Downsides of imperfect impellers?

[Canuckjetboater]

Interesting that temp affects it

Jumping in late here... Those impellers are messed up enough to have that effect, I believe. I would pull the pump again and first take a pliers to the leading edge there. It should be straight (one of them is way bent in one spot on yours). Just even it out and make it straight like the other two. Then I would take a file and clean up that leading edge a bit. You don't want burs on that front edge. Then I would do the same thing on the sides that are against the housing.

The bend will not only cause cavitation, but also may throw your balance off at speed (as that part is going to pull more/less on the water depending on the bend direction). The burs and notches are going to cause cavitation. While cavitation will cause performance issues, note that it also causes damage. Cavitation causes little bubbles in the water when the impeller moves though it. Those bubbles are what cause poor performance (they are air--and the impeller doesn't move air well). But then as the impeller spins, those bubbles run against it and eventually pop. When they pop, they cause a 'rash' over time on the surface of the impeller and weaken it. So, replacing the impeller may be needed, but when I found mine were a bit chopped up (not as bad as yours, granted), I breathed new life into them with my file and my pliers as described.

The smooth path to the impeller is important for that same cavitation reason. If there are gaps or things hanging in the intake path (or immediately outside of it), then that causes turbulence in the water stream, which then makes for cavitation.

As for the scraping on the ring... Cleaning up your outside edge will help a little, but the burr there tells me you are definitely rubbing. That will cause some performance issues. Generally you see more of this when a boat is in salt. But eventually that ring corrodes and expands, impacting the impeller. And it scrapes. If it gets bad enough, you won't be able to start the boat. So that looks like it may be due for a replacement of the ring. Removing the burs from the impeller may buy you a little time, so you can tackle it in the off season.

@madyamaha .....a great reply from @tdonoughue .....and may I add that cavitation is very, very bad for your entire assembly. When using blueprinted propellers for racing applications we had to check tank test criteria as cavitation "bubbles" will erode and destroy surfaces.

 

[Canuckjetboater]

Maybe cooler air temp too? I can see where that would help.

@Babin Farms @BigAbe75 .....interesting thoughts on the potential effects of water temperature. Never encountered that when running high performance engines but they were temperature regulated whereas I assume the Yamaha engines are not or not to the same extent -but - as @Babin Farms states air temperature has a tremendous effect on engine HP and therefore MPH. Colder air (within reason) makes for denser air that combustion engines love - hence all sorts of pre and inter coolers for supercharged engines.

 

[BigAbe75]

@Babin Farms @BigAbe75 .....interesting thoughts on the potential effects of water temperature. Never encountered that when running high performance engines but they were temperature regulated whereas I assume the Yamaha engines are not or not to the same extent -but - as @Babin Farms states air temperature has a tremendous effect on engine HP and therefore MPH. Colder air (within reason) makes for denser air that combustion engines love - hence all sorts of pre and inter coolers for supercharged engines.

I’m no expert on engines. Heck, I’m not even an amateur. I just regurgitate what I’ve read over the years. Lol.

If someone else offers a definitive answer, I’ll try to remember to update my memory banks for future regurgitation of new updated info.

 

[Madcityami]

Just an update: Finally replaced the pictured impeller. Job was easier than expected. As the comments above imply, replacing the impeller did make a noticeable difference. The lake was pretty choppy today, so I was not able to go WOT, but definitely an improvement. The chop was bad enough that I experienced the “intakes out of water cavitation” for the first time...which was kinda cool, but getting tossed around is a bit unnerving for a newbie boater.

There is a slight whine I didn’t notice before. I think I didn’t put enough molly on the splines. I’ll do that when I change the other impeller before the next time out.

I did also order some housing/wear rings, but other than scratches, they do not seem deformed since I’m on fresh water. Any opinions on swapping them out anyway?

 

[Madcityami]

Another thing that is interesting (but makes sense when I think about it)...I had to give the new impeller side slightly more throttle to sustain the same RPM as the side I have not replaced yet. Amazing how inefficient the propulsion gets when the impeller or housing gap is outside of tolerance.