prop rake measurement
 

Am I measuring this right ?

Mercury is supposed to be 15* so even with a angle meter app it`s still 15*
The other seems to be 12*


https://cimg3.ibsrv.net/gimg/www.off...e62f69521b.jpg
https://cimg4.ibsrv.net/gimg/www.off...e5de0122ce.jpg

That is how I measured mine.

You are on a different level than me here so keep that in mind while reading my questions.
Are these new or have you run them already?
If you ran them was there a difference in rpm?
Did all of the blades of each prop match each other?
This may have been answered by the results of a previous question but what made you check in the first place.
Thanks

Not new, at $12k/set new I`m in the used market.
Not sure if there was a question in there but I have cleaver props, it`s not going to be relevant to your bravo stuff.

https://cimg0.ibsrv.net/gimg/www.off...f045ce7a15.jpg

Wow!!! Very Nice!!! I remember a photo years ago I think in "Powerboat Magazine" with the basically the same pic.

I don't think you are measuring correctly. Look at your hub reference point in the first picture. It is elevated above / past the trailing edge of the blade altering your measurement as compared to the lower picture. Imagine the hub shaft extended out another three inches. Your rake measurement would be negative based on your measuring method. This obviously would not be correct as hub length has no bearing on prop rake.

I believe prop rake is measured from mid-chord at the root (hub) extending to the tip and measured from the prop rotational axis. This gets difficult to measure with multiple compound curves associated with many props.

Trash, Dan,

I also think these angles should be measured from the C/L of the shaft bore, not the rear face of the hub. We would all hope that face would be square to the bore, but there's no guarantee it will be.

Thanks. Brad.
(937)545-8991

I was always told on a cleaver prop the rake was a measurement of the trailing edge of the prop in relation to the hub surface
https://cimg5.ibsrv.net/gimg/www.off...3fd7d49ba6.jpg

Agreee ^^^

Dan,

Keep in mind that, when they say, "differs from perpendicular", they are referring to a line perpendicular to the prop SHAFT. The problem you have is that you are trying to measure the angle of a line from a relatively short reference area (the hub). On a prop with an even number of blades, this will be particularly difficult, as the blade opposite will interfere with the resting of any gage longer than the diameter of the rear hub face. You obviously have 5-blade props, so that's not an issue, but you still have a fairly short reference plane to measure from, assuming that surface is perpendicular to the shaft. It would serve you well to find or have made a dummy shaft that fits the bore in your props that you can measure your rake angles from. Remember, the prop is rotating around the shaft, not the rear face of the hub.

In my world of precision machining, I have to eliminate as many sources of error as possible. These habits are hard to shake for me.

Thanks. Brad.
(937)545-8991

That's what Datum's are for......and apparently China don't know what they are! :D LOL we went through a hair pulling exercise teaching a guy in China on how to read drawings! :angry-smiley-038:

Wally,

Yup. And Datum -A- on a prop would definitely be the shaft bore.

Thanks. Brad.
(937)545-8991

in conclusion would you agree the second prop that I thought had 12* of rake has less than the first prop?

I know the first prop is a 15* rake and that what my measurement says so I`m going to pull a PQ90 and burry my head in the sand bacause what I measured and what the prop is matches so why would the second measurement be any different?

Measurements don't matter and we are not supposed to talk about them you should know better. :lolhit:

I concur :)

Lol!!!! I will never be able to live that down. You forgot the "2". PQ290 :D

Great point. Using a few different blades should have different angles if the rear hub face is not square to bore.

I always assumed rake was based on the center line of a blade Vs center line of prop shaft.

if the first pixs are correct then if I trimmed the trailing edges of the blades then I would be changing rake?

If I am wrong and the pixs are correct……, I have been over thinking this for 30 yrs! 🤓

Gary,

Since the prop is rotating on the shaft, everything should be measured relative to the shaft, if you can.

Thanks. Brad.

I assumed from the definition of rake, that the middle of the blade at the root, the angle to the prop tips.

With cleaver style props, it seems to be a little different, and uses the rear flat portion of the blades as the Rake Angle.

Using tHe shaft as the zero point, vs the rear hub surface that mau not be cut square.to the bore.

My '90s Rollas only have about 10° rake. You can see they have less of an angle. Hoping they have enough bow lift.

Tartilla,

The trailing edge of the working face is all that matters. Anything "behind" that is simply there to materially support the working face.

For what it's worth, adding rake just enables additional rendering of thrust from the water as its being forced rearward and slung outward, where it would just be cast off without it. Essentially, you're just adding blade where there's water to wring thrust from. It consumes additional energy to do so, and also changes the lift characteristics of the blade, as well, so it's not always a feasible feature to implement.

Thanks. Brad.

If all the blades are the same, you would hope on these expensive of props they would be.

On bravo style props everyone I have ever had worked on the blades have never been the same.

Wildman,

I don't disagree at all, but, as a MFR, it's not a shocker to me at all. A Bravo is $800. A Herring is $8000. We can buy six or eight labbed, matched blade Bravos for the cost of ONE Herring. Bravos are cast and hand finished. There's GONNA be variances. What would guys like Brett do with their time otherwise, right? :rolleyes:

Thanks. Brad.

Exactly, at least you would hope. Since I am not a buyer of 8K props I do not know, lol.

I get that and wasn’t really even considering that.

My point/question is, based on descriptions above, if I cut/change the angle of the trailing edge of the blades, this should change rake correct?

On variability of “as delivered” wheels, always depends on where from.

I went w/a buddy to pick up his, as delivered 30” Cleavers from his trusted prop shop.

According to his guy, I was front and center, there was +/- 3” in pitch.

These we’re from a west coast guy w/a known name and validated what we all thought.

Gary,

Changing the angle isn’t really what you’re after. It’s blade area in the right place. Think of rake as an extension of the working face of the prop blade, in the area the water will be headed to as it is forced rearward and slung outward.

Without welding material back on, you will only be able to reduce effective rake. If you remove material from the trailing edge toward the tips, that will reduce rake. But if you remove material from the trailing edge toward the hub, it won’t really increase the rake, as it won’t extend the distance the water will stay engaged with the working face of the blade. It will make the prop easier to turn, though, but at the sacrifice of lift (if memory serves correctly).

Thanks. Brad.

Thx Brad.

You did a great job of explaining it and is what I was thinking but couldn’t explain.

Which is different than orig posts.

At least my understanding of them.

If they’re determining rake by putting a protractor on the trailing edge of A blade, that means if I manipulate that edge, I change rake.

Which I did not agree with.

Rake is easy to see.

15* rake :

https://cimg4.ibsrv.net/gimg/www.off...0469984e80.jpg
https://cimg5.ibsrv.net/gimg/www.off...ae275c2f23.jpg

18* Rake
You can see the "bend" where the blade is further from the shaft


https://cimg7.ibsrv.net/gimg/www.off...a608cf5345.jpg

On my heavy straight bottom 15* rake is slower than 18* rake ...it grabs harder at idle during a shift, rpm drop is significantly more than an 18*

18* carries the bow better I`m told and that makes sense , less boat out of the water .(3mph faster) Doesnt drop rpm at shifts, that part is really nice .