Need correct Prop for my 70hp Yamaha on a G3 17/56 SC Jon Boat

[Muddybayou]

I need advice on which SS Pro I should get for my boat. 70hp Yamaha on a 17/56SC G3 Jon Boat. I do not have a Tach on it. So I do not know what my RPM's are running. However I do know there is no difference in speed between 3/4 and full throttle. I'm running 27 mph by my app on my phone. Currently, my motor has a 13.5 x 12" Aluminum prop. I know I need to install a Tach, but was wondering if any of you guys and ladies, could offer any info. Also, I think I would like a 4 blade prop.
Any suggestions? My boat weighs in at 850lb (dry) and right around 1500lb with motor, fuel and people.

Thanks
Muddybayou

 

[WesNewell]

With a properly mounted and running engine and prop, you should be getting close to 45 mph with that setup. But, without knowing the GR of your engine it's hard to give you a prop pitch. Try the mercury or yamaha prop selector.
https://www.mercurymarine.com/en/us/propellers/selector/#!/step-one
http://www.yamahapropselector.com/propSelector/web/
I will say that a 12P prop is probably way to little pitch. I run a 14P on my heavier (995#) same style boat with a 60hp merc 4s with a 1.83gr.and get ~40 mph wot.

 

[Sea Rider]

Could be extremely dangerous for motor to play blindly with props without a tach, install one before venturing playing with props and check current one which max wot rpm achieved as loaded, then go from there...

The motor will definitely rev higher at full throttle than at 3/4, no matter if the speed remains same...

Happy Boating

 

[Muddybayou]

Thanks for the info. Yeah, I figured I should get at least 40mph out of it. If not more. A Tach is on my to do list for sure.
I'll be installing one very soon.
Thanks again.