'Face cup'

[Smokin' Joe]

'Face cup' is is a terrible phrase because misleading. There's camber and there's cup. Cup is
a large camber change over a very short chord length at the trailing edge. Slowly varying camber at
the trailing edge will not have the effect of cup. Why? You have to understand something
about (hydro- or air-) foil theory. A prop blade is a hydrofoil. An airplane wing is an
airfoil. Cupping the trailing edge is like the effect of dropping the aileron on a wing, the lift
on the wing is increased. You can't get the same increase in lift by smoothly cambering the
trailing edge of the wing. The leading edge of an airfoil is completely different than the trailing
edge. At the leading edge, the angle of attack produces the main component of lift.
Camber in the foil section also produces lift. With a surface-piercing
prop, the leading edge must be very thin to reduce drag because each blade enters the water every revolution.
Now, an eddy is created any time there's flow past a sharp leading edge. Just move a paddle in the
water and you'll see what I mean. An eddy shed by an edge is nothing but drag, and that's
what we want to reduce. You can only reduce that by reducing the angle of attack. The
paddle's a rather flat hydrofoil, you can feel the side force (the 'lift') as you move it at nonzero angle of attack.
You can get rid of both the eddy and the side force by moving the paddle blade parallel to the direction
of travel (zero angle of attack). If you put camber in the blade of a paddle or prop then you'll recover
the lift. So 'face cup' is not cup, it's gently-varying camber. If you camber the leading edge
right (with a pitch gauge, and being able to calculate the camber you need) then you can reduce the
leading edge drag and make up for the lost pitch with camber. There's a myth that anything that increases top
speed reduces acceleration, and vv. Correctly cambered, a prop will show an increase in both top speed and
acceleration. That's what I've known how to do since 1978. Tracy Hawkins and Mike Schubert
are running props made like that in F1Sport. Roddy Foreman ran one at Kankakee 2011 in SST 60 (until he broke a piston skirt). Hans is running one
in SST45 now as well. All the factors mentioned above affect the location of 'center of pressure', the average position
on the blade where the side force ('lift') is focused. A center of pressure too near a very thin leading edge
poses an obvious danger, but center of pressure location affects performance as well. PTR, Progress through research,
is Alex Hledin's motto at PTR. It's a damned good motto, we practice ptr too. And it does make a difference to understand physics!

'Dr. Joe'
mccauleyandson.com

[Per]

Thank you very much! What a great article. Not many people want to share their trade secrets that way. Hats off...

[Smokin' Joe]

Thank you very much! What a great article. Not many people want to share their trade secrets that way. Hats off...

I know only one (by accident Swedish) marine engineer who'd know what to
do, more or less. Most prop shops, even with pitch gauge will still not know what
to do. I.e., I'm pretty sure my speed secret remains largely in my head since I didn't say
how to calculate! The marine engineer told me that the surface drive people figured it out independently.

[Mark75H]

Excellent

[Per]

I know only one (by accident Swedish) marine engineer who'd know what to
do, more or less. Most prop shops, even with pitch gauge will still not know what
to do. I.e., I'm pretty sure my speed secret remains largely in my head since I didn't say
how to calculate! The marine engineer told me that the surface drive people figured it out independently.

Is his surname perhaps Bäckmo? :)
I have followed your discussion on "angle of attack" on a different forum...
I'm sure not many can produce a good race prop regardless of what tools and knowledge they have in possesion.:D

[Smokin' Joe]

Is his surname perhaps Bäckmo? :)
I have followed your discussion on "angle of attack" on a different forum...
I'm sure not many can produce a good race prop regardless of what tools and knowledge they have in possesion.:D

Bodo himself, as you know from the other forum.

[rumleyfips]

On your website, it says that diameter is established through experiment. Theory for wind turbines says that power is proportional to rotor area ,ie. square of the diameter. Is largest workable diameter desirable?

When I ran ASH in the 70's I was always trying to run the bigest diameter wheel I could. At the time we had the propshaft about an inch above the bottom and were limited by the ability to get on plane.

Thanks:
John

[Smokin' Joe]

On your website, it says that diameter is established through experiment. Theory for wind turbines says that power is proportional to rotor area ,ie. square of the diameter. Is largest workable diameter desirable?

When I ran ASH in the 70's I was always trying to run the bigest diameter wheel I could. At the time we had the propshaft about an inch above the bottom and were limited by the ability to get on plane.

Thanks:
John
John,

There's no book or paper that tells you how to calculate, at least none based
on sound theory. You can take a look at Gerr's book. 'Large area' comes
from the simplest approximation and we don't know where it breaks down. It
assumes 1-phase flow. Large area is not simply dia.^2, there is asymmetry.
Large dia. and a small chord makes the flow more 2D, large chord and smaller
dia. makes the flow more 3D. You want the flow to be more like 2D to reduce the
drag caused by the tip vortex. In EP 12x23-12-25 was the working solution, in
SE/SST60 12.5x23-25 is the solution. No one can calculate that difference. 13"
dia will not work on short courses in SSTY60, 11.5-12" dia also will not work.

My success (SST45, SST60, EP, F1Sport) is largely in drag reduction. It's a trick to
build a prop that will set a kilo record and also out accelerate all the others
off the beach. Drag reduction.

Joe

[calvin]

Great info on props!!!!

Soooo....how do you measure a wheel with a pitch gage or computer..ie..prop scan?

[Smokin' Joe]

Rundquist pitch gauges, rh and lh. I once had a scan made on an SST60 prop at
Baumann prop to compare with my Rundquist measurements.

[rumleyfips]

Joe:

Thanks. Although I haven't had a prop in my hand for 30 years I still want to know how they work.

After your reply I googled airfoil and there was a bunch of info. My math is from the 60's when we didn't have calculus in high school but I still see things I didn't know.

Again ; thanks:
John

[Smokin' Joe]

Joe:

Thanks. Although I haven't had a prop in my hand for 30 years I still want to know how they work.

After your reply I googled airfoil and there was a bunch of info. My math is from the 60's when we didn't have calculus in high school but I still see things I didn't know.

Again ; thanks:
John

John,

The way a foil (wing, sail, prop blade) works is nonintuitive. There's a 'lift'/side force
on the foil because of an eddy circulating about the foil. This is the origin of the
high lift/drag ratios of a wing. Jim Booe is the only prop man I've talked with
who understands this. I still talk with him, he taught me two invaluable tricks in 2011.
I learned foil theory from Newman's 'Marine Hydrodynamics' in the late 1970s.
More than calculus is required.

Best,
Joe

[Krazy Karl]

I have always felt that prop design was a dark art, but like anything else science and math are always or should be involved. I recently read the book We Were the Ramchargers and nothing they did was by trial and error, solid engineering with all the math was done before they ever picked up a tool.

Your comment on how an airfoil works is non-intuitive is so true. I also sail and the number of people who think the wind pushes on the sail is unbelievable. In fact when going down wind, when this is the case the boat is quite slow. Great article.
kk

[Smokin' Joe]

Yes, wind behind a big sail is a high drag limit. The way the circulation works is subtle.
The thin wake/vortex sheet behind the trailing edge is a velocity discontinuity, that's equivalent to
finite circulation. For engineers and physics students, that's treated clearly in Landau-Lif****z's 'Hydrodynamics'.



I have always felt that prop design was a dark art, but like anything else science and math are always or should be involved. I recently read the book We Were the Ramchargers and nothing they did was by trial and error, solid engineering with all the math was done before they ever picked up a tool.

Your comment on how an airfoil works is non-intuitive is so true. I also sail and the number of people who think the wind pushes on the sail is unbelievable. In fact when going down wind, when this is the case the boat is quite slow. Great article.
kk

[filthy phill]

man this is all so far over my head, anything with an ology or matically on the end is just way too much info.
just want to know what to actually do to a prop to get the effect.

[Smokin' Joe]

man this is all so far over my head, anything with an ology or matically on the end is just way too much info.
just want to know what to actually do to a prop to get the effect.

Ain't it the truth!