Re: Cavitation plate question
Billy: This is a much debated and misunderstood subject. The "cavitation" plate is really an anti-ventilation plate and its purpose is to prevent the top of the prop from sucking surface air into the water.
NOW: The less lower unit that is in the water, the less drag, and the faster the boat will go. BUT if the A/V plate is too high, the prop will entrain air and ventilation or cavitation will occur. The boat will also suffer a lower top speed because of this loss of thrust.
Design of the hull will dictate exactly how high the A/V plate can be set above the bottom. TYPICALLY, even with the bottom to one inch above it will be fine with most general purpose boats. Bass boats and racing boats are designed with a "step" in the hull and typically the engine can be set higher. Some can be as much as 4 inches higher than the bottom.
If you can, picture this: Any planning boat will not ride exactly parallel to the water, It will describe an angle, with the transom being lower. Because of this, the water :rises" as it moves away from the boat. The farther away from the transom (or hull bottom) the engine is, the higher the water and the higher the engine can be set above the bottom. SO, boats with a step or notch at the transom set the engine farther away from the actual vee of the bottom and the engine can be set higher on the transom. Since a jackplate also sets the engine back from the transom, engines mounted on a jackplate can be run higher.
Now, just so this won't be moved: Back in the late 1970s, when Chrysler designed the one piece lower unit, they made the engine one inch shorter so less lower unit was in the water for less drag. This actually did gain about 3 MPH and they advertised it that way. They were clever though: They did not claim to be faster than Merc or OMC, just faster than the old two piece lower unit.
UNFORTUNATELY, most hulls were designed for engines an inch longer and the first generation engines did ventilate a lot. Customers howled. Chrysler rushed into production an Anti-cavitation or Flare washer to fit behind the prop. This was supposed to help. It did look good.
