modifying straight shaft to stern drive

Running a 25' speedcraft custom. Power is a 425 cummins quantum (qsb5.9) turning a 22 x 24 4 blade prop via 1 3/4" straight shaft.
The boat cruises 30 mph at 3450 rpm and wot is 40 mph at 3000 rpm. Burning about 2.75 mpg at cruise. I have been told I am losing alot of performance due to the angle of the shaft into the water. Also the draft is excessive at 40".
Would it be reasonable to install a jack shaft in this rig (Obviously limited room) to improve mpg and reduce draft?
I realize that more info is required but thought this would generate the questions I would need to answer.
Thanks, Carl

 

I just changed my 16 ft mudboat from a straight shaft to an outdrive type surface drive deal. It has taken alot of time and effort. Lots of bugs to work out. Had to modify the entire engine bay and beef up the transom. I don't know if I'll have better performance, but I sure hope so.

I know that its a smaller scale, but the principals should be the same. Take a look at some of the pics for my inspiration.

http://www.blizzarddesigns.com/mudboat/mudboat.html

I'm not sure about a jackshaft, since the thrust bearing is in my tranny. You could do it, but with alot of belts, a good thrust bearing, and way to hold that shaft nice and straight.

 

Unless the boat was built for an outdrive option, the transom is not thick or reinforced enough. The first thing you need to do it build it up to 1 3/4" or so. The engine has to be tilted to be horizontal instead of the 15 degree or so it has now. The rudder has to be removed and the steering system modified for the drive. Check the exhaust manifolds for clearance since the engine will sit higher.

 

Cool mudboat

So, tell us how skinny that rig runs!! That would be a neat setup for red fishing here in the PINE Island sound. Thanks for the reply!
Carl

 

Cool mudboat

So, tell us how skinny that rig runs!! That would be a neat setup for red fishing here in the PINE Island sound. Thanks for the reply!
Carl

 

I know that the boat "Fowl Habit" runs about 75 mph and can run stop and start in mud flats without a problem. Soupy type mud. He's got a 350 in that thing though.

I estimate that mine will do 35 mph max and believe that it will start/stop in soupy mud. Very simmilar drive to the Go-devil surface drives, except I've got a car engine.

The go devil surface drives run about 28 mph and have a little trouble starting to run in mud, but they will also do it. It's better to have about an inch of water if you want to stop. Great little contraptions for extreme shallow running.