Cavitation on top end

I have a Weldbilt 17' long 44" bottom with a Yamaha 40/30 4-stroke jet. It seems like the boat has become slower and at full throttle it seems to cavitate or miss a beat. runs great at all other RPM, smooth or rough water. It is 2 years old, I am not easy on it and push it to the limits on oyster shell, rocks, sand, gravel. probably has less than 100 hours. Does this sound like my blades need to be sharpened?

If so, does anyone have a good link for sharpening the blades on this impeller? I have heard that I won't get much out of a stainless impeller on the 40/30. Do you agree?

I am about to do some maintenance, (oil change, new water pump, ect) and would like to get everything taken care of at once. Am I missing any maintenance items that are specific to a jet pump?

Thanks for your input!

Go to outboard jets website and look under FAQ. They will tell you how to sharpen impeller and you probably need to space it down a little. If the liner is shot order a new one and put your sharpened impeller back. I wouldnt go to a ss, minimal gains imo.

Here's a link to check out. https://forum.tinboats.net/viewtopic.php?f=48&t=36368
The stainless 3 blade impellers hook up better and are normally 1mph faster on top end. I think they're well worth it if not just for the fact they last a lot longer then a aluminum one will. 4 blade stainless are slower and the one I run hurt holeshot some. Both 6 1/8" in my 40/28 evinrude.

If you run in oysters you definitely need a ss impeller. How quick does your boat plane out? I'm about to put a jet on my weld craft 1652 with a 50 2 stroker Yamaha.

Lil' Blue Rude said:

Here's a link to check out. https://forum.tinboats.net/viewtopic.php?f=48&t=36368
The stainless 3 blade impellers hook up better and are normally 1mph faster on top end. I think they're well worth it if not just for the fact they last a lot longer then a aluminum one will. 4 blade stainless are slower and the one I run hurt holeshot some. Both 6 1/8" in my 40/28 evinrude.

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This was my experience when I went from a 3 blade aluminum to 3 blade stainless. VERY noticeable difference in holeshot and gained about 1-2 mph on the top end.

Thanks guys, sure appreciate your help.

Is there any certain brand or place I should be shopping for the stainless impeller? Sounds like the 6 1/8" is the size I need.

The boat runs in the low 20's with 2 ppl and gear, wouldn't mind paying the extra $$ for a little more speed. Holeshot is great. Its a pretty light boat though (looks like .80 gauge aluminum). The bottom has seen better days and is far from smooth (the boat is a 90's model and has had a jet on it the whole time) which im sure isn't helping the speed. Attached are a couple pictures of the rig.

jetdoctor.net seems like the easiest place to get one from. Wooldridge make the 4 blade stainless and I think mercury makes the 3 blade stainless impellers. Get a 3 blade for sure. You should need a 6 1/8" impeller. Probably will want a new liner while your at it.

You may want to also address the bottom. You say it's seen better days? If it's too damaged near the foot of your pump it can cause aerated water to get into your pump causing cavitation. I mashed up my drain plug this past fall, which sits directly in front of my jet foot. My buddy and I straightened it back up and welded up the leaks. The welds were far from perfect. The next time out my motor cavitated like crazy. I took an angle grinder and cleaned up the welds we did. Then took some 220 grit sandpaper and hand sanded the area until it was smooth as glass. All my cavitation issues completely went away. I don't think this is your problem, but while your working on the motor cleaning up the bottom of your hull in front of and around your jet foot will only help.

Thanks for your help. I ordered the impeller and flanged liner today. Will let you guys know how it turns out!

kenter said:

Thanks for your help. I ordered the impeller and flanged liner today. Will let you guys know how it turns out!

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What did it go for?

the SS 3-blade 6 1/8" impeller was about $350 and flanged liner $55 from jetdoctor.net

You might also try some intake fins. I'm able to trim my motor up higher since I installed mine. Little or no cavitation when taking off too.

Your boat looks as if you have a lot of weight towards the front. Add a passenger and miscellaneous stuff, and you maybe be lifting the rear too much. Just a thought.

Riverdog said:

You might also try some intake fins. I'm able to trim my motor up higher since I installed mine. Little or no cavitation when taking off too.

Your boat looks as if you have a lot of weight towards the front. Add a passenger and miscellaneous stuff, and you maybe be lifting the rear too much. Just a thought.

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I found the reason for the cavitation. There is a half dollar sized hole in one of the impeller blades at the top of the impeller (wasn't very visible until I pulled the foot off). Never had any cavitation problems before this recent issue so I'm sure that is the culprit. Otherwise the impeller wasn't near as bad as I expected. The liner had several deeply gouged rings from wear and there was about 1/8" or more of gap between the blades and liner.

I discussed the cavitation fins with the guys at Jet Doctor and they recommended not to use them unless I was having cavitation issues. He also mentioned that it would reduce performance a little.

I like the idea of being able to run the trim higher with the fins. I don't have power trim, so like to leave the lever in the "unlocked" position so when I do hit something the engine will kick up. right now the leading edge of the jet foot is even with the bottom boat (or maybe just a little higher) and the back is about 1.5" below the boat.

Now if I can figure out how to get this stinking impeller to break loose from the drive shaft...

Most of the hours on this boat are from the same river you call home. Must had found a nice sized rock!

Is the impeller stuck to the shaft itself? Or are you having trouble getting the impeller nut to break loose? If it's the nut, try a strap wrench on the impeller to provide some counter force while you work the nut. Might have to take the pump off to get to it that way, but that's how I had to get the impeller off of mine when I first bought it.

Didn't have a problem getting the impeller nut off. The impeller is stuck on the shaft. Since the impeller is pretty much toast I have beat the heck out of it trying to get it off. Going to bring a torch tonight and see if I can heat it up to get it off. Seems like that is the best idea from the research I have done. Also talked to the guys over at outboard jets.

Anyone have any other ideas? It has been on for about a year and a half and has been used in fresh and salt water. Doesn't appear to be corroded.

There should be a plastic sleeve in there that will melt when heat is applied. It is on there for that purpose. It will have to be replaced. Be careful not to get too hot or you may damage water pump impeller on up the shaft.

wshamby01 said:

There should be a plastic sleeve in there that will melt when heat is applied. It is on there for that purpose. It will have to be replaced. Be careful not to get too hot or you may damage water pump impeller on up the shaft.

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Thanks for the tip. I have the spare plastic sleeve that came with the kit. I went ahead and bought a water pump kit. Figured while I was at it might as well put a fresh one in. Also bought some of the synthetic Yamaha oil (the one that the SHO motors use). It only knows 2 speeds, idle and WOT. Should be a little easier on it on the high RPM.

If I had to guess that hole was caused by cavitation burn. When your leading edge gets dinged up or the liner gets chewed up it disrupts water flow causing small vacuum bubbles to form and when those bubble collapse on their self it does so with enough force to slowly erode away at the metal.

it appears to be from impact with something. Will post some pictures of the old impeller and liner once I get it removed.

That would be the first I've ever heard of a impeller getting hole punched in it unless a bolt that holds the pump on backed out and punched a hole on the way out. Boat should run pretty good with then new impeller and liner.