I have built a surface drive with a predator 13.5. I purchased a hill marine 8x4 and started with a 1 to 1 ratio. It just didn't perform like I wanted maybe 12 mph I'm pushing a 1648 I know I need more hp but for now my budget only allows me to have this set up. I have since reduced my ratio to 1.60 to 1. What prop size do you guys think I can push now? Thanks

Your RPM will determine what pitch you need with the new ratio.

Did they sell u a long tail prop if so that will have a lot to do with how it runs_

I have a homemade surface drive on a 16x48 with a 16hp predator that is stock and I started with a 8x6 long tail prop and got 12mph max with a 1 to 1 ratio and alot of slip. Today I got my new 10x8 hill marine surface drive prop put on and went to the river for a test run with a 1 to 1.3 ratio I got 18 mph but I was not able to get the rpms up so im going to a 1 to 1.6
ratio. Both test runs were done with two people and fishing gear. My boat is a 1973 lowe so its built well.

The last motor I built was a 16 hp with a 1.5:1 reduction slinging a Hill Marine 10X8. It spins the 10X8 with no problem and could probably be geared more in the 1.35:1 or so. On the 13 hp I would stick with 1.5:1 reduction and a 10X8. Getting a surface piercing prop on your build will make a big difference. Less drag with a smaller amount of outdrive in the water. Post up some pics of your build if you get a chance.

Mag10

thanks for the info I will post some pics soon.

where is the best place to get a surface piercing prop? thanks

I have had pretty good luck with Hill Marine.

Yea they got some good stuff I'm a hill dealer they are very easy to deal with and they are very knowledgeable. And their prices are very reasonable