I have a ST 75 that I put on my tiger 60. What is the best prop size for this engine? Towers web site says a 12-6 or a 13-6. I have a 12-6 on it now but the plane has never been in the air. what do you think is the best prop is for this set up?

The recommended props are a good place to start. From there, if you need more speed or more pulling power, then you can change the prop to get what you need, but if a 12-6 is recommended, then it will at least get you in the air and you can adjust your prop from there.

You can check out page 14 of the manual:

http://manuals.hobbico.com/sup/sup-manual-v1_1.pdf

The recommended break-in props are 12x7 or 12x8, and the manual says the recommended range is 11x8~11x9 or 12x7~12x8. Based on this information, I think the 12x6 will be fine for break-in. You might play around with a 12x8 or 13x6 if you want more power after you get a gallon or so of fuel through your G-75.

http://www.rcuniverse.com/community/...section=models

I believe I am using a MAS 12x6 or 12x8

The manual shows a range from 11 x 8 to 12 x 8, calculating the load of a 12 x 8 I can figure it out that a 13 x 6 can easily be handled by your engine.
Good luck!!

I flew one for years with a MA 13x6. Perfect combo.

David