Pat I would say that neither DA, Taurus, or posters to this thread is spelling it correctly:

prop-An object placed beneath or against a structure to keep it from falling or shaking; a support, One that serves as a means of support or assistance, A theatrical property

The correct spelling:

Propeller-(synonym propellor)-a mechanical device that rotates to push against air or water, A machine for propelling an aircraft or boat, consisting of a power-driven shaft with radiating blades that are placed so as to thrust air or water in a desired direction when spinning.