Does Your Prop Have Secret Cavitation?
Sep 21, · Cavitation if the formation of air bubbles on the propeller. Cavitaion usually occurs as a result of damage and you'll feel it in the form of vibrations. Cavitation is bad for three reasons. First it can physically damage the propeller. Secondly, it causes those vibrations which makes your . Dec 13, · Overpowering or wrong prop causes cavitation. It's kinda like doing "burnies" on a hot rod. Ventilation is cause by exhaust gases or surface air causing an air bubble around a prop. To me, either one is just a symptom if this problem so try duraruna mark and look at your engine/trim setup.
Subscribe Now Log In. In a recent e-mail, a reader requested help with what he believes is a cavitation problem. He described his boat as a foot center console with a model horsepower engine. His boat has been problem-free until recently. 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 degrees, but if you reduce the atmospheric pressure sufficiently, what are the kia soul animals 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 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 what causes outboard motor cavitation 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. 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 how to connect fiber optic cable to cctv camera 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. 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 how to configure snmp in linux, 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 how much weight to gain during pregnancy pulled closer to the surface. 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 theboatdr yahoo.
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Outboard Motor Cavitation
Sep 15, · Cavitation is a word to describe the creation of gas bubbles underwater that results from a fast propeller in low-pressure environments. At certain low pressures, a vacuum can cause water to “boil” at low temperatures, thereby causing the bubbles that we see with increased propeller speed. These bubbles then pop, creating noise and small implosions and shockwaves against your propeller’s metal. Jan 23, · The true cause of cavitation can arise from several factors, but we see them occur most often as a result of leading-edge imperfections, such as nicks, dings, scratches, bends, etc. That said, they can also be caused by improper geometry or upstream imperfections related to the hull, such as hull design/configuration, hull imperfection, incorrectly installed accessories/appendages, etc. Jan 30, · However, I believe the problem here is air being sucked from the surface. When in a sharp turn and under power, small outboard boats often "cavitate". What's actually happening is that the suction side of the propeller gets too close to the surface. This commonly happened on my Dad's outboard boats and smaller I/O boats.
Have you ever run your finger along the face, or edge, of a prop and felt a pitted area? If you have, then you know what cavitation can do to a prop. Given enough use, that pitting will only get deeper and you will end up having to replace the prop. The causes of cavitation have always been somewhat of a mystery to us, so we asked some prop experts to give us the technical explanation for this phenomenon.
Cavitation very commonly occurs on damaged, or imperfect, propellers and can occur if an attempt is made to transmit too much power through the prop. At high rotating speeds or under heavy load with high blade lift coefficient , the pressure on the inlet side of the blade can drop below the vapor pressure of the water, resulting in the formation of a pocket of vapor, which can no longer effectively transfer force to the water.
To our thinking, cavitation could be defined as the phenomenon of the formation, rapid collapse, and subsequent implosion of vapor bubbles of a flowing liquid in a region where the pressure of the liquid falls below its vapor pressure. As these bubbles collapse, energy is released in the form of a shock wave that can, and often does, damage the surface of the material in question. Stretching the analogy to a screw, we might say the water thread 'strips'. The true cause of cavitation can arise from several factors, but we see them occur most often as a result of leading-edge imperfections, such as nicks, dings, scratches, bends, etc.
Since there are so many possible causes, diagnosing the problem should really be done on a per-case basis. A similar, but quite separate issue, is ventilation, which takes place when air is introduced to water around, or to the water flow fed to, a propeller.
This can be caused by a number of different sources, such as porting, over-and-through hub propellers, tunnel hulls, extreme motor elevation, excessively high trim angles, stepped hulls, or even hull appendages transducers, pick-ups, and the like. As the propeller meets the aerated water, the water's flow attachment to the propeller's respective working surfaces is detached.
As the water detaches, the propeller's loading is reduced, and the torque applied to the propeller through the shaft can more easily spin the propeller to a much higher rpm even at the same given power input. Specifically what happens in cavitation is this: Vapor gases evaporate into the cavity from the surrounding medium; thus, the cavity is not a perfect vacuum, but has a relatively low gas pressure.
Such a low-pressure cavitation bubble in a liquid begins to collapse due to the higher pressure of the surrounding medium. As the bubble collapses, the pressure and temperature of the vapor within increases.
The bubble eventually collapses to a minute fraction of its original size, at which point the gas within dissipates into the surrounding liquid via a rather violent mechanism, which releases a significant amount of energy in the form of an acoustic shock wave and as visible light. At the point of total collapse, the temperature of the vapor within the bubble may be several thousand Kelvin, and the pressure several hundred atmospheres.
Cavitation can significantly damage moving parts, and is usually an undesirable phenomenon. It is specifically avoided in the correct design of props and that is why prop fabrication and prop repair must be relegated to the experts.
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