Prop cavitation

[Darylle]

Hi all
When returning from my last outing in fairly sloppy conditions I noticed we had distinct prop cavitation at low speed. With the drive right in the existing 141/4 x 17p 4 blade was loosing "bite" and would slip with higher rpm and see no increase in speed. I was told once that my existing prop looked "small" So I am looking to go to a 15" x16p 4 blade aluminium to see if that cures my issue. The existing prop is not the rubber bush type and in good condition and the coupler is brand new with no evidence of burnt rubber smell or debris in bilge. I must point out that I have no interest in WOT only slow-fast cruise at best.
Thanks in advance.
Darylle

 

[Creekwood]

That sounds more like a spun hub to me. I think 14.25 is the normal size for an alpha prop so its not small.

 

[Lazy Daze]

Hi all
When returning from my last outing in fairly sloppy conditions I noticed we had distinct prop cavitation at low speed. With the drive right in the existing 141/4 x 17p 4 blade was loosing "bite" and would slip with higher rpm and see no increase in speed. I was told once that my existing prop looked "small" So I am looking to go to a 15" x16p 4 blade aluminium to see if that cures my issue. The existing prop is not the rubber bush type and in good condition and the coupler is brand new with no evidence of burnt rubber smell or debris in bilge. I must point out that I have no interest in WOT only slow-fast cruise at best.
Thanks in advance.
Darylle

Confused... when exactly is the issue happening? And, I assume you mean ventilation or excessive prop slip as you can only observe cavitation by looking at the prop as it spins. But those are oftenly confused terms.

I'm not sure what you mean by: "With the drive right in the existing 141/4 x 17p 4 blade". Do you mean with the drive in the "right" position... meaning all the way down when starting out?

If you've had the boat for a while and this is the first time you've had this issue then the prop size is not the issue. Besides, if it's not the "perfect" size for the boat, it's still close enough not to cause this issue.

I would still pull the prop off (easy and quick) and disassemble the hub to take a real good look at it. If nothing else, it will eliminate that as a variable.

If it's not the prop hub, you're trimming the drive properly, and you're positive it's not the coupler, the only other thing it could really be (based on what you've written) is having some weeds/debris caught on the drive or possibly excessive growth on the drive/prop or damage to the prop.

You DO NEED to be concerned with WOT. It's important to prop correctly to get the engine into it's WOT RPM range for good engine health. It doesn't mean you have to actually travel at WOT, but it's important that the ability is there.

 

[Strecker25]

This happened to us on the way across lake ontario last year. It was nasty out, 3-4' chop on the beam with occasional 5's (observed at the buoys), and the boat was pounding pretty good. It turned out to be the hub in the end, but I have a feeling that ventilating the prop was what did it in. As we went over the top crest of a wave the transom would come over the top, revs would jump and then quickly go down as the prop reentered the water. I have to assume this puts a ton of stress on the plastic hub, and it sounded like I had something rattling in the bilge. I took the prop off at the marina and sure enough I could see shaved edges on both the hubs. New hubs, problem fixed.

Now, you say the prop is not the rubber bush type, does it not have a plastic hub?