Prop Question

[Carnivalride]

It will be fun working together to dial in and improve performance on our favorite past time and our obsession, our boats!

No question is too big, to small or too repetative. Let's see what we can acomplish. [smilie=042.gif]

Welcome aboard and if your serious about tackling prop questions a have a few that might be a little different.

What effect is blade “rake” is supposed to have? From my experience on my v-drive flat “rake” seems to give me better low end and more transom lift. I can accomplish the same thing (transom lift) by putting spacers in my coupler to move my less “raked” round ear prop back. What is the idea or design intention between these two styles of propellers (high rake pointed ear and a traditional raked round ear)?

Isn’t the leading edge supposed to have more effect on acceleration while the trailing edge is supposed to have more effect on top end?

What is done to the prop to give more burn on a dead water start (that instant slip to say 7000rpm) and then have it hook up and pull the motor down to a lesser rpm?

Any ideas?
Thanks,
Loren

[bbladesprops]

OK, I thought I lost this post but I see it was moved to its own thread.

Rake is a interesting and very involved topic. Rake angles are being redefined in the industry as we speak. Rake can be straight, true, parabolic, and changing.

Rake angle is measured in degrees perpendicular to the centerline or propshaft of the propeller. Rake can be neutral, negative or positive.

The effect rake has on hulls is as different as the rake angles themselves. In "general" more and more positive rake will carry the bow higher. This is actually done by leveraging the stern down. In turn the desired effect is to create less wetted surface area therefore decreasing the drag and increasing the hulls speed. Great idea in theory. Problem is, not every hull can handle nor needs this much bow help or stern pressure. What that means is high rake propellers can actually slow you down in the "wrong" application. If a boat will carry itself well (perfect running attitude) with lower rake angles, it will typically have better top end speeds because the higher rake propeller actually can create more propeller drag.

Low rake props can help low end performance on applications that are heavy in the stern or in Carnivalrides situation, a propeller under the hull. This will bring the stern up quicker and move the hull forward in a quick manner.

As far as round ears props versus cleavers. We have a similar scenario with round ears creating higher stern holding power and cleaver ears creating quicker surfacing attributes. Therefore an quick explanation would be round ears act as if they have more rake. But, we must take into consideration the actual rake angle of the propeller.

Yes on the LE versus TE question.

We can depitch the LE to increase turn up time and add cup and pitch to the TE in specific increments to add top end speed and efficiency.

As you might be able to tell. Each specific application is different. Every identical application, for that matter, is different. Perfect propping any given boat package requires individual attention. Starting with a quality casting is key. From that point, the individuality of the finished product is as varied as the paint schemes.


Brett A. Anderson
BBLADES Professional Propellers, LLC.
920-295-4435
www.bblades.com

[Satisfied]

Brett welcome this is Jeff Zitzner from LaCrosse. Glad you found the site. We will be needing them 5 blades this year to try again hope we can get ti done this time. Again glad to see you on here and thanks for for keeping this site up as a sponsor. [smilie=071.gif] [smilie=071.gif] [smilie=071.gif] [smilie=071.gif]

[Carnivalride]

Brett,

Thanks for the earlier response, how about a few more questions?

In "general" more and more positive rake will carry the bow higher. This is actually done by leveraging the stern down.

This is meant for an outdrive or an outboard boat where the propeller shaft angle actually drives into the water like when they trim “up” right? But with my boats running attitude it has a 7.5 degree prop shaft angle that actually drives to the surface so instead of leveraging it is trying drive out of the water correct?

If a boat will carry itself well (perfect running attitude) with lower rake angles, it will typically have better top end speeds because the higher rake propeller actually can create more propeller drag.
What do you consider low rake and high rake? My round ear propeller has 15 degrees, while my cleaver/round ear hybrid has 25 degrees rake.

In your experience which blade style has less drag, long thin blades like an airplane propeller or short wide ones like a competition ski boat? Or is blade area more of a factor than shape? In general which style is more efficient as far as “slip” is concerned or do cup and other factors come in to play so a generalization can’t be made?

I’m just trying to understand propeller theory a little better so I can make some educated guesses on what changes I need this go around.

Thanks for the input,
Loren

I just spent 20 mins. writing a book only to loose it trying to put one of those damn bouncing faces on in the more emoticons area. So your getting the quick version this time.

No matter where the propeller is, its geometry still affects its personalty sas far as moving to the surface or holding water longer. Your 2 propellers have different geomtry that in affect are trying to get to the same place. Lower rake round ears, higher rake with tighter tips etc. (of course I haven't seen them yet, so I'm generalizing). The best prop you have will be determined by a stop watch and your butt. Then with good records, small changes may be able to be made depending on the existing props condition.

0 degrees is low rake. 25 degrees is high rake.

True surface piercing cleavers the least drag. In general.

Efficiancy can not be generalized. A set of 14.5x34 KILO cleavers won't get that Baja Islander on plane. but will make my 32 Doug Wright hall the bacon. Also, the Mirage Plus special Lab cut we do stinks on a 32 Skater with 525's and shorty sporty's. But they make a great day of boating on that Baja.

Sorry for the quicker short story answer, the other one was better but I have some race props that need to get done for a race in New Zealand next weekend.

I hope this helps.

[Lazy Moose]

Brett,

I am trying to squeeze out a couple more HP'S in my 496 on a 23' islander. I have a K&N air filter, picking up the Dana Marine EX-2000'S and then wanted to determine the best prop for this. Either a labbed or whatever you might recommend to stream line my top end (maybe add a 1-2 mph).

What will you need from me to get this done?

Gear ratio
Current prop w/ pitch
RPM'S I am currently running

Anything else?

[bbladesprops]

First,

The guest response was mine. Not sure what I did there. (I'm a prop guy not a computor guy :lol: )

Lazy Moose,

Did we talk on the phone? If not we need to. I think we did. A special recipe Mirage+ or B1 or even a Rev 4 will help your boat. As far as top end though, It is a Labbed Mirage +.

Thanks [smilie=042.gif]