Propeller Blade Rake on Inboards

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kens
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Propeller Blade Rake on Inboards

Postby kens » Fri Feb 18, 2011 6:37 pm

Does anyone have experience with different props , specifically blade rake, on inboard props?
I am reading that aft rake lifts the bow. Who can confirm/deny this with inboard driveline experiences?
Please, no outboard nor sterndrive opin here.

I have been thru a dozen props looking for that 'majic' wheel and I cant understand why some props just turn out to be duds. I am starting to believe that blade rake on the 'duds' is just neutral rake, whereas my better props have a bit of positive rake.
I am hesitant to invest in a wheel with extreme aft rake.
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

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Mr Hot Rod
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Re: Propeller Blade Rake on Inboards

Postby Mr Hot Rod » Sat Feb 19, 2011 6:12 pm

We removed our propeller today and measured its rake. Here are a few photos :


    Image

    The V1.0 engine ran a used unknown brand 12x15p LH cupped 3 blade Nibral propeller with 5° rake.


    Image

    The improved V2.0 engine was fitted with a new OJ 12x16p LH cupped 3 blade Nibral propeller with 7° degree rake.

We're running a Casale v-drive with 12% overdrive. The V2 engine is putting out significantly more HP and torque than the engine we ran in 2009 (more efficient aluminum heads, hotter hydraulic roller camshaft, ignition and carb tuning).

Below 3500 RPM, both props ran identical MPH with respectable slip numbers. At 4000, the higher rake prop picked up 8 MPH and produced better slip numbers (from 51 MPH with 20% slip to 59 MPH with 10% slip). On our last run in rough water we turned 69 MPH @ 4500 (peak torque for this engine).

We we're just starting to have fun with it but Winter caught up to us. We'll have to retest with the old propeller next season. Reducing diameter by 1" to get the engine into its power band is the next item on the list.

Keep trying props until you hit that magic wheel !

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kens
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Re: Propeller Blade Rake on Inboards

Postby kens » Sun Feb 20, 2011 5:28 am

Paul,
Have noticed any difference in bow trim (hull wetted surface) between the 2 props?
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

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Re: Propeller Blade Rake on Inboards

Postby Mr Hot Rod » Sun Feb 20, 2011 9:23 am

We didn't get much run time on the old prop, but it felt like it was dragging the bow and steering was heavy. The new prop felt like it was providing more bow lift and lighter steering. When comparing the two props side by side on the bench, I thought that the rake difference would be greater but our measurements showed that they're only two degrees apart. If in doubt, measure it !

We'll file this one as a 'keeper' and move on to the next prop ...

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Re: Propeller Blade Rake on Inboards

Postby kens » Sun Feb 20, 2011 9:48 am

I found this tidbit of cupping info on the internet.
I asked 2 different prop shops about cupping the tip for rake. They both agreed.
Attachments
propellerrake1.gif
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

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Re: Propeller Blade Rake on Inboards

Postby Mr Hot Rod » Sun Feb 20, 2011 11:07 am

With so many pages out there on choosing a propeller, it's a treat when you find a nugget or two that aren't included in most pages.

Is this the reference for your diagram ?


Here's another reference :


So ... which propeller in your collection are you going to tweak ??

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Re: Propeller Blade Rake on Inboards

Postby DaveLott » Sun Feb 20, 2011 11:44 am

Ya'll are killing me. :roll:
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Re: Propeller Blade Rake on Inboards

Postby vettepilot » Sun Feb 20, 2011 2:33 pm

DaveLott wrote:Ya'll are killing me. :roll:
Me Too think my head gonna explode!! :lol:
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Re: Propeller Blade Rake on Inboards

Postby kens » Sun Feb 20, 2011 2:51 pm

Mr Hot Rod wrote:So ... which propeller in your collection are you going to tweak ??

I dunno. The one I want to tweak is a brand new OJ Legend 14x18 4blade. It is steep raked prop for a ski-boat. But, it is 1 1/8" bore, and the tweak would be to bore it up to 1/1/4".
At that point I couldnt even re-sell it or anything if it dont work. I might re-sell it now and tweak something else.
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

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Re: Propeller Blade Rake on Inboards

Postby Mr Hot Rod » Sun Feb 20, 2011 3:54 pm

I can feel your pain ! It takes a while but you eventually get over tweaking new, expensive parts. We cut up a really nice set of 4" aluminum exhaust 'S' pipes last week. Also cut off a good chunk of the Glenwood header elbows to make way for 4" OD aluminum collars. It's off to the welding shop next week ...

We can probably sell all our tweaked parts that don't work out at the G5 swap meet !

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Re: Propeller Blade Rake on Inboards

Postby kens » Tue Feb 22, 2011 7:49 pm

I got a wheel headed my way with massaged tips (cupping)
It is a 15x15 4cup that I already ran, just a so-so prop.
He is working it to a 15x14 4cup plus tip cup.
He telling me that 'tip cup' also adds pitch.

If this tip cup adds pitch and changes trim then why wasnt I informed of this before?? :?: :roll:
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

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kens
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Re: Propeller Blade Rake on Inboards

Postby kens » Sun Mar 13, 2011 6:11 pm

I ran the prop with 'tip cupping' it is supposed to raise the bow.
I cant quatitively say it raises the bow, but the bow remained same attitude as power went up from cruise to WOT, whereas previously the bow would flatten down as power went up.
Not a real speed prop by any means, a bit slow @WOT, but here is the kicker........a 60 mile trip offshore cut off 3 gallons fuel in 60 mi roundtrip.
prop guy added tip cup and took out 1" pitch. now it is a 15x14 with cup massaged all way out to the tips.

edit:
this is another 0% slip prop.
at 3150 cruise theortcle speed on 14"pitch is 21.8mph, I got 20.7
@4000rpm WOT theor speed is 27.76mph, I got 28

looks like the bow raised a tiny bit, or, refrained from lowering at least. slip numbers got better as speed/rpm went up. You guys with more HP/RPM/Speed might see bigger gains than I am.
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

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Re: Propeller Blade Rake on Inboards

Postby psychobilly » Mon Mar 14, 2011 5:56 am

That's awesome kens! You're defiantly the wheel guy!!!

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Re: Propeller Blade Rake on Inboards

Postby BruceDow » Sat Jul 02, 2011 4:55 am

Prop Rake definitely was "the charm" for me.

I was running a 12x15, and my WOT RPM were just about perfect, but I had porpising in the top 20% of my RPM range.

Earlier in this thread, Mr. Hot Rod described a neutral rake prop that was dragging his bow down, so he swapped it out for one with more aft rake. (Look for the picture. It's the one with the red paint on it)

He sent me that prop to try.

Putting the props side by side showed a clear difference in the overall geometry. My original prop had pretty severe aft rake. The new one was "neutral". When I next bring the boat out of the water, I'll post pictures of the two props side by side.

And it did the trick. No more porpising. I did not need to modify my hull. WOT RPMs are still almost perfect for my engine spec.

B.
Bruce.

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