3 BLADE vs 2 BLADE PROPS

[Murry]

I have owned boats for many years and when I was young (eons ago) Two blade props were always faster than the three. At that time the boats were smaller and the motors had less power. Just wondering why the two blades are no longer used.

[baddjonny]

I'm no prop expert but I do go back far enough that my old 1100 with the early style prop shaft used to twist off props with big stock or elephant ear props also the two blades had terrible steering torque and sometimes vibration problems jacked up.
That's what I remember from 66-67. In late 66 merc improved the shaft no more lost props.

jon b

[Mark75H]

I don't know if anyone really knows why current 3 and 4 blade props are faster than the old 2 blade props, but the biggest difference is blade area. With a 3 or 4 you can have a lot of blade area without excessive diameter. Diameter has to be tuned to the weight of the boat, but you can usually add blade area within the usable diameter by going to more blades.

[afr]

all the v drive racers i talked to still run the two blade props and if im not mistaking DE set a 1/4 mile record with his i know he set the record but with what prop im not sure of

[DanielC]

Everything being equal, a two blade propeller will run faster. One less blade disturbing the water. But they tended to vibrate more, and be slower accelerating, because of the smaller blade area.

There are many factors that change how a propeller runs, but we generally only talk about number of blades, because it is the most obvious feature to notice.
Almost everything in propeller design is a compromise. They could make the blades really thin, but then the blades would not have as much strength, and would get damaged more from hitting a object in the water. But the propeller would have better performance, until the first time it hit something.
We now have more powerful engines that can turn three and four blade propellers, and we also have many more years of research on propellers that had not happened yet, when a big outboard motor had maybe 75 horsepower

[Instigator]

Great thread.
Couple points to consider. I ran kneel down hydro's and runabouts for nine yrs and in the 70's we ran 1 2/3 mile courses w/long straightaways (top end speeds being paramount). During the gas crunch the courses shortened up and speed through and acceleration out of the turns became more important than straightaway speeds. When this happened we all went to 3 blades which have an advantage there.
Unlimiteds did the same thing. They used to run on (I think) 1 1/2 mile courses and all ran 2 blades (w/near 200 MPH straightaway speeds!). The courses shrank and then came the 3,4, and 5 blades. The OPC's went from 3 to 4 and 5 blades.
My theory is that the 2 blades are still faster WFO but no one has tried it. I have a 28" 2 blade hanging on the wall waiting for the right opportunity
In pleasure (and performance) applications the 3 (and more) blades are smoother, carry weight better and accelerate better than 2 blades. Every one wants top speed #'s but the reality is that we spend way more time doing everything but.
PS, Scott is right about the drag boats too. Most of them still run 2 blades.
G

[afr]

daniel c if any prop is viberating its out of balance or the shaft is bent

gary there was pile of two blade stuff at the old prop shop lot of brass to
shame its all gone now

[Jimboat]

There are a number of reasons why 3-blade, 4-blade (and more blades) props have become more popular. More blades certainly provides a more balanced distribution of the loads, making for less vibration and less opportunity for the onset of cavitation or efficiency losses.

More significantly, the development of surfacing propellers caused a significant increase of pressure (load) on the propeller blades. So the load on each blade was reduced by increasing the number of blades. (3-blade prop carries 33% less load than a 2-blade prop of same total area). It was found that this was a real advantage to propeller design.

The reduced load on each blade allows the propeller designer to make the blades thinner and lighter, which is more efficient, while still being strong enough to carry the required load. The result is a recovery of more RPM available to the propeller, less drag and more efficiency. In many applications, this also results in more top end speed.

Normally, more blades will result in better low-medium rpm performance and better rough-condition performance, with slightly reduced top RPM and so, slightly reduced top-end performance. The multi-blade props tend to overcome the "reduced top rpm" issue in high-rpm applications such as surfacing applications, high-engine mounting applications, and also in disturbed water conditions (such as with tunnel hulls, stepped hulls and vee-pad hulls), and hence can produce better overall results in such cases.

[Mark75H]

Thanks, Jim.

That means its all about blade area and things related to it.

[Jimboat]

Thanks, Jim.

That means its all about blade area and things related to it.

Yup. Blade area needs to carry the required load. Too much area means too much drag. Too little area means blades are overloaded.

But as you know, propeller design and selection is a major-league compromise, so it's never only a formula to meet! In particular, those loads on the blades are different at the different stages of the speed range, which of course, changes everything for the propeller.

[Murry]

Thanks Guys for a lot of information. I ran two blade brass for a long time but that was with 100 to 150 H/P motors.

[Unchained]

I run a two blade on my Vdrive. They do vibrate more than a three blade.
I have one of each and they are both 11" x 16 pitch, 4340 tool steel.
I overdrive them 60%. The prop is turning about 11,000 rpm.
The three blade is a better all around prop but the performance of the two blade is better.

Here's a video from 30 to 115 mph in 4-5 sec. with the two blade.
The back of the boat pulls to the right from the engine torque and the prop is slipping up to about 75 mph.
You can see the boat straighen out at the point the prop hooks up,
http://s621.photobucket.com/albums/t...arblelake3.mp4

[ddipert]

Unchained that looks like a run ride.

[Instigator]

I run a two blade on my Vdrive. They do vibrate more than a three blade.
I have one of each and they are both 11" x 16 pitch, 4340 tool steel.
I overdrive them 60%. The prop is turning about 11,000 rpm.
The three blade is a better all around prop but the performance of the two blade is better.

Here's a video from 30 to 115 mph in 4-5 sec. with the two blade.
The back of the boat pulls to the right from the engine torque and the prop is slipping up to about 75 mph.
You can see the boat straighen out at the point the prop hooks up,
http://s621.photobucket.com/albums/t...arblelake3.mp4

WOW! Awesome. Love the sound of the turbo's. I watched a couple of your videos. That thing is evil! Would love to see it in person.

[99fxst99]

Jim touched on it but one of the often overlooked factors in props is "DAR" (Disc Area Ratio), the amount of blade area compared to the surface area of the circle measured by the prop diameter. On 3 blade pleasure boat props (yachts) it can range from about 50% for a standard 3 blade to around of 120%. The higher numbers made possible by 5 or 6 blade props with overlapping blades. The more DAR the more hp can be absorbed into the water for a given diameter. If you think about it the prop dia has not changed much since the inline days but the available hp has doubled.
I remember standing in the stands in the Miami Marine Stadium for the '75 OPC nationals. 3 blades were becoming popular in the production classes, but many were still running the 2 blade Merc props. When the SJ boats came by you could point out the 2 blade boats with your eyes closed by the sound. I still think they were cool props!