4 blade prop for 70 horse Johnson

[marcusknight]

I am wanting to try a four blade prop. I have a 1979 70 Johnson. What size 4 blade prop for this?

 

[batman99]

What size of 3-blade prop do you use today? Is that prop's pitch properly matched for its WOT RPMs?

If 3-blade is optimal for your current setup, then "rule of thumb" is to go 1 less pitch for a 4 blade replacement.

However...

Some prop brands have a slight cup on their 4 blades. Thus, best to go -2 pitch lower. For example, I find solars aluminum brand has a slight cup on their 4 blade design. Thus, I went -2 pitch when going from 3 blade to 4 blade (when using a 4-blade solars amita aluminum prop). re: 17 3-bade down to 15 4-blade - for my Johnson 115 V4.

Hope this helps....

 

[steelespike]

Your present prop size should be near the base of the blades or under the prop nut.
If you get us the prop size and the wot rpm and gps speed we can help tell how your setup works.