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I just finished making my 9.5 Kroler mud motor. I have never owned a mud motor. I am having problems with the motor bogging down. I have a 8x4 stainless weedless prop that I bought. The site said that this was the right size for this motor. As soon as the prop is under water it digs in and boggs down. I am running a chain with a 3.5 sprocket on the engine and a 2" on the prop shaft.

Is this too much prop or too much ratio between the sprocket sizes? What is the preferred sprocket ratio?
How deep should the prop be in the water?

I think the bigger sprocket needs to be on the prop shaft.

Definitely have your gears backward. What rpm is your engine turning WOT with no load? What size chain are you running and why chain?

You defiantly need to change the gear ration. I am running a 1.5 to 1 gear ration with a chain drive and a 16hp engine and a 10x8 prop. If your running a chain use the teeth as a guide for the gear ration. Example, and top gear with 15 teeth will require a bottom sprocket of 22 or 23 to gain the 1.5 to 1 ratio. I found gears are cheap and you can play around with them to get the ratio correct. The best ratio can be anywhere from 1.2 to 1 all the way up to 2 to 1.