cavitation

Hi All
The million dollar question.
I know what cavitation is what are its causes beside prop problems
Thanks

Drive height amd setback (positioning of the prop).

Not sure about water delivery to the prop (turblence from fittings ahead of the prop) and if that would play a factor (struts, etc.)

[tom28571]

PT, are you talking about cavitation, or ventilation. Your Bayliner is probably not subject to cavitation.

[PT Man]

Hi all
I hope I do not sound stupid or inane,I have received some interesting responses. I do not know difference between cavitation & ventilation. Either way this boat used to plane with relative ease, I even have photos. WHAT COULD HAVE CHANGED?
For late comers that have not seen my past post, I have a 28 ft Bayliner Command Bridge with an OMC 330hp King Cobra outdrive. Please Help I am very discouraged & can not afford to get out of it.
Thanks
PT

[Jeff]

I do not know difference between cavitation & ventilation.

“Propeller ventilation is caused by air from the surface or exhaust gases being drawn into the rotating propeller blades. This results in the propeller slipping more than usual due to the reduced water load on the blades. The obvious symptoms of this are a sudden increase in engine RPM (over-reving) as well as a possible loss of speed. This commonly occurs when trying to turn the boat at high planing speeds or if the outboard or sterndrive is trimmed out too high.
In racing conditions this can also occur when following another boat too closely. The small bubbles in the water created by the leading boat can cause ventilation of the propeller of the following boat, with a subsequent loss of speed. This is why you rarely see high speed race boats following directly in the line of the leading boats, even if they are far enough back to be out of the spray and wake.
Most drives and outboards (not including surface drives which utilize props designed to work in a ventilated condition, e.g. cleaver or chopper style props) have a plate above the propeller area designed to keep surface air from entering the blade area. This plate is correctly called the "anti-ventilation plate", although you will often see it referred to as the "anti-cavitation plate". Through-hub exhaust systems also have specially designed hubs to keep exhaust gases from entering the blade area.

Propeller cavitation is less obvious than ventilation, but can be far more damaging. When the propeller blade passes through the water at an increasing speed, the pressure that holds the water to the sides of the blades is lowered. If the water is sufficiently warm, and depending on the speed of the boat, formation of water vapor (boiling) can occur. During normal propeller operation, low pressure exists on the blade back. Normally, the pressure does not drop low enough for boiling to occur. However, poor blade design or selection, or blade damage can cause an unusual pressure drop on a small area of the blade. Boiling can occur in this small area. As the water boils, air bubbles form. As the boiling water passes to a higher pressure area of the blade, the boiling stops and the bubbles collapse. The collapsing bubbles release enough energy to erode the surface of the blade. This entire process of pressure drop, boiling, and bubble collapse is called "cavitation" and the damage caused by the collapsing bubbles is called a "cavitation burn". It is important to remember that cavitation is caused by a decrease in pressure, not an increase in temperature. One of the advantages of stainless steel propellers is that due to their superior strength they can withstand cavitation damage better than aluminum and can also be produced with thinner blades to reduce the occurrence of cavitation.”

--from about.com and Jim Linden

[Jeff]

PT - what happened with trying another prop on your boat or having your prop (hub) inspected?

[PT Man]

Hi all
I have tried several brand new props from prop shop still not grabbing
Thanks

[Willallison]

Bloody hell, this is perplexing!



Wait, Ive got it!!
PT Man - you gotta stick the thing in gear!!

[PT Man]

Everything I touch turns out to be perplexing. What ever is the most bizzar or unbelievable is my normal state when it comes to problems
Any other IDEAS
Thanks

Could it be slip at the engine coupling -- the coupling which reduces vibration - I've not experienced one of those giving, but it could be possible -- I'm not a mechanic though.

I don't think it's the clutch in the outdrive - I've heard of those going where you suddently don't have any forward, but I would think the RPM would hold it once in gear so you would not lose power as rpm's increases. But I wonder if there are other fittings which might reduce vibration or be made to give on impact which might be slipping.

Maybe it is time to take a local marine mechanic for a quick spin to get their opinion... do you have a local marina with a mechanic who you might be able to take out for 5 minutes or so?

[PsyWarrior]

Have you added anything to the boat? Did you install a fishfinder/sonar to the hull which could be causing a turbulent wake affecting the prop? Have you grounded the prop and possibly bent the skeg? The new props that you are trying, are they the same pitch and duration as the original?

I'm new here and not too swooft, but have read up on cavitation and aeration and all of these could lead to cavitation. Just trying to help...

[JORGE LANG]

Here's another reason for cavitation. A poorly designed underwater exhaust. If the exhaust outlet is in the stream of the props, you will be moving more than water with the blades. Something to think about.

[Evolution Yacht]

Probably a silly responce but makesure the outdrive is all the way down and the cavitation plates are in line with the bottom of the boat. Also if you changed the weight distribution that could have an effect. Good luck
SG