Measuring mechanical propulsive power includes establishing a first power and a second power. The first power is associated with a first rate of energy conversion by a device in a fluid, where the device is in the wake of a self-propelled conveyance moving with respect to the fluid. The average distance between the device and self-propelled conveyance may remain substantially unchanged. The second power is associated with a second rate of energy conversion by the device in the fluid without the self-propelled conveyance, where the other physical factors are substantially similar to that associated with the first power. The difference between the first power and the second power is calculated. The mechanical propulsive power for the self-propelled conveyance is determined using mappings, where a mapping associates the difference with the mechanical propulsive power.