Before I can answer your two questions, I need to give you some background on the phenomena we are going to be discussing.
First, the "engine" does not cavitate; a propeller blade can have cavitation. Cavitation is different from ventilation.
A propeller cavitates at the tips leaving a spiral trail of small bubbles. This phenomenon is not particularly cured by engine mounting height, as the air bubbles are being created from the water, they are not air sucked downward from the water surface above. Cavitation occurs when a propeller blade is rotating in deep water. To reduce blade tip cavitation you need to redesign the propeller blade tips to reduce the creation of cavitation. A propeller blade is designed to accelerate water. As the water moves faster the pressure decreases, and small bubbles appear. The cavitation is the water boiling due to lower pressure. The low pressure side of the blade causes cold steam or water vapor to exit from the water in the form of very small bubbles, more or less boiling off at the blade surface. This causes long term erosion of the blade, particularly on blades of softer material like aluminum (aluminium in the U.K.). If you look at almost any aluminum propeller with black painted blades you will see erosion of the paint at the blade tips due to cavitation.
Second, a propeller ventilates when the propeller mounting height is too high, the blades are very close to or even above the water surface, and the blades draw air into the water, causing the water to become very aerated around the propeller. This tends to quickly reduce the load or the power needed to turn the propeller in the airy water instead of solid water.
In old fashion engines in which the engine speed was regulated entirely by the throttle plate and fuel was dispensed based on flow of air over a carburetor, a reduction in load at a particular throttle setting would immediately cause the engine to accelerate to a higher engine speed, often significantly higher. The higher engine speed would result in higher propeller shaft speed, which tended to draw in more air. This positive feedback system quickly produces a very large reduction in thrust produced by the propeller, which is now spinning in a very airy water mixture. The remedy is to grab the throttle, reduce engine speed, and let the airy water move away from the propeller.
With modern engines, the engine rotation speed is controlled electronically, and at a particular throttle lever setting the engine speed will not suddenly run away to higher speeds if there is a slight change in the load from the propeller. This behavior of the engine tends to mitigate or prevent ventilation in many situations.
Third, many modern boats are equipped with modern propellers that have been designed to operate at elevated mounting heights, with their propeller blade tips running very close to the water surface.
Fourth, modern outboard engines are designed with ANTI-VENTILATION plates that help prevent air being drawn down into the water by the propeller. In many instance of engine mounting height, the ANTI-VENTILATION plate can be run with the upper surface of plate just above the water flow around the engine gear case.
Taco1818 wrote:Q1: If I make an engine bracket that adds 0.25-inch height to the transom, will [the propeller that I will install on the 20-inch-shaft Mercury 90 FOURSTROKE] never cavitate?
With regard to changes in engine mounting height by 0.25-inches, I would expect such a small change in engine mounting height to produce very little noticeable effect on propeller operation. A propeller is going to produce cavitation as a function of the design of the blade tips and the rotation speed. Generally are standard boat propellers are always producing some cavitation when they are rotating at high speed.
Taco1818 wrote:Q2: where will the [anti-ventilation] plate end up [relative to some undefined other fixed location on the boat]?
The only particularly important distance relationship in choosing engine mounting height is the vertical distance between two surfaces:
- the water surface flowing around the engine gear case when the boat is on plane at higher speeds and the engine is propeller trimmed, and
- the bottom of the engine anti-ventilation plate.
Measurement of any other vertical distance to some fixed object on the boat transom is of no real importance.
The desired vertical separation between the top of the water flowing around the gear case and the bottom of the anti-ventilation plate is 0-inches. That is, as long as the bottom of the anti-ventilation plate is just at the surface of the water or just above the surface of the water when the waters flows around the gear case when the engine is on plane and engine trim is proper, that is the probably the best height--if you are interested in getting the least drag and highest boat speeds.
Proper engine trim usually means the plane of the anti-ventilation plate is parallel to the plane of the water flow around the engine.
The anti-ventilation plate can be running wet in the water, that is, the engine mounting can be lower, and the result will be the top speed may lose a small amount, perhaps five-percent reduction (for example from 55-MPH to 52.75-MPH, but there may be improved resistance to ventilation, particularly when operating in rough sea states where the pitch of the boat will change significantly from its normal statis trim. Big down-by-the-bow pitch changes in the hull will cause the engine to be raised out of the water several inches more than at normal trim, and this can immediately induce ventilation.
The choice of engine mounting height is a trade-off. If you never want the propeller to ventilate, mount as low as possible. If you want the highest boat speeds in calm water, mount the engine as high as possible for the propeller to avoid ventilation in calm seas. For general use, some mounting height between those extremes is probably more useful.