The prop I need doesn't exist

[oldslowandugly]

It may well be worn, it came with the motor. I'm sure it's shaved the beards off a lot of clams in it's day.

 

[Texasmark]

NEVER remove pitch, you can increase pitch (Efficiency) but you can not drop pitch without loosing bite.
Also, a 15pSS should turn roughly the same rpm as a 17p alum. That said, The ss prop cup is likely worn substantially. To properly test pitch, you need to stay with same design, only change pitch, not material or manufacturer.

Nice to hear from you again. Been awhile.

Couple of items here I question:

What difference in "bite" does it make which way you go other than obviously more pitch makes for more "bite" since you are moving the prop forward at a slightly faster rate per rpm.

To wear the pitch off a SS wheel you have to have a lot of miles on it or spend a lot of time grinding sand. Opinion!

Agree on the "To properly test pitch, you need to stay with same design, only change pitch, not material or manufacturer".

 

[oldslowandugly]

[FONT=Verdana, Arial][FONT=Verdana, Arial]So today I went fishing and made some observations. This Whaler 15 will hold on plane at 3000 rpm, but cruises best at 4000 or so. At 4500 it's going pretty fast, and at 5000 I don't let my Grandson go that fast. At that point the motor is maxed out and thankfully so. At that speed, wind, waves, current, nothing slows this baby down. At 4000 rpm and above the bow wave is at the 2/3 point of the hull length. At 3000 just barely planing it is at the mid point. No GPS yet, maybe tomorrow, then I can get some honest speed numbers. I know in a little boat like this with such little free-board any speed seems exaggerated, but other guys in boats have commented "man that thing moves!". Maybe I'm just getting old, but I'm curious to know for sure. Also my Evinrude factory manual says to get the rpm's right in the middle of the range, which is where I am. [/FONT][/FONT]

 

[Texasmark]

Well pops, time to spend more time fishing with your grandson and less time worrying about what needent be worried about. Wink!

 

[walleyehed]

Nice to hear from you again. Been awhile.

Couple of items here I question:

What difference in "bite" does it make which way you go other than obviously more pitch makes for more "bite" since you are moving the prop forward at a slightly faster rate per rpm.

To wear the pitch off a SS wheel you have to have a lot of miles on it or spend a lot of time grinding sand. Opinion!

Agree on the "To properly test pitch, you need to stay with same design, only change pitch, not material or manufacturer".

Good to see you on here too!
When you re-pitch a prop down, in order to properly set the geometry you have to literally cut the blade loose from the hub and re-attach at a lesser angle of incidence so you don't have the center section near the hub pulling more water than the outer third of blade. There is a formula for root pitch to tip pitch that has to be taken into consideration during the design process. We ran into this issue with the prototype Apollo in testing. It was an easy fix but takes time to recalculate and re-mold. If you just flatten out the outer half, even though you may keep the cup (which takes some hellish work) the prop will never bite as good because you have effectively removed the "progressive" blade geometry. You can't trim much without blow-out, and even though you may get more RPM, your slip ratio will go up substantially, and you have just set yourself up for cavitation burn. Think of how the best props we have are designed.....Thin profile, not only trailing edge cup but tip cup as well, which helps create parabolic rake and a true progressive design with low slip numbers. These items are added in certain amounts to create the best combination and some manufacturers (Getting very popular) are using the same blade attach point for 2 different pitches, 1 is a great prop and the next pitch up is just flat amazing.

 

[Texasmark]

Good to see you on here too!
When you re-pitch a prop down, in order to properly set the geometry you have to literally cut the blade loose from the hub and re-attach at a lesser angle of incidence so you don't have the center section near the hub pulling more water than the outer third of blade. There is a formula for root pitch to tip pitch that has to be taken into consideration during the design process. We ran into this issue with the prototype Apollo in testing. It was an easy fix but takes time to recalculate and re-mold. If you just flatten out the outer half, even though you may keep the cup (which takes some hellish work) the prop will never bite as good because you have effectively removed the "progressive" blade geometry. You can't trim much without blow-out, and even though you may get more RPM, your slip ratio will go up substantially, and you have just set yourself up for cavitation burn. Think of how the best props we have are designed.....Thin profile, not only trailing edge cup but tip cup as well, which helps create parabolic rake and a true progressive design with low slip numbers. These items are added in certain amounts to create the best combination and some manufacturers (Getting very popular) are using the same blade attach point for 2 different pitches, 1 is a great prop and the next pitch up is just flat amazing.

Alright. Thank you. If anyone knew you surely would.

 

[oldslowandugly]

Finally. GPS says 28mph with 2 big butts and lots of fishing gear. 30mph just myself traveling light. This is an average of 2 passes, one going against wind and current, the other going with, Wind/current made a 2mph difference.

 

[Texasmark]

Check out: www.go-fast.com/Prop_Slip_Calculator.htm.

Plug in your lower unit gear ratio....how many turns does the engine make for the prop to make one. Probably going to be around 2.3. Insert whatever it is like 2.3 as an example, forget the :1 part.
Plug in your prop pitch, 15 or 17 whitchever you're using.
Plug in your 5000 rpm.
Punch the calculate button for theoretical speed. This is the speed you would travel if your prop were turning in a solid, like a screw in wood.
Leave that there and skip over to your actual speed.....28 or 30 which ever one you want to use.
Punch the calculate button for prop slip. Since the screw is turning in a semi incompressible solid and not a solid, this is the percentage you are missing out on. The bigger the number the sloppier your boat performs.
If you are in the 10 give or take region you are a hot dog.
A 15 give or take and you are getting sloppy.
Over 20 and you might as well go home.....grin...little pun here.

Run the chart and get back with your numbers. If they come back low then you are getting about all you are going to get out of this combo. If high then you have room to improve, setup, prop pitch and type prop. I just googled your boat and with the load you mentioned, at 5k rpm, I don't think you are doing all that bad.

Just for grins, engine turning 5000 with a 2.3 for example, prop speed is 5000/2.3 = 2174 rpms. Taking the 15 that's 15" of forward movement per rev = 2174 x 15 = 32610 inches per minute. Taking a statute mile at 5280 ft. and per hour, you first change your inches to ft. = 32610/12 = 2717 ft per minute, or 2717/5280 = 0.51 mile per minute x 60 minutes for miles per hour = 60 x 0.51 = 30.8 mph.

So, if you are getting 28 with your 15 then 28/30.8 = 9.4 % slip.

That sir says you are geeting all the whoopie out of that rig that you are going to get at 5000 rpm and your current 15" prop.......as I see it. On the calculator, it basically does what I did real fast and it's accurate. I've used it for years and never let me down.

Getting back to your earlier comment, why not just leave well enough alone? Well, why not?

 

[oldslowandugly]

I worked it out-slip factor 9. Methinks my biggest problem is too much crap on board, but this is a serious fishing boat and you know how that goes. I believe I will leave well enough alone for now and just go fishing. Thank's Mark, that calculator is incredible and I learned a lot from you. You have my gratitude Sir!

 

[Texasmark]

I worked it out-slip factor 9. Methinks my biggest problem is too much crap on board, but this is a serious fishing boat and you know how that goes. I believe I will leave well enough alone for now and just go fishing. Thank's Mark, that calculator is incredible and I learned a lot from you. You have my gratitude Sir!

My pleasure sir.

 

[oldslowandugly]

Ok, let me tell you what happened before this topic gets too stale. I took my 15" SST to Lawrence Johnson Machine Works in Freeport NY and asked them to change the pitch. They said they don't do that for outboard props, but they can cup it. I explained my problem and they said the cupping would make the 15" act like a 16" pitch. They were correct. Now my max rpm is 5600, it planes easier, and the motor sounds quieter. WOO HOO! Now I will take the 17" prop and have them uncup it so it acts like a 16". Much thanks to all who contributed, I learned a lot.