"Cupped" prop blades...

[justrfb]

Hello All.
I am right on the verge of hitting the "Order" button on a new prop but have one outstanding question... I also have this question in my "Online parts" thread but not sure people are seeing it because of the title of the post, of which I can't seem to edit... I hope you don't mind and if this is an issue (sorry if it is...), please just delete this thread.
Regarding "cupped" blades on a Mercury Blackmax 3 bladed prop, what is your experience with cupped blades and will it also cause a significant RPM drop? Thank you for your help.

Sincerely,
Rich

[GXPWeasel]

I couldn't tell you for sure, as I don't believe any of my props are cupped. I've heard that with an aluminum prop, you have prop flex, especially if you have over 150hp. One would think that a cupped prop would then possibly flatten when under load, to more evenly reflect the actual pitch that the prop should be.

If you don't have at least 115 hp motor, I don't know that I would spend extra on the cupped prop. If it's the same price, I'd probably purchase one.

[Bryden24shp]

Cupped props will give you more stern lift, you may loose a little trim range. Depending on the amount of cup, you might see a 150 to 200 rpm drop. A cupped prop will run faster than a non-cupped because of its reduced slip, by grabbing more water and throwing it out faster. My experience is pontoons love cupped props. Everyone of mine is cupped to some extent, some have had extra cup added.

I agree w/ GXP, A small outboard on a twin tube would probably not see much gain. But may see better fuel economy due to the reduced slip.

I just read your "online parts"... left a comment for you.

[justrfb]

Hello and thank you for your help...

I do have a description below my signature, I guess not enough info... We have a nice "plain Jane" two toon 2007 Princecraft Vantage 20L. It is a 20' toon with a 2007 Merc 75 4 Stroke hanging off the back. I am currently running a BlackMax Un-cupped 11" pitch prop. Running 6,200 RPM @ 21.8 MPH (GPS) with 2 - 3 adults and half tank of fuel (14 gallons or so...).
I am looking to go to the Cupped 13" pitched Blackmax prop (next size up and the suggested Mercury Prop Selector pick) but am wondering if it is going to be "too much" prop. Thanks again for the help and advice.

Sincerely,
Rich

[Bryden24shp]

Ya, our posts crossed. I didn't see your signature in my reply pane. Happens all the time. I read to fast and forget what I just read just as fast. Kinda like hearing the wife talk about the outlaws, then smacking me when I ask her something about what she just told me.
I use the Bamm and Merc calculators. I like the Merc because you can compare props so easily.
You'll probably drop 350-500 rpms by pitching up and adding cup. An inch of pitch is usually 150 rpm's. You are realistically adding 3 inches of pitch y stepping up to a 13 cupped.
What are your top recommended RPM's?

[justrfb]

I do believe my RPM limit is 6,000 RPM's. The rev limiter doesn't activate so maybe my gauge is off. I think I am going to do it. What is the worst that can happen?... I am going to use Eb-y too as you suggested... I see one for $30.00 less. Thank you for all of your help!

Sincerely,
Rich