hey guys i have a homebuilt 7hp sd and i have a 8x4 3 blade prop(from lt) and i have tried multiple gear changes(from 1:1 to 1:7) and im still getting slow speeds, around 10mph which is pitiful on my 12x32(110lbs) riveted jon with just me but if i add gear and friend it makes very little difference. my questions are: is my non surface drive prop costing me my lack of speed? what would be a good prop to try for my motor? i see dixie mm runs a 9x9 would it be a good prop to run? also where would be the best on the wallet to get one of these props?
thanks in advance for the input, any help greatly appreciated

It not being a true sd prop is a big factor.

also, if it is a old 3 blade its prolly pretty worn.