Is this cavitation??

(t_brown) · (t_brown) **Join Date: Feb 2006** 03-19-2006, 9:15 PM

I was going about 30mph and tried to turn around and it I slowed to about 20 and the rpm's when to about 4k until I completed the turn. In my other boat, with a 4 blade pro, I never had that happen. This is a 3 blade standard alum prop.

(yosquire) · (yosquire) **Join Date: Jun 2005** 03-19-2006, 9:48 PM

yep, my old I/O would do that..

Sometimes it helps to pull back on the throttle.. then the prop sinks back into the water and you can juice it again.

(midwesty) · (midwesty) **Join Date: Aug 2003** 03-19-2006, 9:56 PM

NO POWERTURNING!

(attila916) · (attila916) **Join Date: Oct 2005** 03-19-2006, 10:15 PM

My bayliner would do that as well. If you lower the outdrive all the way you should be able to avoid cavitation. After I swapped my prop from a 3 blade 21 to a 4 blade 19 I had less problems with cavitation.

(t_brown) · (t_brown) **Join Date: Feb 2006** 03-19-2006, 10:47 PM

I'm not trying to power turn, just turn and stay on plane. I was able to do exactly what I did with my 18.5 Bayliner with a 4 blade prop and never had that happen. It just sorta freaked me out.

(humboldtboarder)

quote:
I'm not trying to power turn, just turn and stay on plane

sounds like a power turn to me

(boarditup) · (boarditup) **Join Date: Jan 2004** 03-20-2006, 5:52 AM

It is ventillation, not cavitation. You are getting air around the prop. A dinged up prop will make the situation worse. It is a hull/outdrive/prop interaction issue. Some mfgrs will raise the outdrive to get extra speed and sacrifice some of the handling. Other times, it is hull design.

(tpyle) · (tpyle) **Join Date: Feb 2006** 03-20-2006, 6:39 AM

What is your definition of a power turn?

(t_brown) · (t_brown) **Join Date: Feb 2006** 03-20-2006, 9:41 AM

My definition of a power turn is punching the throttle down once you enter a turn to swing the stern of the boat around at a fast speed.

(tuneman) · (tuneman) **Join Date: Mar 2002** 03-20-2006, 10:48 AM

Todd, have you tried turning the other way? It may only do it in one direction due to the rotation of the prop.

Also, are you using ballast? A loaded down boat will have a greater chance of cavitating that an unloaded one.

(t_brown) · (t_brown) **Join Date: Feb 2006** 03-20-2006, 11:07 AM

No, I do not use ballasts and I only turned right so I can try turning left and see what happens. I had 4 people in the boat at the time.

(rodmcinnis) · (rodmcinnis) **Join Date: Sep 2002** 03-20-2006, 11:10 AM

It could be cavitation, or ventilation, or a combination of both.

Cavitation is something that is happening pretty much all the time. It only becomes a problem when it becomes so bad that the prop slips. Cavitation is the result of the "forward" side of the prop trying to "pull" the water towards it, verses the back side that pushes. You can't "pull" water, all you can do is create a hole that the water flows into. Creating the "hole" causes a major decrease in the pressure. When the pressure drops low enough the water will actually boil at lake/river temperatures. This "boiling" is the cavitation.

While the backside of the prop can generate unlimited amounts of "push", the amount of force the front side can exert is limited to the water pressure, which is a function of the depth. When you are in a turn, any tilting of the boat will cause the prop to move closer to the surface, decreasing the water pressure on the prop and increasing the cavitation.

If the prop creates a hole that the water won't fill in before the next blade rotates into that position then the blade doesn't have anything to push and it slips. "Cavitation" is the boiling of the water due to the pressure drop, which happens all the time. Only when it gets so bad the the prop slips do you notice it.

Ventilation is another issue that can happen. This is when the action of the hull forces air down into the water in the path of the prop, then the prop slips in the air bubbles.

In general, more blades on the prop reduces the amount of cavitation. The condition of the prop can also have a major effect. A dull, dinged or bent prop will cavitate a lot more than a sharp true prop.