Hi every one

I am new to the mud motor thing so I am looking for some help. I have a 17 foot canoe that I would like to make a mud motor for, I have a 3.5 hp Briggs and Stratton motor that I plan on using. The canoe is 38 in across at its widest point so it's very stable. I am looking for some help on figuring out how long the shaft should be and anything else that might be important.
Thanks for the help!