Zip 20” Transom 50 HP Modification

Questions about modifying a design

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Location: Hopkinton, MA

Zip 20” Transom 50 HP Modification

Post by Shelboss »

As I started getting into my Zip build, I knew that I wanted a Mercury 50 HP Redband outboard (1968-1972) and that a long shaft would be easier to come by. It also leaves the door open for a newer 4 cycle outboard in the future. The long shaft modifications that I have seen don’t look totally integrated, rising up from the rear deck to the clamp surface in a straight line. That’s when I decided to scale all the frames and create a CAD layout. The drawing shown in my gallery is a result of that exercise. By modifying the transom radius and the #2 frame radius, the top line can be maintained and the rear deck can meet the clamp surface more naturally.
The second part of this modification is dealing with more horsepower mounted higher on the transom. With a long shaft motor, the prop stays in the same position and the motor moves up about 5”. The motor torque acts to pull the top of the taller transom back. I decided to use SS tubes with rod end links and brackets carriage bolted through the transom and battens. I went with two additional battens to give me more parallel placement of the reinforcement tubes. The further forward the tubes mount, the more strength that is transferred longitudinally into the floor. This can be done with the sides of the motor well extending forward as well, but I wanted a clear view to the transom when the back seat is flipped down. So yes, I am not putting a motor well in per Vupilot’s comments.
My other departure from the tradition build is doubling up on the transom plywood as opposed to adding a frame and motor board to the single sheet. I did not want a break in the strength across the width of the transom. It was originally designed that way because the short shaft transom jogs down so much the frame member can’t continue across. The doubled up transom adds a little weight, but gives tremendous strength where eyebolts attach as well. I have tested screwing into the plywood for the planking. I am using ¾” Sapelle plywood, which is 11 ply. I could not strip the standard length screw out, and I intend to go ¼” longer anyway. Since this plywood is actually 11/16” thick, I recessed all my carriage bolts and capped the back with ¼” Sapelle plywood, for a total thickness of 1 5/8”. My transom knee was made with 3 layers of ¾” and is 2” larger in both length and height than the original. The additional width allows for counter boring, recessing the carriage bolts, and plugging the holes.

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