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Prop Cupping


racer808

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Can't really find the answer I was looking for.  What would changing the cupping on a prop from .075 to .105 do?  More power higher RPM, lower RPM lil less power?  I am taking my prop off to just have it once overred by the prop shop since we drag through soft sand often & was debating trying a new cupping or maybe pitch.  Currently running Acme 2951, 15" x 10.5" w/.105 cupping. 

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As I understand it, increasing the cup is like increasing the pitch.  There may be some subtleties, but I personally believe that those are esoterics for guys that also found their boat uses less fuel when you wax the bottom of the boat.  IIRC, the guys at ACME told me that increasing that cup .030 would be like adding 1 inch of pitch.  

Can you change the pitch on a pre-existing prop?  I am not so sure of that.  

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Don't hesitate to call the folks at ACME! I found the call well worth my time and the engineer was eager to talk props.  When I spoke with ACME, they explained that .050" cup was roughly equivalent to 1" of pitch.  Going up to .105 cup will give you a tiny bit of more top end speed/less RPM.  Only 1/2" of effective prop pitch, so not a ton...

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13 minutes ago, braindamage said:

It’s not the same as pitch. When I bought a new prop I had the pitch for the hole shot but added cup to maintain the high end speed.

This is correct.  Cup only becomes effective above ~30MPH.  So you can reduce the pitch for board sports, then gain it back with cup for cruising.

Edited by MadMan
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@racer808:  If your prop cavitates, adding cup will help that also.  As noted, I have had good luck adding cup to drop higher end RPM's and to curtail some cavitation.  Acme can run you through the specific gains for the particular prop & they are good to exchange information.

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22 minutes ago, Lees23 said:

What’s the drawback of adding cup?  If there were only advantages, then wouldn’t all props come with aggressive cup?

Maybe price? 

One more advantage is cupping make the blades stronger.

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45 minutes ago, Lees23 said:

What’s the drawback of adding cup?  If there were only advantages, then wouldn’t all props come with aggressive cup?

As Woodski noted, cupping lowers WOT RPMs and, correspondingly, top speed.

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16 minutes ago, John I. said:

As Woodski noted, cupping lowers WOT RPMs and, correspondingly, top speed.

This is the opposite of what Dave Gerr states in "Propeller Handbook".  Basically you can lower the pitch by an inch or 2 for low speed activities where the cup has no effect, then gain it back at cruising speed (or above).

Edited by MadMan
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14 minutes ago, John I. said:

As Woodski noted, cupping lowers WOT RPMs and, correspondingly, top speed.

Wait a minute... I thought THE advantage to adding cup is to decrease RPMS at cruising speed (<30MPH), RPMS decrease, but speed doesn’t change.  

No one said anything about decreasing WOT speed.  If adding cupping doesn’t change your hole shot performance but decreases your WOT speed, why WOULD you want to do it?

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2 hours ago, MadMan said:

This is the opposite of what Dave Gerr states in "Propeller Handbook".  Basically you can lower the pitch by an inch or 2 for low speed activities where the cup has no effect, then gain it back at cruising speed (or above).

Seems like if this is really true every tow boat would benefit from a cupped props. 

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26 minutes ago, Lees23 said:

Seems like if this is really true every tow boat would benefit from a cupped props. 

I believe most, if not all, tow props are cupped, just different amounts.

http://www.acmemarine.com/prop-list_ski-boat.php

A couple years ago I changed props, ACME 537 to ACME 1161.  Both are 13.5 x 16, but went from a .105 to a .135 cup.  My cruise RPM @ 30mph dropped from 3400 to 3200 and there was no change surfing with >4k ballast.  The only reason I made the change was that I acquired the 1161 very, very cheap.

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12 hours ago, MadMan said:

I believe most, if not all, tow props are cupped, just different amounts.

http://www.acmemarine.com/prop-list_ski-boat.php

A couple years ago I changed props, ACME 537 to ACME 1161.  Both are 13.5 x 16, but went from a .105 to a .135 cup.  My cruise RPM @ 30mph dropped from 3400 to 3200 and there was no change surfing with >4k ballast.  The only reason I made the change was that I acquired the 1161 very, very cheap.

You are correct.  I believe I have the 2277 on my boat(17 22 VLX) and it appears to be heavily cupped at .150.  Some people have a theory that different props change wake/wave characteristics, I wonder if cupping effects wakes.  

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Maybe not clear, but I did not mean to imply that adding cup lowers top speed, it simply drops RPM at that speed.  Available power is a key to determine whether your particular vessel can accommodate the added cup so in my case at the time top speed actually increased as the boat stayed in the better power band.  There is a point where the efficiency drops off when you add cup so you have to stay in the sweet spot, just as there is a range for what pitch is appropriate.  Also, prop spinning speed is important which is where I think the Acme 3 blade does a good job of blending speed to blade area so a 4 blade is not as needed compared to the older prop designs.  

Glad to see another Crew member has the Dave Gerr book on the shelf, if you want to excellent propeller literature, I suggest the book.

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On 12/28/2017 at 8:00 PM, Woodski said:

@racer808:  If your prop cavitates, adding cup will help that also.  As noted, I have had good luck adding cup to drop higher end RPM's and to curtail some cavitation.  Acme can run you through the specific gains for the particular prop & they are good to exchange information.

Sometimes I wonder if I am getting cavitation or porpoising.  It's hard to tell, most times there is nothing there, sometimes it feels like the boat is porpoising.  Doesn't sound like cupping will do much.  If anything I would like a small drop in RPM's but I still want the tractor prop I have & wouldn't mind it actually being more aggressive.  I still need to yank it off, who knows it may be slightly off from just dragging it & a tune up will make it right as rain. 

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