Alum vs. SS w/numbers

[FreeBeeTony]

I had an aluminum 14.5X17P 3 blade prop installed, wanted to try a SS and found one on ebay........a 15"..........2" less in pitch which is what I heard the rule of thum was when going from al to SS. here are the results:

PROP 15" Stainless 17" Aluinium.
RPM SPEED SLIP SPEED SLIP
3200 20.00 33.00 27.00 21.00
3400 24.00 25.00 28.50 22.00
3600 26.00 24.00 30.00 22.00
3800 28.00 22.00 32.00 22.00
4000 29.50 21.00 34.00 21.00
4600 did not record 38.00 23.00


Seems like I lost ~4MPH comparable RPM's and WOT shot to 5000+.........was so high I stopped at 5000 and still had a bit more throttle left. I then checked the other prop, thought maybe it was really a 19" instead of a 17"......it's stamped 14.5X17P.

Any comments? Now I wish I had a 17" SS to try!!

 

[FreeBeeTony]

Re: Alum vs. SS w/numbers

data did not transfer well from an excell spreadsheet.

hope you can decipher it.............

 

[junior1113]

Re: Alum vs. SS w/numbers

never heard of that rule (drop 2" pitch going all. to ss) unless its a very aggressive prop w/ lots of cup or going to 4 blade. prop for prop ss to all. there will be very min, change in wot rpms u need a 17 ss. besides a 15 for a 350 mag is way to small of pitch on that setup good luck

 

[Tail_Gunner]

Re: Alum vs. SS w/numbers

There could be more than a few things going on here...Perhaps the easist is the SS is a high rake prop with blades setup for bow lift....and the alum is a big eared pusher which would function better in your situation..Take some pics Tony this could be intresting.

While your at it take a very close look at the ss blade's..does it look like any "body" work has been done..that is is the material all the same in overall smoothness and or are the blade edges all the same..any grinding to take out imperfection's

 

[MikDee]

Re: Alum vs. SS w/numbers

I think the hub maybe slipping on the 15" SS prop, if your revving to 5k, or more, once you put it under maximum load.

 

[FreeBeeTony]

Re: Alum vs. SS w/numbers

I will try to to take some pics today.

I used the same hub from the 17" Al so I don't think it's slipping.

Might even try the 4 blade today.

I will say this.........while cruising @3800RPM w/ the SS prop the engine was definitly under less stress............although I was not happy with the speed I think I may have used less fuel.

I bet the WOT would have been ~5300 if I opened it up.........didn't want to do that though.

What do you guys think? I put a post on craiglist to see if someone want to trade props!

 

[MikDee]

Re: Alum vs. SS w/numbers

I don't get it, there's usually always an increase of speed with a SS prop by design? And usually there's only a 400rpm difference when going up, or down, 2" of pitch? You say, you used the same hub? Is it a flow torque hub system? Could this system be defective in this case?

Anyway, I wonder if there is something called prop "blowout" because the boat is too heavy, or difficult to get up, or keep on plane?

These results blow my mind?

 

[steelespike]

Re: Alum vs. SS w/numbers

While your motor seemed less stressed at 3800 with the ss you were going 4 mph slower I would think you would be using more fuel because it takes longer to travel a given distance.While it takes less throttle to reach 3800;
I think the slower speed eats up any saving. boat mileage is figured using GPH (gallons per hour)only way to figure best cruising speed(mileage)
is to run a specific time at a given rpm/speed. then top up the tank.
There is a balance point that produces the most distance at the least gph.
Best cruising may be anywhere from just on plane to a signifigantly higher rpm.A flow meter simplifies testing.

 

[Tail_Gunner]

Re: Alum vs. SS w/numbers

I will try to to take some pics today.

I used the same hub from the 17" Al so I don't think it's slipping.

Might even try the 4 blade today.

I will say this.........while cruising @3800RPM w/ the SS prop the engine was definitly under less stress............although I was not happy with the speed I think I may have used less fuel.

I bet the WOT would have been ~5300 if I opened it up.........didn't want to do that though.

What do you guys think? I put a post on craiglist to see if someone want to trade props!

1. Post the hub dia along with the pics...
2. Did you notice a noticeable increase in hole shot
3. Polish the SS up highly...can you see any type of wave in the SS indicating it has been reworked. ...There is very little if any wave or imperfection in a stock SS prop.

 

[FreeBeeTony]

Re: Alum vs. SS w/numbers

Went out today and tried the 4 blade 15X16P prop and added the numbers:

PROP 15" SS 17"" ALUM." 16" 4 blade
RPM SPEED SLIP SPEED SLIP SPEED SLIP
3200 20.00 33.00 27.00 21.00 23.50 27.00
3400 24.00 25.00 28.50 22.00 26.00 24.00
3600 26.00 24.00 30.00 22.00 28.00 23.00
3800 28.00 22.00 32.00 22.00 30.00 22.00
4000 29.50 21.00 34.00 21.00 31.00 23.00
4600 XXXX XXXX 38.00 23.00 XXXX XXXX
4800 XXXX XXXX XXXX XXXX 33.00 32.00

WOT with the 4-blade is 4800 but the speed is lower than the 3 blade 17" and the slip appears very high.

Sorry about the data............I can't get it to line-up!!!!!!!!!!!!

looks to me like the 17" is best..........what do you guys think?
FYI.........the same hub was used for all 3 props.

I also took pics of the props for comparison:

 

[junior1113]

Re: Alum vs. SS w/numbers

i dont like any of them #'s how much does this boat weigh. i'd look for an agressive 3 blade 17p ss

 

[MikDee]

Re: Alum vs. SS w/numbers

What the hull kind of a hub system do you have, or use, on those props??? I'd seriously suspect that! The new SS prop looks pristine, and perfect, for your boat

 

[Tail_Gunner]

Re: Alum vs. SS w/numbers

Nothing to surprising here...depending on your monitor the 3 blade alum is a big eared pusher and the ss is a raked prop with fairly good sized ears..Tony stand the props on there ends so all can see the curvature of the blades.

Remeber this..those props are pushing a hull..a big blunt nosed wave busting hunk of fiberglass....A man's best frend in heavy swell's..

 

[FreeBeeTony]

Re: Alum vs. SS w/numbers

So I guess I should just keep the 17" aluminum.........4600 @ WOT is about right and the speed is better. Will probably try to sell the SS on ebay.......

The HUB is the standard "replacable" hub used on Merc's.

 

[MikDee]

Re: Alum vs. SS w/numbers

So I guess I should just keep the 17" aluminum.........4600 @ WOT is about right and the speed is better. Will probably try to sell the SS on ebay.......

The HUB is the standard "replacable" hub used on Merc's.

You do know that it's harder to spin a SS prop then an aluminum, just the shear weight of it, and the fact that it is more rigid. I think your hub system is not up to the task!

That looks like a good prop for your boat, I bet dollars to donuts that it was not spinning at 5k rpm in the water, but your shaft might have been! I would not call it prop slip, but shaft slip. A SS prop won't cut you any slack if there's any weakness in your hub. Why don't you mark the prop, & shaft, and check to see if it moves, before hastily trying to get rid of it!

 

[FreeBeeTony]

Re: Alum vs. SS w/numbers

If the hub/shaft was damagaed/spun it would be evident when I moved it to the other props........don't think there is any chance that this was happening.

 

[45Auto]

Re: Alum vs. SS w/numbers

You do know that it's harder to spin a SS prop then an aluminum, just the shear weight of it, and the fact that it is more rigid. I think your hub system is not up to the task!

Technically, it's the inertia of something, not the weight, that makes it harder or easier to spin. Compared to the load of pushing a boat, there's not enough difference in the inertia of the aluminum and stainless props to have any affect on the hub. If it's slipping on one, it's going to be slipping on the other one.

From the pics the props appear to be about the same profile, so they're going to have about the same efficiency (slip). Basically what has happened is you've dropped 12% in pitch from the 17 to the 15 pitch (15/17 = .88). Magically, your speed at the same RPM has also dropped about 12% (29.5/34 = .87). The same thing but different percentages on the 17 vs 16.

What are you trying to achieve??? Sounds like the 17 pitch aluminum is propped about perfect for your boat if it's turning 4600 RPM at WOT. You're at the HP peak of the motor, so you're not going to get any more speed by wasting your money diddling with props. If you want a better holeshot, you're going to have to give up some speed and risk over-revving the motor. About the only benefit you're going to see in the stainless is better damage resistance.

 

[MikDee]

Re: Alum vs. SS w/numbers

If the hub/shaft was damagaed/spun it would be evident when I moved it to the other props........don't think there is any chance that this was happening.

Yes, it is evident to me, do you seriously think you were spinning that 15" prop to 5k+ in the water? Not! That hub system may work for an average boat, but you've got a whale of a boat, with a powerful Magnum 350, there is a weak link somewhere

 

[45Auto]

Re: Alum vs. SS w/numbers

do you seriously think you were spinning that 15" prop to 5k+ in the water? Not!

BZZZZZZZZ - wrong answer.

He's turning 4600 RPM with the 17 pitch. Dropping 12% in pitch is going to raise the RPM by 12% ......

4600 RPM x 1.12 = 5152 RPM

 

[FreeBeeTony]

Re: Alum vs. SS w/numbers

Appears to me that the 17" Al is best suited at the moment.......

Just thought I would experiment with a SS to see If I would gain some speed at the same RPM and, from what I read on here it sounded like you should drop pitch when going from Al to SS..........seemed to make sense.

Anyway, will try to sell the SS on ebay/craigslist.........

Don't want to waste anyone's time either.............
But, it is kinda fun fooling around with this stuff............except it costs $$$!!