The standard propeller for the 1003 is the is two bladed version part number 1917-00. The two bladed propeller is the best choice for most users of a 1003, and gives higher efficiency when used on a typical small sailing boat or tender for a yacht.
There is also a three bladed propeller available and this is the original Torqeedo propeller design that was used on the earliest models such as the 801, the early Cruise 2, and used to be standard with the 503, which has a slower turning drive-shaft than the 1003.
For certain easily driven vessels, such as long narrow canoes or kayaks, or very light multihulls, the three bladed propeller can give a higher top speed at full throttle. If you find that when running at full throttle the power used by the motor is only 800 or 900 watts, that is much less than its full power rating of 1000 watts, then you know that it is reaching full RPM. (This is like red-lining in a car, and you could do with a higher gear ratio.) For this kind of boat you could think of the three bladed propeller like an over drive gear — with the three bladed propeller easily driven hulls can reach a higher top speed. If this is important to you the three bladed propeller, part number 1901-00
For most users, choose the stnadard two bladed propeller.