From what little I've read on contra-rotating props you're generally looking at a +10% boost compared to a single prop of the same size, so if your single prop would get you 30Kn, you're probably looking at 31.5Kn on a contra-rotating prop. Also, as per Ezgo, I'd recommend a line to separate the parts of the prop - it looks a little like a single casting at the moment.
do note that ship resistance (and especially not wave resistance!) is not a linear formula. a 10% boost in power does not give an 10% boost in speed. for 35 knots you will need at least 136% of the original power (keeping wave resistance as quadratic too, wave resistance gets exponential in reality which is why there is an at least there, so most likely 150% or more is not out of the question) which is keeping into account that the propulsion train and hull shape are updated for the higher speeds. (adding to that, if you go over hull speed, you will need an even larger amount of power)
the shape of the propeller blades is also different between 2 contra- rotating props, because the second one works in a more turbulent, higher speed waterstream then the first. do note here that I think you are forced to fit an fixed propeller here instead of the controllable pitch propeller that is on most warships, because there is no space to really turn the propeller blades without hitting the second one.
I am wondering what the power levels would be for this setup. you basically fit 2 propellers of the same size as the original one on the same powerplant, so while more efficient, will the propellers still work? or should 2 propellers be powered by twice the powerplant?