Proper engine height on 18'??

The motor bracket or mounting plate is designed to give you roughly the proper height. If the arms of the mounting plate lie on or close to the top of the transom you are good assuming a roughly 20" transom and a motor designed for a 20" transom. As Jim said what you heard about 5" above is for high performance boats with surface piercing props. On my Venom I run the prop shaft centerline about even with the bottom of the hull. I have a hydralic jack plate to fine tune it while making speed runs. Comparing high performance boats to a Dyne is apples and oranges. The Dyne hull was never designed to go fast - but rather to leave a minimal wake at any speed - to include planeing at 7-8 mph. Transom wedges would also help. Different boats for different folks with different strokes. That's why I have 3.

 

My motor (95 Johnson FastStrike) is mounted 1 inch above a 20 inch transom making the top of the mounting bracket 21 inches above the bottom. I am using a 5 degree wedge for extra tuck because I need slow speed performance.

When the motor is trimmed up so the cavitation plate is parallel to the bottom, the plate is about even with the bottom. The higher the motor, the less the boat tends to bank its turns, which is good. When the motor is trimmed fully in, the front of the cavitation plate is above the bottom, and the rear of the plate is below the bottom. This allows the plate to create lift.

Since the boat rides at a slight positive deck angle referenced to the water surface, the water is above the bottom at the cavitation plate. I have often thought that it might be better with the motor a little higher. My boat handles so well that I doubt if I will ever try it.

jim