
General and Administrative Expenses: Corporate. Corporate-related general and administrative expenses for the three months ended September 30, 2021 were $4.3 million, an increase of $0.2 million from $4.1 million for the three months ended September 30, 2020.
General and Administrative Expenses: Commercial and Chartering. Commercial and chartering expenses are the expenses attributable to our chartering and commercial operations departments in connection with our spot trading activities. Commercial and chartering expenses were $0.8 million for the three months ended September 30, 2021 consistent with $0.8 million for the three months ended September 30, 2020.