The Anti-Ventilation Plate and other Nomenclature

Optimizing the performance of Boston Whaler boats
jimh
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The Anti-Ventilation Plate and other Nomenclature

Postby jimh » Wed Jun 10, 2020 6:59 am

The lower wing appendage on an outboard engine gear case is known as the anti-ventilation plate. Its function is to help prevent propeller ventilation. A propeller ventilates when air from the water surface or exhaust gases are drawn into the propeller and the propeller blades no longer operate in solid water.

Above the anti-ventilation plate often there is a second foil appendage called a spray deflector. Its function is to deflect spray outward and keep spray from being directed further upwards and spraying on the engine mid-section and cowling.

To avoid confusion, forum discussions use proper names for boat objects so that we don’t have multiple names or incorrect names or misleading names used for the same object.

Propeller cavitation is a different phenomenon than propeller ventilation. To understand propeller cavitation, please read the article at

An introduction to propeller cavitation
https://www.iims.org.uk/introduction-pr ... avitation/

Reference to vertical distance between the anti-ventilation plate and the lowest part of the transom—nominally the center of the transom and the hull bottom centerline or keel—is not particularly useful in determining optimum engine mounting height. Optimum engine mounting height is determined by visual observation of the position of the anti-ventilation plate relative to the water flow around the engine gear case while the boat is on plane at higher planing speeds and the engine trim is set properly so that the anti-ventilation plate is aligned with the flow of water.

For some examples of the anti-ventilation plate and the flow of water, see a separate article at

A-V Plate Pictures
http://continuouswave.com/ubb/Forum4/HTML/006491.html