Suzuki
- Thread starter Stampr
- Start date
In thinking of this from afar, I would be more inclined to suspect "ventilation" rather than "cavitation". On an obvious major change such as a repower the most obvious suspects are mounting height on transom, is the leg length same as previous properly performing engines, are the engines much lighter causing the hull to carry the engines higher in relation to the water surface, are the propellers mismatched to your particular craft, are the props designed for the particular lower units exhaust discharge to prevent exhaust gases being sucked back to the blades. Here is an explanation I found that speaks to this....
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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.
>>>>>>>>>>>>>>>
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.
Thank you, thank you, thank you, Half! Saved me a lot of typing! Yes, cav and vent are VERY often confused/misused. The height issue is the first one I would think of, as well.
How far back is the engine from the aft end of the running surface of the hull bottom (typically where the garboard drain is)? And, how high above the hull bottom (use a straight edge) is the anti-VENTILATION plate? A very general rule is 1" above for every 1' back.
You may just need to lower the engines a hole or two. It could still be a mis-matched prop, but start with the easy things. As far as the prop is concerned, I would assume that you bought this from a reputable dealer who would know what prop to put on it?
How far back is the engine from the aft end of the running surface of the hull bottom (typically where the garboard drain is)? And, how high above the hull bottom (use a straight edge) is the anti-VENTILATION plate? A very general rule is 1" above for every 1' back.
You may just need to lower the engines a hole or two. It could still be a mis-matched prop, but start with the easy things. As far as the prop is concerned, I would assume that you bought this from a reputable dealer who would know what prop to put on it?
I remember seeing a thread on here where someone re-powered with DF 250’s on the same boat and there was something with the factory set negative trim. Once he adjusted that the boat ran a lot better. Just a thought
I repowered with DF250's and have not had any issues...WAY different prop though...went from a 19 to 23 pitch prop due to the different gearing...and had probably 75 more HP
I have DF300's on my 2003 330, they need to be LOW on the transom, I think lower than the original Yammies.
Other issue could also be prop - I have Suzuki 3x16x17 props on my boat.
Other issue could also be prop - I have Suzuki 3x16x17 props on my boat.