You have a bad vibration and/or severe propeller damage. Is the cause cavitation? It is very difficult to determine without more information. Diagnoising and solving a cavitation problem, at times can be a very tricky affair. There are different types of cavitation and ways that is shows itself. There are also sources of vibration and propeller damage that are not cavitation.
Over the years, cavitation has gotton a bad rap for causing vibration and propeller damage, while it is true that cavitation can cause these problems, the more common type of cavitation is what is called "back sheet" cavitation, which is non destructive. Back sheet cavitation is a very stable pocket of water vapor (not air) that occurs on the trailing edge half of the propeller blade on the suction face (side of propeller closet to bow) of the prop. This area of water vapor grows in size from the tip of the propeller towards the middle of the blade as propeller RPM increases. This type of cavitation can actually be beneficial because it reduces the surface drag of the propeller which increases the efficiency of the propeller. Back Sheet cavitation can and does exist without being noticed so long as it does not become excessive. When a boat speed stops increasing with increased RPM or a vibration develops at a hig RPM, it is an indication that back sheet cavitation has become excessive. By adding some trailing edge cup, the above problems can usually be solved and boat performance improved.
The type of cavitation that is usually associated with propeller damage or severe vibration is an unstable cavitation that is caused by a defect on the leading edge of the propeller blade, which causes a bubble of water to form. As the bubble of water vapro moves along the blade surface it collapses back into the blade causing propeller damage and noise. This type of vibration can usually be solved by a close inspection of the leading edge of the propller and making sure that is smooth, consistent and free of any damage. If the propeller is in good condition and the problem still persists, the cause may be a hull appendage (water intake, strut, etc) up stream of the propeller that is causing the problem.
The results of cavitation can be very severe. Deep pitting of the propeller blade can occur in a very short period of time and may even result in a thrown blade. Cavitation can also show up as a discoloration of the propeller blade, which can be an early indication that cavitation is occuring. Damage caused by cavition usually occurs in a well defined pattern on one or all the blades depending on the source. If the pitting or discoloration appears to be random, with no indentifiable pattern the cause is probably not cavitation but rather Galvanic Corrosion. Another phenomena that is commonly misidentified as cavitation is ventilation. Ventilation occurs when the propeller blade is able to suck air from the surface down into the propeller. Ventilation is much more common on boats powered with outboards and I/O's since the propeller is operating much closer to the surface of the water. Ventilation usually causes a severe vibration and sometimes a sudden jump in engine RPM. When a propeller "blows out" in a tight turn, the cause is usually ventilation and not cavitation.
The moral to the story is to make sure you provide as much information as possible about the problem you are having, or you may wind up trying to solve a problem that never existed.
This article was reprinted from the quarterly publication "The Leading Edge", written by John Althouse - Naval Architect.
