This paper proposes a semi-empirical model to predict a ship\'s speed loss at arbitrary wave heading. In the model, the formulas that estimate a ship\'s added resistance due to waves attacking from different heading angles have been further developed. A correction factor is proposed to consider the nonlinear effect due to large waves in power estimation. The formulas are developed and verified by model tests of 5 ships in regular waves with various heading angles. The full-scale measurements from three different types of ships, i.e., a PCTC, a container ship, and a chemical tanker, are used to validate the proposed model for speed loss prediction in irregular waves. The effect of the improved model for speed loss prediction on a ship\'s voyage optimization is also investigated. The results indicate that a ship\'s voyage optimization solutions can be significantly affected by the prediction accuracy of speed loss caused by waves.