New Prop - Cavitation

  • Thread starter Thread starter Anonymous
  • Start date Start date
A

Anonymous

Guest
Got a new prop recently Bakesan 17 pitch alum. prop for Yamaha 90 4 strk.

Now I get cavitation above 4600 rpms.

Anyone know why? My 15 pitch Yamaha prop and my 17 pitch Yamaha (both aluminum) did not do this cavitation thing.

Even adjusting the trim tabs would not stop the cavitation/slippage. Sell the prop? Can it be fixed?
 
A Fishin C-

What's the diameter of this prop vis-a-vis' the original two? Larger? Smaller?

Does this new prop have any special features?

3 or 4 blade?

Who recommended it to you?

Why did you feel you needed another prop? (Just wondering.)

I'd switch the three props on and off the motor and check the clearance between the prop blades, the anti-ventilation plate, and the trim tab (rear fin) on the motor. Look for any differences in clearances between the old props and the new.

Are there any "burn" marks on the new prop where paint and/or metal has been pulled off the new prop? Where are they?

Does the prop fit properly up into the gear housing hub to avoid exhaust gas escaping up there?

What was the recommended thrust washer arrangement for the new prop? Was it followed?

If the new prop is sucking air or creating cavitation bubbles, there's a reason for it somewhere!

Joe. :teeth :thup
 
That was my thought too. Spun Prop. Could well have happened, even to a new prop/hub. Will act just like cavitation until it totally wears out, then you won't go anywhere fast, or even at all!! :shock: :roll:

Charlie
 
Captains Cat":26pei8q5 said:
That was my thought too. Spun Prop. Could well have happened, even to a new prop/hub. Will act just like cavitation until it totally wears out, then you won't go anywhere fast, or even at all!! :shock: :roll:

Charlie
Click to expand...

I thought of that, too, but my experience has always been that when a hub lets go, it's done, and you can't get more than about 1500 rpm out of it without just reving up off the scale!

Joe. :teeth :thup
 
This is taken from the Boatfix.com Prop site:

Ventilation occurs when surface air or exhaust gases are drawn into the propeller blades. The load on the propeller is reduced by the mixing of air or exhaust into the water steam causing over revving.


Cavitation: The aeration (bubbling) and boiling effect of water caused by creation of a low pressure area. Generally caused by a solid shape (propeller blade) passing through the water, in such a position and speed, that a low pressure area is formed due to the inability to move through the water in nonresistant manner. An example is, a propeller blade that has a rough edge would not cut efficiently through the water, thus creating a low pressure area. If the pressure drops below the vapor pressure, a cavitation bubble will form in that region. These bubbles will collapse when they reach the higher pressure region of the blade. This causes a rapid change in pressure and can result in physical erosion. You may notice burns (erosion) at some area on the face of the blade.

Is there actual burning of the surface of the prop--rough edges etc? If not there is ventilation. This can be the engine too high for that prop, a depth sounder or other hull appendage causing disruption of water flow.

You may need to drop the engine down one hole--but that may increase the drag slightly--and I wonder if it will be the best plan. Also the configuration of the prop--cup and rake, which are not usually measured can increase turbulence around the prop. First check the edges of the prop, and be sure it is spinning true. I don't think it is the hub.
 
Captains Cat":3ibfdevy said:
That was my thought too. Spun Prop. Could well have happened, even to a new prop/hub. Will act just like cavitation until it totally wears out, then you won't go anywhere fast, or even at all!! :shock: :roll:

Charlie
Click to expand...

I thought of that, too, but my experience has always been that when a hub lets go, it's done, and you can't get more than about 1500 rpm out of it without just reving up off the scale!

Joe. :teeth :thup
 
I bought a new prop because I chipped mine a 2nd time and they said it would cost almost as much to fix as the cost of a new one.

It is ventilation I guess cause no visible marks on prop. Too bad because my fuel burn is very low on this prop. I had wondered if the rake/cup could be changed to stop the ventilation. Maybe I need to get a Yamaha prop. Yes Joe it is the one in the picture.
 
Is it possible that the engine is too high for the BSK Pro 17 pitch 13 1/4 prop
but not for the Yamaha 17 pitch 13 1/4 prop?
 
You can look at the ventilation plate on the motor in relation to the bottom of the hull to see what your motor height is relative to the bottom. If you trim the motor such that the ventilation plate is in a parallel plane to the bottom of the boat you can eyeball it to see if the motor is above or below that plane. Generally speaking you would want the ventilation plate even with or slightly above the hull. The reason being when the water separates from the hull it tends to rise up. I have had a prop that was really prone to ventilation when run with a Permatrim due probably to the way the Permatrim affected the flow of water around the lower unit. If it is ventilating, then lowering it could help, but at the expense of increased drag as Bob indicated. Another crude test would be while underway. If there are two of you on board, send the heaviest one to the stern and see if the prop performance changes. While there look at the ventilation plate and see where it is in relation to the flow of the water under the stern. Just shifting weight can tell you a lot on a 22.
 
Prop is now "spun"

It happened to me day before yesterday in Tahsis Narrows. I swapped out the prop at a nearby dock, and then everything was fine.

Prop bought brand new one year ago, less than 100 hours and never damaged or hit.

Doesn't seem right,

any thoughts?
 
Back
Top