So i bought a 21p 3 blade prop today, not impressed at all

[jokergerm]

I am kinda stumped here, i went out and bought a 21p 3 blade aluminum prop today, swaped it out from the high 5 19p.

The funny thing is my rpms only dropped 100. but i lost 5mph.

I can only go 44 with the 21p.

for some reason my boat likes the heavy stainless steel high 5 19p. I can go 49.4 with it.

So i am taking the prop back tommorow.

I am really suprised, i thought that the RPMS would drop alot more than they did, and the 3 blade cavitates like a mother.

Im sure some one can tell me why this is, i just had to try it for my self.

 

[steelespike]

Re: So i bought a 21p 3 blade prop today, not impressed at all

Not a prop pro but going from stainless to aluminum is a step backwards
in effeciency generally all things being equal the stainless will resist ventilation better than aluminum and will produce more speed.Just thinking logically the high 5 will out perform what appears to be a generic 3 blade aluminum.
I believe the high 5 not only pushes better but raises the boat as well.

 

[MikDee]

Re: So i bought a 21p 3 blade prop today, not impressed at all

I find this hard to believe? But, sometimes you may lose some with the wrong brand prop. If it's a Michigan Wheel, I might suspect it, but I prefer a Mercury Black Max Aluminum, they've always worked well for me. They're usually smaller in diameter, & blade area, and made for speed, whereas the MW is made more for heavier boats, & power. Mercury props usually work better on Merc. outdrives, not to say other brand props are not better, but there's a lot of variables to consider in prop design.

 

[Dhadley]

Re: So i bought a 21p 3 blade prop today, not impressed at all

Like Steel says, you went from a prop with lift to one without. The loss of speed and the venting shows that. Technology costs money and the prop you bought costs about 1/2 of the one you had.

The prop you had was intended to run a fairly high X dimension. The one you bought is meant to run fairly low. It doesn't appear that you lowered to motor to put the new prop at or near where it was designed to run.

You probably can't use as much trim with the new prop either.

 

[K.P.]

Re: So i bought a 21p 3 blade prop today, not impressed at all

You may not be impressed but you learned what makes stainless props so great. You can't switch from a stainless prop to an aluminum prop of a larger pitch and expect an increase of speed or performance. Just increasing pitch does not automatically mean you go faster. There are a lot of other factors in the props design and material than just pitch.

For example, aluminum flexes and stainless doesn't. At several points through the power band, the aluminum will change shape and pitch causing significant loss of performance. A stainless props shape is consistent from hole shot through full throttle. As long as it can keep grabbing water, it will keep moving. But that's where stainless props change in their performance ability. Some stainless props just start churning water after the RPM exceeds it's ability to move the water through it.

The strength of a stainless prop enables the designer to create more radical shapes in the blade like increased cupping or more rake angle. Those shapes literally lift the boat further out of the water so you have less of the hull dragging along the surface which slows you down. I'm not sure if you noticed the spray that comes off at the sides of your boat. I'm sure the spray was further forward with the aluminum prop than the stainless prop which means there was more hull in the water slowing you down. When I upgraded from 17 pitch aluminum to 15 pitch stainless, the spray went from being off to the side of the drivers consol to behind the rear passenger seats. Less hull in the water means less resistance which equals more speed.

I know you want to go faster. You would have been better off going with a 19 or 20 pitch 3 blade stainless with a lot of rake angle or progressive rake. The lower pitch will maintain good hole shot. Increased rake will lift your boat higher and get you moving faster.

I-Boats has excellent infomation on how props work. I attached the link below. Scroll to the very bottom of the page on the left.

http://boatpropellers.iboats.com/

 

[DeepV]

Re: So i bought a 21p 3 blade prop today, not impressed at all

How much cupping did the aluminum prop have?

I have changed between cupped and non cupped and there is a night and day difference.

 

[tmh]

Re: So i bought a 21p 3 blade prop today, not impressed at all

Joker, you need to re-read your thread from a week ago about props and performance. To be honest, i'm not sure how you could have gone out and bought a 21"P ALUMINUM prop after that other thread. IF you're looking for a cheap prop that far underperforms a high five but maybe is safer in rocky, shallow waters THEN some may have recommended that prop. However, for performance you're looking for, SS is the only choice - then you need to look at other factors for various SS props.

As has been explained, you lost speed due to many factors, but one big one is more drag from the boat sitting heavier in the water - not lifted like with the H5.

As was said in the other thread, the high Five is a high tech, well made, expensive prop that many folks swear by, especially for water sports. You may get a few more MPH out of a high tech 3 blade SS prop - 20" or 21" and sacrifice some on the low end vs. the High FIve. You're talking $350 and up for this type of prop - some are a lot more than that.

Prop testing is fun if you like to learn what makes your boat happy. It won' be easy at all for you because you started out with one of the best props so most changes will be a failure compared to the H5. Think how fun it would have been if you started with that aluminum prop then magically transformed your boat with the H5! Likely the previous owner found that out.

 

[QC]

Re: So i bought a 21p 3 blade prop today, not impressed at all

I am with tmh and others here dude. You went backwards. Aluminum props flex and their blades are thicker, both bad . . . Oh, and forget weight, it is just not a significant factor AT ALL when you compare it to the efficiency benefits AND the load of twisting a prop through the water at 3000 RPM. Spinning that thing takes very little extra effort compared to a lighter prop when you add in the inertia that is also included when it is spinning. The weight concern is commented on often, but I guarantee there are very few race boats that run Aluminum because they are lighter

 

[jokergerm]

Re: So i bought a 21p 3 blade prop today, not impressed at all

Ok i got it, im a big dummy. I just had to see for myself. Im going to stick with the H5 and be happy with it, and this winter i might to some motor work. Ill stick to what i know. LOL

I can do engines. Im just new to the boat and prop world and have been reading alot of about pitch and what not, and figured i could get more speed with 3 blades and more pitch.

thanks for the help

 

[K.P.]

Re: So i bought a 21p 3 blade prop today, not impressed at all

I wouldn't say you are a big dummy. If propping boats was easy, we'd all be happy boaters. I've gone through a lot of props to find what I like best. I've even gone the opposite direction a couple times like you did. I now have 3 different stainless props I use depending on the season and load conditions. The prop that works best on my boat in the summer (15 pitch Ballistic), over revs my motor in the spring and fall. Lower humidity and air density makes boats go faster. The spring and late fall is the only time of year I can get over 40 MPH with a 15 pitch Rapture. That same prop is a Dog in July and August.

If you have the chance to ever try a 3 blade Ballistic in the 21 pitch range, try it. It's unique blade geometry helps with getting a higher pitch prop out of the hole. It's rake angle really lifts the boat and deep cupping lets me run my motor very high. The blades are unbelievably thin too. Yet all these benefits of the prop design are a detriment over 5500 RPM.

Take a look at the below article of various props tested on the same boat. It's amazing how different they can be.

http://www.trailerboats.com/output.cfm?id=1069057&sectionid=315

Consider youself lucky to have such a good prop to start off with.