I didn't check your math but that looks about right. In simple terms, for simple people, like myself:fallguy1000 wrote: ↑Tue Nov 30, 2021 8:43 pm 20.5 is my math
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Cut a transom at 20.5 inches, stand it on the ground at 13.5 degree angle and measure the vertical distance from the top of the transom to the ground. Should be(according to your math) 20 inches, which looks about right. So, if your motor measures 20 inches from the bottom of the vent plate to the top of the inside of the bracket, and you want a perfect fit, cut the transom to 20.5, not 20. If you cut it to 20, you'll either have an extra 1/2" of lower unit in the water, or will need to mount it 1/2" up the transom.
That's the way I see it. Since my motor is 22.5+ and my transom is 20", which is 19.58 of vertical height at 13.5 degrees, it's a much bigger issue, beyond what can be adjusted and still use the clamps, which I was. I'm remounting with bolts which will work but there is a lotof motor above the top of the transom. So mine is an extreme case, and it looks like the same is true here, though it's unclear why at this moment.