Shaft length needed?

I'm designing a boat 4'wide by 16' long and a 70" beam 24" sides 1/4" plywood covered with 3 layers of 7.5 oz. fiberglass. The transom will be 1 1/4" thick, double gussets in the middle from transom top to floor. There will be gussets on the top sides to the transom also.
This is the question: I was hoping I could us at least a 15hp motor but what length shaft and how much do I need to cut out of the transom, if needed?
MichaelG

 

Shaft length is equal to the distance from the top to the bottom of the transom. A long shaft is 20" and a short shaft is 15". You can dictate the shaft length by how you trim the transom.

 

You have confused us. You say that the boat will be 4 feet wide but you say that it will have 70 inch beam. Which is it? Why three layers of glass on quarter inch ply? The boat will be stiffer if you use more wood and less glass. 3/8 ply and one layer of glass will be a more practical method. 24 inch midship sheer height may be a bit tall for a 16 footer. The bow height should be at least that high but probably more. Post some sketches for additional critique.

 

The bottom is 48" wide and the top sides are 70" wide. (sorry)... The reason for the 1/4" ply and 3 layers of fiberglass is... I believed that for cracking reasons in the finsh coating I would rather have a good thickness of eboxy, instead of less expoxy and more wood. I thought I would have the same strength in the boat but more flex for the finish coating. The design of the boat mimics your basic Jon Boat.

 

Most epoxies are quite flexible, quite comparable to many woods. But for best results the wood should be sealed with epoxy inside and out.