Clark Craft's Jeckle9 motor question

Planning on getting started on a Jeckle9 soon, and being a small inboard, I was thinking about a suitable motor. I like the idea of using a 6-8hp single four stroke like found on go karts. A 6.5 weighs 42 pounds. With the lower speed of 3600 rpm, should I use a reduction of 2-1 and an agressive prop, or just direct drive? With the idle of that type of motor be enough to keep running hooked straight to the shaft, of should I use some type of either clutch or in-out setup?
Second, where do I find the piece that holds the shaft to the hull down by the prop?? Never built an inboard before.
Thanks, Greg

 

I too am building an inboard of sinular size. I plan on using an 18 or so HP aircooled lawn tracktor engine. The underwater "bracket" is called a strut. Glen L Marine (online) has them, but they are mainly for the 15 and up footer boats. The degree of angle is 12 and uses a 12" dia prop. Thats a bit big for small boats. I'm planning on a 8" prop from an outboard and building my own strut and shaft log along with the rudder and driveline.
I was planning on using a jet ski motor coupled to a prop shaft Jeckel style but the water cooling system became a stumbing block.
My 8 foot boat will be using a 3 foot shaft on a 10 degree angle running V drive style direct with no reverse.

 

With lawn mower engine conversion like this, be careful about the shaft angle as very much and you oil starve the engine which typically us "slingers" not pumps. It doesn't take very much angle to starve a slinger.

 

A lot of todays engines use an oil pump system. In my design, the engine is level as in a large V8 V drive ski boat.
With a forward engine design such as the Jeckle, a powerful 2 stoke type engine such as a motorcyle or snowmobile would work just fine.
Problem I find with using a snowmobile engine is that it will turn a prop CCW. It will be harder to find a cheap prop that turns left.
A motorcycle engine will turn a prop right only if the drive sproket ran down the right side of the bike. (Harley sportster)
Jet ski engines turn CCW and also need a cooling system.
Therefore, the "lawnmower" engine is a cheaper choise.
My design also utilizes a "torque tube" design for the underwater shaft. It will eliminate the need for a cutlass bearing in the strut and will also eliminate the need for a shaft log and shaft packing gland.
The rudder will be mounted on the rear of the transom like a hydroplane.

 

Whatever system you make, it needs thrust bearings for the shaft. Otherwise the crankshaft won't last long.

 

My design is fashioned after a V drive where the engine is in the rear with a drive shaft running forward to a chain drive "V" box. The box will be at a 10 to 12 degree angle, therefore the drive shaft will have a universal on it at the Box end. The prop shaft will be inclosed in a tube that will have a cup and cone bearing at the propeller end for thrust. I have designed this as an assembly (sans the underwater strut) that can be completely built on a work bench, and even test run on the bench before installing into the boat.
I also have made a provision for gear changes to the V box. Changing the shaft speed for optimum performace is cheaper than buying a bunch of propellers.