Re: Motor Height-Stepped Hull-Cavitation
OK! Let's use a little reasoning and logic here.
Pretty boat by the way----
Carefull! Thinking required here!
When a deep vee planing hull is on plane it does not ride on the surface of the water like a flat bottom hull but cuts a groove the shape of the hull in the water. As you apply more power to the hull, it rises higher and the groove becomes smaller. Less hull is in contact with the water and thus resistance is less and speed is higher. This is where a deep vee is different than a displacement hull which never rises higher no matter how much power you put into it. (Too much power for the deep vee hull design and it rises too high for stability and "Chine Walks," falling off from side to side. Good for racing, not good for pleasure boating.)
As the deep vee moves through the water, some of the displaced water "springs" back or rises as it leaves the transom. This is partially because of the angle of the hull to the water surface. It is also due partially to fluid mechanics but I don't want to get too deep. For all practical purposes the water "rises." This is why on a standard transom, the "cavitation" plate can be set about one inch higher than the hull bottom.
The "cavitation" plate is really an ANTI-VENTILATION plate. Its PRIMARY function is to prevent the top of the prop from "sucking" air and ventilating which will lead to cavitation. It mechanically "seals" the surface of the water so air can not enter the prop cone. In terms of mechanical efficiency, the best place for the plate in a normal, non surface-running prop, would be right on the surface of the water. Thus: the approximate one inch height which will vary with hull design and weight etc. (My cuddy has a flat riding plate about one foot wide at the transom, tapering into the keel about six feet up the hull.--one inch engine lift is too high on this hull--ventilates wicked bad even with a cupped prop.)
A step or notch designed in the hull like yours tries to accomplish two things: It tries to reduce the surface of the hull in contact with the water AND it tries to act as a set- back or jack plate for more leverage and higher bow lift. This is also an attempt to get more of the hull out of the water, and air entrained under it, for higher speed with a given horsepower.
With the engine set so far back from the effective transom--the point where the water leaves the hull bottom, the surface of the water is higher, so the engine must be run higher to position the plate ON the surface of the water. --AGAIN: this height will vary with hull design etc.
SO: the proper height (for maximum performance) for your engine will be that height which positions the plate as close to the surface of the water as possible, but not above it, while at full throttle and trimmed properly. And as little as 1/4 inch difference can have a dramatic impact in performance. Of course, with the plate set this high, when its angle is varied by turning the engine, the prop will ventilate some. If this ventilation in turns is objectionable, then you must lower the engine OR trim in. It's always a trade-off. Note that because of the angle of the hull to the water, and the angle of the rising water, trim will usually be best trimmed out some from parallel to the hull bottom. Again, this varies with hull design. Get the picture? Nothing is set in stone; everything is variable. That's what makes it so much fun and why everyone has a different opinion.
