Hi that is a good question! I use plastic props to find out how the boat performs, I mean in a realistic fashion not how fast. Plastic props are relatively cheap and a good way to work out what you want from the model and then if brass is your choice a similar brass one can be obtained.
The standard 2 blade plastic props are designed with a certain ratio of prop diameter to pitch. I think the coarse pitch ones are 1.7 : 1 and the less so 1.3 : 1, don't quote me!
You will find that Prop shop and also Deans supply brass props and you should get an equivalent without any problem.
Depending on what type of boat you have fast boats have small fast turning ones and the more sedate liners and cargo boats have bigger but slower turning ones. The slow turning props are a little more efficient but this is not very noticeable in models. I have a couple of slow boats and you can watch the large prop turn and propel the ship as well.
My criteria for models is can I stop it quickly as you will find you are surrounded by idiot drivers! Radio assisted collisions happen more often than they should and operators of boats can be distracted by watchers, many of whom will try and stand between the model and yourself!
So be prepared, the more experienced modeller will await your return to the bank before telling you what a wonderful creation you are driving.
Good luck, survival is the watchword.