Is it cavitation or ventilation?

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In a recent e-mail, a reader requested help with what he believes is a cavitation problem. He described his boat as a 1994 16-foot center console with a 1994-model 70-horsepower engine. His boat has been problem-free until recently.“Lately I have had a problem with what seems to be cavitation — the boat gets up on plane just fine, levels off, gets to cruising speed, then it feels like the prop is losing its grip,” he said. “The RPMs go up, and the boat slows down.”

He thought he may have a striped hub in the propeller, so he had the hub replaced. There was no improvement with the new hub. He also checked the lower unit oil, and did not find any signs of metal debris in the oil, which would have indicated gear failure.

According to his e-mail, he has not changed anything in the setup of his boat. He did read my article on ethanol in the fuel, and was wondering if low octane could cause such a problem. He told me that he has recently developed a leak in his hull from running up on too many oyster reefs.

Cavitation is often confused with ventilation. Cavitation occurs when there is an extreme reduction in pressure on the back side of the propeller blades. Under normal conditions, water boils at 212 degrees, but if you reduce the atmospheric pressure sufficiently, water can also boil at room temperature.

As your propeller begins turning through the water at an ever-increasing rate of speed, the pressure on the back side of the blades is reduced, and if that pressure is reduced low enough, the water will begin to boil and form water vapor on the blades. This usually occurs near the outer or leading edge of the blade.

The water vapor bubbles migrate toward the center of the blade, where the pressure is higher, and the boiling stops. The vapor bubbles implode against the blade’s surface. The resulting energy release is so great that it can chip away at the blade surface, leaving a cavitation burn.

There are numerous possible causes of cavitation. Nicks or damage to the leading edge of the propeller blades are one of the most common causes. If your propeller cannot cut through the water smoothly, it will cause disturbances in the water flow, and many times will result in cavitation.

A popular myth is to sharpen the leading edges of your propeller blades so they can cut through the water like a sharp knife through butter, and this will make your boat run faster. A sharpened leading edge can actually cause cavitation, which will increase the slippage of your prop, and the boat will run slower. Propeller blades that are bent or have pieces broken off the edges will also suffer from cavitation.

Probably the sneakiest propeller problem is worn blades. In our environment of sand, oyster shells and hurricane debris, we are constantly wearing down the edges of our prop blades. This process comes on slowly over a long period of time, so we don’t always notice it, but when the propeller diameter gets small enough, you will experience performance problems.

Ventilation is the result of air or exhaust gases being pulled into the propeller blades. This causes the blades to lose their grip on the water, the engine RPMs go up and the forward speed of the boat is reduced.

Sometimes when the propeller RPMs increase, it can also cause a massive cavitation, which only compounds the problem. Most outboard and stern-drive engines have anti-ventilation plates made onto the lower unit housing directly above the propeller. Many people mistakenly call them cavitation plates or possibly anti-cavitation plates, but that is incorrect.

The anti-ventilation plate does not stop or help prevent cavitation. Its sole purpose is to prevent surface air from being sucked into the negative pressure side of the propeller blades.

Ventilation can also come from exhaust gases being introduced into the blades of the prop. Most propellers have some sort of ring around the trailing edge of the exhaust hub. This ring creates a high-pressure barrier that prevents exhaust gases from being sucked back into the blades.

Other sources for ventilation include the leading edge and sides of the motor’s lower unit and the bottom of the boat. Inspect your lower-unit housing for any imperfections in the surface directly in front of the propeller. Knicks, gouges, scratches or barnacles can all disturb the water flow to the propeller, and cause ventilation.

The same is true for the bottom of the boat. Holes and chips in the bottom surface from such things as oyster reefs can cause disturbed water flow to the propeller. Depth-finder transducers and live well or wash-down water pick-ups or drains could disturb the water flow to your propeller.

In the case of a boat that has a little age such as the one mentioned in the e-mail, you should check the condition of the bottom. The boat bottom should be perfectly straight for at least 5 feet forward of the transom.

The boat can be checked with a long straight edge placed along the bottom from transom forward. If the bottom is concaved upward from the straight edge, you have a condition called a hook. If the bottom is convex, the condition is called a rocker.

A hook in your bottom will cause the boat to have excessive transom lift. As your speed increases the pressure of the water flowing under the hook in your bottom will cause the transom of the boat to lift out of the water and the bow to be pushed down. Ventilation occurs because the propeller is being pulled closer to the surface.

A rocker will generally cause your boat to porpoise at running speed.

Ventilation or cavitation, they are both bad for your boat. I hope that I have helped you to understand what each is and you will be able to keep either of them from interfering with your boating pleasure.

If you have any questions about your boat, motor or trailer, drop me an e-mail at