[Fr. Frank]

Steve, check the height of the cavitation plate above the hull with the cavitation plate as perfectly parallel as possible to the keel line. I'm wondering if you have wake flow over the top of your cavitation plate, which pushes the stern down.

What happens then is the wake pushes the cavitation plate down (and the stern with it) and so levers the bow upward, until the weight of the hull out of the water over-balances the downward thrust on the cavitation plate. Then the bow falls until the thrust on the cavitation plate is greater than the lofted hull weight, and the cycle begins again.

Ideally, a straight edge extended straight back from your keel should pass midway between the water inlets on the outdrive and the cavitation plate on an Alpha or Bravo outdrive. On a TRS or TRS II drive, the cavitation plate should be about 1" to 2" higher than the keel line.

At lower speeds this isn't a problem, but the higher speeds create a lot more downward pressure on the cavitation plate.