Trigonometry refresher
You should refresh your memory of high school trigonometry (just a little, we only need two functions). You will need to be able to do calculations using the sine (sin) and tangent (tan). The Tangent of an angle is the rise over the run (or opposite over adjacent). This assumes that you have a right triangle where the angle opposite the one you are trying to calculate is 90 degrees. The angle you are trying to calculate is the prop shaft angle. That is the angle of the big hole you are about to drill in the bottom of the boat.
The Angle we are interested in is at corner A
Now you might be thinking: "I don't know the angle - that is the the part I need to figure out" . This is where you will use the inverse functions of arctangent (arc tan or inverse tan or tan -1). So if you are using the windows calculator . Set it for view->scientific and make sure the button for degrees is selected. For example: run = 20", rise = 5.75" 20/5.75 = .2875 Now hit Inv and then tan and you get 16.04 degrees!
Next question is if I know the angle (or at least a reasonable assumption), and I have the rise, how long will the shaft be for this segment? In the drawing, the shaft would be the hypoteneuse of the triangle. For this you need to use the Sine. On the scientific calculator this is the sin key. So for a rise of 5.5" which is the height of the engine side of strut:
Strut Length = 5.5" / sin(16) = 19.8"
Back to Prop Shaft Meaurements and Caclulations
For a more complete review take a look in Wikipedia