Prop info

[lgpjr49]

I'm sure this sounds dumb but I just got my first outboard. This is a whole new ball game to me especially when it comes to the prop. I have no idea what people mean by pitch and what not? Can anyone help me out with some info or a forum that already has that?

[Ron Fogelson]

Most of this info can be found around the web, but I used http://bblades.com/ as a reference and because it's easier to copy past then type this all out from scratch..... 

The two things most talk about are Diameter & Pitch

Diameter how big the prop is, measured from the outer circumference of the propeller across the center of the prop,,,,, showing how big it is.

Pitch is the forward movement in inches & is stamped on the propeller.  The pitch tells you how far propeller would move if it was turned one revolution in the water minus any slip.


A few other things to read up on are Rake, slip, Cupping & the Number of Blades you might use based on the set up of your boat and/or need of the operator.

Rake angle is measured in degrees. Rake is the amount of degrees the propeller blades angle perpendicular to the propeller hub. Rake can be slightly negative (leaning towards the boat), or positive (leaning away from the boat). The range of degrees could vary from -5 to +30 degrees. An average rake angle for most outboard propellers is 15 degrees. Rake can be progressive meaning it increases as you move out from the propeller hub to the blade tips. Or rake can be flat maintaining its degree of angle continuously from propeller hub to blade tip. Higher rake angles help to improve some boats ability to operate in aerated water situations by causing the water to adhere to the blades better than a prop with lower rake angles. The centrifugal force the propeller creates, throws the water outward and the higher and/or progressively raked propellers can hold and control that water better, increasing the thrust rearward. Higher rake angles can also help to hold a boats bow higher in the air, reducing hull drag and increasing speeds. However, low rake propellers can be a better choice for light weight fast hulls that carry the bow on their own since they direct thrust with less drag and can increase efficiency.


Cupping
When the final trailing edge of a propeller blade has a "curl" away from the boat, it is referred to as being "Cupped." Cupping on propellers was not always used as a standard propeller design. When cupping was first used, it was done to propellers as an easy way to increase the pitch or rake without bending the entire blade. Once propeller shops began to do this, they realized other benefits that came along too. One of those benefits was improved performance. Because cupping can increase pitch, rpm will generally decrease. Depending on where and how much of a cup is added, determines the rpm and the boat personality changes.

Number of Blades most bass boats run 3 blades with 4 the 2nd most used there are a few high speed hulls and or tuned motors that preform the best with 5 blades.

A propeller with one blade would be the fastest and most efficient propeller if it weren't for the fact that the vibration could not be contained. The reason for its great speed would be the lack of blade drag caused by metal in the water. As blades are added to propeller hubs, the drag is increased. However, the ability to grab more water and decrease the time frame between load and unload is shortened. This means the propeller will "feel" smoother as it grabs water or loads the blades. The number of blades does many other things to the performance and control of a vessel. Performance type hulls tend to show the greatest response from adding and subtracting blades. Propellers typically come with 2 to 6 blades.

Slip
Prop slip is the true forward movement in the water; so if
Propeller Pitch is 26
Gear Ratio is 1.75
Engine RPM is 5,000
Actual Speed (MPH) is 70.343mph
Propeller Slip would be 0%

On mine right now running a 26 pitch tempest plus at 5,500 rpm's 67mph I'm at 13.42 prop slip

Going up or down in prop pitch will reduce or increase your max RPM's respectively also going from a 3 to a 4 blade will drop your rpm's as well.  You need to find out your motors max rpm recommendation, and prop it right to get within those operating numbers and balance your boats handling needs / speed desires.  Talk with folks that run your boat set up, see how high and how much of a set back they run and find the best set up for your boat and load and handling that makes you the happiest.  If you have a good boat shop local talk with them and read, read read some more on line and ask questions and play with your set up until you get her dialed in,,,,,,, while doing this make sure you are not raising the motor so high that the required water pressure isn't reduced under the minimum required amount. 

Good Luck
Fogy

[TWilson]

What would you recommend for a 1996 Ranger 391 XT with a johnson 200 Venom?

[Buzbait88]

What would you recommend for a 1996 Ranger 391 XT with a johnson 200 Venom?

Most people with Johnson engines are running the raker props, because they are splined for it.  Meaning the props will slide right on without any aftermarket parts.  I would try a 24 or 25 Pitch Raker prop.  Find the Max RPM's for the motor and when running WOT(wide open throttle) and fully trimmed out make sure you are running as close to the max RPMS as possible without going over.  I would guess your motor would be about 5600 RPMS.  You can buy props like the mercury tempest + with no hubs in it and buy a hub kit with the splines to fit on your motor if necessary.

[TWilson]

Try again. We had the motor at 5800 last week and that was 3/4 throttle.

[Buzbait88]

Try again. We had the motor at 5800 last week and that was 3/4 throttle.

Just trying to help...When I say MAX RPMS I'm referring to how many RPMS the motor SHOULD be run at...not how many you can make it run.  Every motor is different and the specs should be on the side of it, I was just going off of my first motor that was a 93 175 Johnson and the max on that was 5600, figured yours would be similar, but I could be wrong.  Look it up to know for sure.

[lgpjr49]

So where would you find specs on how many RPMs your motor runs at and how would you know that mines a 87 Johnson 25hp short shaft. This is all new to me the only motors I've ever used till this year have been transom trolling motors.