Specifically, using an old trolling motor to mount to? I don't know what I should use, or what size to make it. A pic would be great. Thanks.

Are you adding the rudder to a kayak?  

No. My boat is a 16' flat jon, with a 48" wide floor.

 

I'm going to bolt a plywood rudder to the shaft. I'm just trying to determine what size to make it. I don't know how may inches of the shaft will be under water.

Specifically, using an old trolling motor to mount to? I don't know what I should use, or what size to make it. A pic would be great. Thanks.

Try looking in this thread - many of us that have small pond boats made rudders - I'm sure any of these would work on a jon boat too - I made mine out a old road sign, attached it to a steel pipe I bought at Lowe's for like $3.00 - it's 3.5 feet long and works perfect - I found a MK TM mount on E-bay for $30.  Here is also a picture of it.   However I did cut it down some cause of the weight and it was too big. 

 

http://www.bassresource.com/bass-fishing-forums/topic/25442-any-pelican-bass-raider-owners-out-there/

Jim, I would use an old transom  trolling motor mount with a cut off whitewater rafting or canoe paddle mounted in it. Leave it long at first. you can shorten it to finished lenght after you see how long it has to be.  Lynn

Thanks guys. I wound up bolting a 12" X 20" piece of plywood to an old trolling motor shaft. It's been too hot to try it out though.

I tried out the rudder a few days ago, and it helped quite a bit. Between the rudder and moving battery to the rear, it is a big improvement.

I tried out the rudder a few days ago, and it helped quite a bit. Between the rudder and moving battery to the rear, it is a big improvement.

Mine worked great yesterday first time in a very strong wind and I had no problem with it.  However I think I may step up the size of my TM - got a 30 lb and probably need a 50-60lb.