Prop causing cavitation...how to resolve

[spacerust]

It started with a spun hub with the original prop. That prop was a 17 pitch. I had just recently bought the boat and I believe the prop is the original from the 80's. Motor is great though. So I ordered one from iboats, 17 pitch, and when I took the boat out I was running at 4500rpms. I wanted to gain rpms so I ordered another one, this time a 13 pitch, to give me approx. 800 rpms. I took the boat out and it was running where I wanted it at 5300 rpms. It was real windy heading out so I couldn't really tell or hear anything but rpms were good. On the way in, wind had died down and rpms were spiking and dropping like it was slipping. New prop and I didn't hit anything so I'm confused. From what I've read, is this cavitation? and if so, how can I resolve this? I've already spent a couple hundred just on two props with the second being right where I want to be as far as rpms but what about this erratic rpms? Any suggestions?

 

[Bondo]

Re: Prop causing cavitation...how to resolve

It started with a spun hub with the original prop. That prop was a 17 pitch. I had just recently bought the boat and I believe the prop is the original from the 80's. Motor is great though. So I ordered one from iboats, 17 pitch, and when I took the boat out I was running at 4500rpms. I wanted to gain rpms so I ordered another one, this time a 13 pitch, to give me approx. 800 rpms. I took the boat out and it was running where I wanted it at 5300 rpms. It was real windy heading out so I couldn't really tell or hear anything but rpms were good. On the way in, wind had died down and rpms were spiking and dropping like it was slipping. New prop and I didn't hit anything so I'm confused. From what I've read, is this cavitation? and if so, how can I resolve this? I've already spent a couple hundred just on two props with the second being right where I want to be as far as rpms but what about this erratic rpms? Any suggestions?

Ayuh,.... What motor,..??

'n Exactly What props,..??

 

[spacerust]

Re: Prop causing cavitation...how to resolve

The motor is an 1988 Nissan 90HP and the props I ordered were both aluminum Solas Amita 3's.

 

[jestor68]

Re: Prop causing cavitation...how to resolve

Check your motor height. You want your AV plate even with the bottom.

 

[steelespike]

Re: Prop causing cavitation...how to resolve

Just to nit pic a little; very likely what you are experiencing is venting,the prop getting air fouled because the motor is too high or
something on the boat disturbing the water ahead of the prop.
Cavitation is a different animal and you don't feel it.It occurs in the low pressure areas on the back of the blades
usually eroding paint and metal usually of no consequence and can be an indication the prop is working really well.

 

[spacerust]

Re: Prop causing cavitation...how to resolve

I went outside to check on the boat and I noticed that the jack plate was up a few inches. I don't remember putting it up but the last place we left was pretty shallow so I must have moved the jack up a bit to get out and I am thinking that this is what caused the ventilation that I heard coming in because it ran fine throughout the day till then. I will taking it out this weekend and double checking but I am pretty sure that this was the problem. I remember looking back at the motor when I was coming in but since I was on a leaning post and have a raised console (I reanacted the scenario before I put the jack down) from where I was standing the jack looks like it was down. I have to kneel down a bit to see it more from a side view. I'll post my conclusions but thanks for your replies.

 

[Texasmark]

Re: Prop causing cavitation...how to resolve

You just solved your own problem. If you want to run high in the water you need some amenities on/in the prop to do that. Most standard alum props are not designed to run high. So jack her back down to where Jestor said, at least for starters.
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All my life the horizontal plate just above the prop was referred to as the "anti-cavitation" plate. We just didn't pay attention to what we were saying. I got on here and started to see the term "anti-ventilation" used and started to think about it. Which was correct? Ventilation is trading ambient air for ambient water near the surface. The plate helps to keep the two separated. Sounds reasonable.

Cavitation, as spike said is air developing around the blades due to a low pressure area associated with the prop and how it rotates through the water. The air bubbles are pulled out of the water via the vacuum created by the low pressure area around the blades.

I had one boat with real cavitation damage to the prop. It was a '71 Chrysler 85 hp with the standard issue prop. The erosion, on the front side of the blade just behind the leading edge not only ate through the factory paint, but also etched into the alum. The blade had a funny reverse curve in that area and apparently caused it.

Frank A. over in the Chrysler/Force sections has pictures of several Chrysler props and at least one of them shows this reverse curve. I never saw that in a prop before or since.

Mark