Studies of ship strikes on whales often focus on large vessels (>20 m), with attention to their speeds and the resulting risk of lethality. Smaller coastal vessels also co-occur with whales, resulting in collisions that merit study. To cast light on injuries caused by vessels of all sizes, we used knowledge of right whale anatomy and Newtonian mechanics to construct simple models that predict the mechanical stresses experienced by whales during collisions. By comparing our predictions with published models and with data from ship strikes on various whale species, we developed a model for lethal injury as a function of several vessel and whale properties, finding that collisions that create stresses in excess of 0.241 MPa were likely to cause lethal injuries to large whales. Furthermore, this model has revealed that (1) vessels of all sizes can yield stresses higher than this critical level, and (2) large vessels produce stresses much larger than this even when travelling at reduced speeds (i.e., 10 knots). The model is fast enough to power an interactive GUI-based tool (in R) and flexible enough to simulate strikes by vessels of different masses and speeds upon whales of different species, sizes, and physical conditions.