I am making a sd with a modded gx 390 clone it will be turning roughly 5200 rpm I have been researching to see what prop gear combo I should run I have the option to run a 2:1 engine to prop ratio and have been thinking somewhere around a 9x7 prop for that setup does this sound about right?

1.3:1- 1.5:1- 1.63:1

Which ratio for which prop

1.5:1 with a 9x9

1.3:1 with a 10x8
Or 1.56:1 with a 9x9 or 9x8
Make sure you get a SURFACE DRIVE PROP

10X8 with a 1.3:1

I built one with a stock 16 duromax, 1.5:1 reduction and a 10x8 last summer that runs about 22 mph at 3900 rpms on a 1436 jon.
This is with the hill marine prop

Mag10

Thank you guys for your input. I am going to call backh2o tomorrow and inquire about props. It does not appear from their website that they sell a surface drive prop so I am going to call and double check as i have a new 8x4 that i was told by an employee there i could trade in for full credit on a different prop.. I also believe that i am goin to be purchasing their lower bearing assembly as I have had a bastard of a time trying to find a .75 id x 1.375 od bearing. I wanted this size because i had some 1 3/8 id dom i was going to make a housing out of and mcmaster has seals that would fit this application. In any event i was unable to find a bearing that had a high enough rpm rating so backh2o will be getting some business from me.


Once again thankyou for the valuable on ratios vs prop size