Alum vs. SS w/numbers

[MikDee]

Re: Alum vs. SS w/numbers

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

Ok, so then according to my rough paper calcs. I get 48.08mph at that rpm, minus 10% slip = 43.28mph, or 20% slip = 38.46mph, or 30% slip = 33.65 mph
Do you think 30% slip is normal?

Or, is there something else going on here???

For the 17" prop, I get 49.06 mph, minus 10% slip = 44.15 mph, or 20% slip = 39.25 mph, or 30% slip = 34.34mph
Here we have a little more then 20% slip

 

[Tail_Gunner]

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 $$$!!

Yes it is fun Tony..Just a thought here, if you look for another prop in the futher look for a big eared prop..or a pusher prop. Similar to a toon prop.If you compare all those props that 17 should have a larger flatter blade with less rake effectivly pushing it harder..

 

[Tail_Gunner]

Re: Alum vs. SS w/numbers

Ok, so then according to my rough paper calcs. I get 48.08mph at that rpm, minus 10% slip = 43.28mph, or 20% slip = 38.46mph, or 30% slip = 33.65 mph
Do you think 30% slip is normal?

Or, is there something else going on here???

For the 17" prop, I get 49.06 mph, minus 10% slip = 44.15 mph, or 20% slip = 39.25 mph, or 30% slip = 34.34mph
Here we have a little more then 20% slip

It might be Mike..It would probably take twin drive's with high HP to lift that hull up..thats a lot of boat there

http://findarticles.com/p/articles/mi_m0BQK/is_5_7/ai_91085604/


Grady-White has always preferred powering its boats with outboards, although the Offshore 241 model was offered with gas and diesel engine options. With a single 200 hp outboard, about the smallest engine I would recommend, performance is marginal with a full load of fuel and four adults. The 225-hp Yamaha was a popular choice for years and engines in this range can cruise at 22-25 mph with top speed of over 30 mph when not overloaded. Twin outboards of 115 to 150 hp each are a good choice.

 

[hwsiii]

Re: Alum vs. SS w/numbers

Tony, my program says you should be using a 17" prop for your boat and it should be doing about 42 MPH, and I believe it 100%. BUT, MikDee is 100% RIGHT, you are showing HUB slip starting at 3,800 to 4,000 RPM and it just gets worse as you give it more throttle. That is why I think you quit showing any numbers for the stainless at 4,000 RPM because it was obvious in your mind that you could push it to too high RPMs, and you were disappointed. I don't know which hub you are using, but I know that one of them has changed their guidelines and is now recommending that you torque the hub to over 80 ftlbs, I just can't remember which one.
Your THEORETICAL prop slip for both props that you pushed past 4,000 RPM show a rapid increase in SLIP between about 3,800 to 4,000 RPM, from there it starts increasing. And props just don't work that way on normal boats, as RPM increases the Slip goes down, NOT UP. There is not enough aluminum deformation in a prop below 45 MPH to cause an increase in prop slip from that as there is less than 8 pounds per square inch of pressure applied to the aluminum from the thrust coefficient. The 4 blade shows the most increase in Theoretical slip by an increase of almost 50% slip, as it increases from 21% to 31% in only 1,000 RPM. It is very easy to see on the True Slip Percent chart where your hub starts slipping at about 4,000 RPM.
I ran some numbers in my program and then I converted them to charts, so everybody can see what happens as well as I ran some individual charts for each propeller showing Theoretical Speed versus Real Speed. I decided to also chart some Propeller Curve and Motor Curve charts, which I just do for friends, so you could see what is theoretically the difference between your Torque and HP output from your boat and motor combination and the power required for each individual propeller that you are trying, sorry I had to use my outboard power curves for the motor as I can't find my inboards right now. When the two curves meet in the Chart it will show you that motor torque and HP and the individual propeller power requirements have equalized and met each other and it shows the RPM required to do that. The distance between the two curves shows you the reserve power available from your motor to give you an idea of how fast you will be able to increase speed at any RPM setting to a higher RPM, as you look you will see that the stainless prop gives you the best advance speed increase. I also included a Prop Comparisons with Real Slip chart as well.

Chart 1

Chart 2

Prop Comparisons

H

 

[FreeBeeTony]

Re: Alum vs. SS w/numbers

WOW.....that'a alot of data!!!!

So it does show that the 17" is best suited right now..........and I don't understand prop slip all too well but, I really don't think that the hub is slipping.

Thanks for all the work you put into that, I will look at it in more detail tonight.

 

[QC]

Re: Alum vs. SS w/numbers

The hub is not slipping. . . .

With the plastic hub, there'd be HUGE evidence of even one revolution of slip, let alone 1000s, it would be a perfectly lathe turned cylinder with no shoulders or the inside splines would be non-existent in a nano-second. There are no other options and it would be immediately evident on removal and installation and it wouldn't go 2 MPH let alone 34 . . .

And Mik, please take 45Auto's attempt at debunking the SS is hard to spin thing seriously. There is no way that at 250 shaft horsepower, with the propellers themselves spinning at better than 2000 RPM, that the difference in load due to the weight difference between the SS and Aluminum could even be calculated. I am thinking 167,232 decimal places before a digit other than Zero would even show up . . . and if it did you'd also have to calculate back in the lower resistance due to the thinner blades. If it was harder to turn why would stainless usually be faster? Yes the lack of flex does cause the slip % to go down, but regardless, the horsepower is still being applied to the hub whether it is SS or Aluminum. The hub doesn't know or care what happens after it transmits the power from the shaft to the blades. All of that power is transmitted either way.

 

[MikDee]

Re: Alum vs. SS w/numbers

QC, I don't know if you ever had experience with older boats, but I've had both a slipping hub, and a slipping prop, though not on the same boat, and sometimes it's not "Cut, & Dry" as you say "Going 2 mph at 1,000's of rpm". On my old 72'- 18' Well craft Airslot, with a 165hp 6cyl Mercruiser, all was fine with the usual load (2 adults, or 1 adult & 3 kids), till I put a 1/2 a dozen good size adults in it, then the engine coupling would slip,,, once back to it's normal load no problem. Then I had a similar experience to this on another boat with a defective prop hub, so I've been there, done that!

Besides, Tony has some kinda different hub system? I don't even see splines in the props he has pictured? It appears to be a rubber bushing, on a brass collar with splines, that needs to be torqued up a certain amount, to work properly, if I'm understanding it right? Who's to say it can't slip a little

As far as Aluminum to Stainless, it appears to me it's like a heavy flywheel, compared to a light flywheel on a track car, Aluminum will spool up quicker, but flex under load, and deliver less torque, and SS will be harder to spin, but rigid at all times, and deliver more torque! Add the load of boat weight, & the force it takes to drive a prop (push/pull) it through the water, and I'm sure more power is needed for a SS then Aluminum, that's all I'm saying, Sure the results will always be better with SS because there is very little flex, and the blades are thinner. When planing at WOT, I'm sure the flexing, & slipping, of the Alum. prop, cancels out the extra weight of the SS prop, but a heavy boat with more of a displacement type hull, rather then a planing hull, like this, creates a lot of variables

 

[QC]

Re: Alum vs. SS w/numbers

Besides, Tony has some kinda different hub system? I don't even see splines in the props he has pictured? It appears to be a rubber bushing, on a brass collar with splines, that needs to be torqued up a certain amount, to work properly, if I'm understanding it right? Who's to say it can't slip a little

It's Merc's FloTorq system and it cannot slip without evidence. That's the point, it is a plastic removable hub, and if it slips, it won't work ever again. He is changing it when he changes the props so would see any damage. They bugged the heck out of me when they came out, but once you get used to them they're OK. It just can't be slipping AND be reusable.

The large plastic piece fits tightly into a corresponding female "bore" in the prop. The four slots on the inside match the four ridges in the brass coupler to the left (that's where the splines are). For a Flo-torq hub to slip those four slots need to be gone or the rounded shoulders need to be gone. If it looks like it does above it cannot be slipping.

First time I saw one BTW I was changing a prop in the water in the dark for a friend. I had the large plastic piece in backwards. It's tapered so it cannot work backwards, so I gave up. Threw on my spare and we went back in. The next morning with some light, it was all clear. Trust me, I understand your confusion.

I'm sure more power is needed for a SS then Aluminum, that's all I'm saying,

There is no more power available though and yet the boat goes faster. Ultimately the load on the prop shaft is the same. Max power.

The point is that newbies or less experienced boaters may come to the conclusion that SS is bad, or that it is harder on the drive system. With the exception of impact damage that can't be true. Similar with the hub thing. Poor guys may be going off thinking that they have a failure when their tach is off or their boat is just plain heavy (Tony).