Pitch verses Cup

[mmainelli]

Does cupping add to the rated pitch of a prop? Would a cupped 17P still be a 17P if the cupping were ground off? I read that cupping was primarily for "bite" in turns and rough water, and did not affect pitch. True or false? Would a plain 17P have the identical blade countour as a cupped 17P?

 

[WillyBWright]

Re: Pitch verses Cup

Pitch includes cup on props manufactured that way. Removing cup will essentially reduce the pitch a little. Following that logic, I'd assume that a 17 is a 15 with cup. So a rebuilder would be using a 15 pitch block to shape it before cupping (for illustration, don't quote me on that). Besides adding pitch, the blades hold the water better preventing ventilation.

 

[Dhadley]

Re: Pitch verses Cup

A 17 cupped and a 17 non cupped of the same manufacturer and style should have the same blade shape. Until you get to the trailing edge and tip. I believe cup adds bite (reduces slip & increses efficiency) and as you go further around the tip, to a point, it adds lift. Lots of folks say it adds to the actual pitch. I dont necessairly buy into that. If it added to pitch then at some point a 17 would be so efficient that it would have negative slip but would work out like an 18 pitch. I havent run into that yet.<br /><br />The kicker is that it's really hard to measure a prop's pitch in the field. Especially todays props with progressive pitch. So we rely on what's stamped on the prop. <br /><br />Then think about marketing. When Rakers showed up OMC wanted them to act like a larger prop. IE -- get the slip % down. They wanted a 24 to run as good as a 25 or 26 of other brands. Kind of a "our 24 is better than your 25" attitude. It backfired. As bass boaters and performance guys sat around the conversation became "Wow, you can only turn a 24? I'm turning a 25! You got a weak motor dude!" Guys like to talk about how big a prop they can turn. Bigger is better, right?

 

[Texasmark]

Re: Pitch verses Cup

I think the rule of thumb is 100 rpm drop for identical props with one being cupped. (But you get a performance improvement with that loss) You could probably get this comparison with aluminum props as I am noticing more "standard" type aluminum props cupped and my experience with them is that they tend to cater to the general purpose market.<br /><br />But when you attempt to compare a stock aluminum with a stainless steel, which are usually high performance, you are comparing apples to oranges. You have hi rake, thin blades, progressive pitch, cupping, thick trailing edges for strength, reduced bending, highly polished finish for reduced drag and all that stuff. <br /><br />You have to stay in the same type/mfgr to compare props. Go to the top of this section and brouse through the testing that Kenny did with a couple of dozen props. If you want to know what the props look like just www the prop name in and you can see what they are all about. And if you want to know what all the terms mean that I mentioned either go to the Merc or Michigan wheel sites for your answers. I'm sure OMC has a similar digest.<br /><br />HTH<br /><br />Mark

 

[Silvertip]

Re: Pitch verses Cup

Most prop calculators tell you add 1-inch of pitch to the rated prop pitch if the prop is cupped. In other words if you are calculating theoretical speed using a cupped 15P prop, you would enter 16 in the pitch box. 15 if were uncupped. Whether or not that actually translates to 1-inch of pitch is questionable but it apparently has a fairly significant effect or it would not be considered by those who develop create prop calculators.

 

[gatorred]

Re: Pitch verses Cup

if a 25 has cup work done is the tip of blades done top 1/3 and shapped tip blade's to,ok he put it in a mercury box with prop type's and chart on back need four blade trophy plus was close to what i had