Prop cavitation

[Islandboy1974]

I have recently put a new prop on my Yamaha four stroke 75 hp, to find out that they gave me the wrong thrust washer, I took it off, put the Yamaha prop on it, and now it’s cavitate over 4200.
But I replaced it with Lower Unit last year, with a unit from a 90, but it fits three different engines, I put the original thrust washer back on it, when I just put this new Yamaha prop on it 13 25×17 which goes with the engine, and now wit cavitate‘s, I’m wondering if I have the wrong thrust washer.
This never happened until I put the original thrust washer from the Lower Unit back on.

 

[Chris1956]

Cavitation is when the prop spins so hard that a vacuum is created behind the blade. This vacuum can erode the blades and is unusual.

Ventilation is when a prop suck air in from the surface. It can also pit the blades somewhat, and is quite common, especially in turns. A change in thrust washer is not usually responsible for either. A change in prop can often cause ventilation. Given your HP, ventilation is most likely.

You should observe the antiventilation plate on your motor at planning speeds. perhaps the motor is mounted too high, or your new prop has less cup or none, compared to the old prop.

 

[Sea Rider]

When on plane visually check at which lower leg's plates is the water flow passing by, if it's too close, slight over the AV plate, there's your thrust problem. Will be very notorious on choppy, wavy, windy water conditions and at close fast turns...

Happy Boating​

 

[Texasmark]

I don't see the thrust washer as being a cause for ventilation. It keeps the prop from eating up the lower unit casting. You have some other problem. I have a bad habit of getting "too close" to a problem. Need to back off and see what "else" you changed from when things were working right.

 

[Faztbullet]

How about a pic? Too much clearance between edge of prop and housing can allow exhaust to exit into prop stream.