Impellers: Aluminum vs. Stainless

[Lil' Blue Rude]

Figured I'd start this thread up to show some difference between the 2 and to help explain where I get my sharpening method from. Here's a picture of a stainless and a aluminum 3 blade 6 7/8 impeller, neither one have been sharpened. The stainless is on the left and the aluminum on the right. First thing you will notice is the stainless is thinner then the aluminum. The next thing you will notice is the leading edges are profiled differently. The stainless impellers leading edge tapers a little farther down then the aluminum and the aluminum has more material on the bottom side of the impeller. Since the leading edge of the stainless tapers a little farther down this allows it to get a little better bite in the water and that is why they have a better hole shot and perform a little better the aluminum impellers do even though they are a heavier impeller. The same thing goes with jet skies and there impellers. If they want to tune their impellers for a better hole shot they have their impellers repitched on the leading edge for more bite to improve hole shot (same thing as sharpening the top to bring the leading edge down a bit) or if they have too much bite they remove pitch by bringing the leading edge up. ( same thing as sharpening on the bottom side of the impeller like OBJ instructs you to). Most of the time I see better hole shot and 1mph gain on top end with a stainless impeller.

 

[rktman]

Good info, thanks.
I'm pretty sure if I set my impeller down on a flat surface the center part would hit first. Is this a wore out impeller?
I've tried to find pics of how you sharpen your AL impellers, do you have any more good close ups?

 

[SaltyGhost]

When my aluminum impeller wears out I will be replacing it with a stainless one. They do cost more but they are vastly superior in the wear department. The increased strength of stainless over aluminum allows the blades to be thinner too. The Yamaha 150/110's come stock with stainless.

 

[flatboat]

Lil Blue Rude
...You know you must be a mind reader .... I was trying to make up mind to buy a stainless impeller . I have a merc 3 cyl 59 ci that has been tweaked a bit w/ a medium 6 1/8"outboard jet pump .After running it a while to get to know the limitations ,now I have more questions ! Curious about 4 blade or 3 also, if the leading edge can be repitched in jetskis , how about in outboard jets ? What about repitching the whole thing. thinning blades or cupping ? The reason I ask is , I can easily hit 6400 rpms with an aluminum impeller, and can easily cavitate it if I nail it trying to get up . space is right in the sleve , height is right on the money . This was my first experience with a jet ,thought i'd go with what was recommended for it . now its time to experiment on performance .

 

[Lil' Blue Rude]

I've never really messed with trying to have a impeller repitched other then the little bit I do when I sharpen them like I do. I did send one off and it was butchered to the point I couldn't use it by a prop guy that said he did jetski impellers too.
flatboat I'm guessing you've got your motor upgraded with 60hp carbs, exhaust, reeds and maybe some porting? If so you needed the large series pump with a 6 7/8 impeller. Guessing it started out as the 40hp on the smaller mid. The stock 60/40 mercs come with the larger pumps. They do make a 4 blade 6 1/8 stainless but the one I tried sucked. It was on my built 40/28 omc and I went from around 40mph to 35mph with the same hole shot if not worse with the 4 blade stainless vs my 3 blade stainless. In my opinion the 4 blades from Wooldridge are for grinding gravel in Alaska, not going fast. The 4 blade should bring your rpm down but there should be some way to get your motor to hook up with the 3 blade. If I was going to have a impeller done I'd probably send it impros.com If you do I'd send them a liner to go with it so they can keep the angle and size true. Trailing edge will effect your top end rpm and the leading edge will effect your holeshot. You might notice some impellers are marked as "CB" like (6 5/8" CB) those are cut back impellers. They've had the trailing edge cut down to allow for more rpms.
What kind of speeds are you getting from your boat? Have you tried dropping the motor to see if that stops the cavitation? I'm sure your probably over powering the 3 blade aluminum but it's worth a try. Just a 1/8 to high can cause them to cavitate out of the hole.

 

[flatboat]

well its been modded a little lightened flywheel , reeds and body ,porting, drilled lower and 60 carbs . that motor runs pretty good with a prop 59.9 . on a 16x56 Edge . With a pump it will spin so fast on holeshot it cavitates so I have to feather it. Tried dropping it down just made more spray . once its running its fine I can trim it up with no cavitation . the nice young man at Outboard jets said had to run the 6 1/8" one. I guess I should have gotten the stainless impeller to begin with but I thought i'd try it with an aluminum first ... well, that thing will let my motor rev to 6400 rpms in a heart beat. With 2 big fishermen and all the crap we carry +12 gallons of fuel I get 32 mph at 5900 rpms . it will go faster ,just not fond of 6400 rpms . it aint gonna stay together long at that rate . that's why I was looking into the stainless to slow the rpms down a little thanks for the info about impros I will contact them for sure

 

[BigTerp]

Most of the time I see better hole shot and 1mph gain on top end with a stainless impeller.

I replaced my aluminum impeller with a stainless one this summer after destroying my foot on a rock. This is on a 1994 Johnson 50/35 that has 30+ hours after a rebuild. I noticed almost exactly the above. I gained 2-3mph on the top end and the seat of my pants noticed a MUCH better hole shot. Seems to run a bit better with a load as well. I also replaced my reeds with Chris Carson performance reeds at the same time. Probably why I noticed an extra mph or 2 compared to what Rude typically sees. All in all a definite upgrade!!

 

[flatboat]

Big Terp was that a 3 or 4 blade impeller?

 

[BigTerp]

Big Terp was that a 3 or 4 blade impeller?

3 blade

 

[airbornemike]

If you mod a 40hp to a 50hp do you have to change the pump or impeller?

 

[Lil' Blue Rude]

If you mod a 40hp to a 50hp do you have to change the pump or impeller?

Depends on the motor but probably not. Most 40-50hp motor use the medium pump with a 6 1/8" impeller.

 

[airbornemike]

If you mod a 40hp to a 50hp do you have to change the pump or impeller?

Depends on the motor but probably not. Most 40-50hp motor use the medium pump with a 6 1/8" impeller.

Thank you

 

[Lil' Blue Rude]

I checked with impros and they won't mod a impeller for a outboard jet. I'll let you know if I find anyone that will.

 

[flatboat]

airbornemike
a tweaked 40 puts a little more hp ,or so I've been told . all the mods are to parts that attatch to or in powerhead nothing done to lowerunit .... Lil Blue Rude that's what i'v found out as well about tweaking the impeller