I'd suggets to group the propeller with itself to create a new transform node to it. Then parent the group (not the propeller shape) to the submarine. This will allow to transform the propeller freely while it still moves with the sub. Then create the expression to the propeller's (not the group) rotation. For example:

propeller.rotationZ = time * (some value);

I used rotationZ just for an example. Your may vary...