I have been around a few mud motors, longtails and surface drives, but never owned one. I got a good deal on a 31hp Vanguard and want to build it into a longtail. it will be used on a 16x50 boat

What size prop should I use?

Will a 3/4" shaft be enough?

and what is the advantage of running an angled shaft with a U-joint over a longer shaft run directly off of the engine?

dont quote me on thise but i believe that the longer the shaft the more power loss ive been told that 30 hp long tail runs as if it was 15hp also alot of motors have an low oil shut off and if the motor is on two much of an angle it wont run or you could blow the motor up by the oil running on one side of the motor

My suggestion is to run what all the major manufactor's run, and thats a U joint shaft. I think 3/4 will be plenty big. and I would go with a 9x7 prop. post pics of the build when you get it going.

The big 31-35 lt shafts are 7/8 not 3/4

I would get a 10x4

For sure use a 7/8 shaft but I prefer the 10x5 You give up a lot of speed with the 10x4