The Graupner 600 motor can run hot especially under load so I would suggest something like a brass 35mm 3 blade prop would be a good starting point. if you have or can borrow a Wattmeter this would help measure the current whilst holding the model in the water. There are different variants of the motor so you need to Google your actual motor details to get the correct specs.
If you have a selection of props then you could just try different ones on short runs to see the performance and check the temperature.
I do believe you corresponded with Robert on this site regarding his NoStar set up, perhaps he could advise which prop size he used.