number of prop blades????

[netik racing]

Im looking at getting a friend to make some prop's for me and have seen 4 and 5 blade prop's at stores ie prop shop...

What i need to know is has anybody tryed 4 and 5 blade prop's....

I normally run a 3 blade x440 over a 2 blade seems to handle better in both my rigger and my tunnel...

Idea's or answers would be great....

[m4a1usr]

Depending on the RPM range desired you should lean towards props with less blades. Not more. The most eff prop technicaly is one with only one blade. When you add more blades you increase blade surface area. Thats all fine an dandy if your turning low rpms. A simple rule to remember is props with more then 2 blades do better for acceleration but up to the water pressure/ cavitation limit. Once that occurs your creating a pressure bubble where the prop no longer has any more bite. I.E. normaly a prop needs to be wetted on both sides of the blade to operate correctly.

John

[netik racing]

Depending on the RPM range desired you should lean towards props with less blades. Not more. The most eff prop technicaly is one with only one blade. When you add more blades you increase blade surface area. Thats all fine an dandy if your turning low rpms. A simple rule to remember is props with more then 2 blades do better for acceleration but up to the water pressure/ cavitation limit. Once that occurs your creating a pressure bubble where the prop no longer has any more bite. I.E. normaly a prop needs to be wetted on both sides of the blade to operate correctly.

John

Much thanks John....

I have a better understand of it now

[sailr]

with that in mind, how would you extrapolate the change from say an X446/3 to a two blade? Would you use the same size (X447) or go up in diameter? I understand the concept on airplanes, 2 blade vs 3 blade. The old rule is if going from a 2 blade to 3 blade you go down an inch in pitch and diameter but now sure about boats.

[tiqueman]

with that in mind, how would you extrapolate the change from say an X446/3 to a two blade? Would you use the same size (X447) or go up in diameter? I understand the concept on airplanes, 2 blade vs 3 blade. The old rule is if going from a 2 blade to 3 blade you go down an inch in pitch and diameter but now sure about boats.

I knew the answer to this at some point. If I remember correctly, you drop the diameter by 10% (or raise) So a 2 blade 42mm would go to a 3 blade 37mm. I think thats how it went

[m4a1usr]

More blades is more surface area. More surface area equals more friction. That can effect the flow of water in to the prop. That doesnt mean dont run 3/4/5 blade props in our boats. Just think of it as possible restriction or limit in QUANTITY of water passing thru a prop. This can effect cavitation. A prop acts like a pump. Too many RPM's and not enough water passing thru= vaporization or a prop with no wetted surface. We need water to move forward.


If all things are equal, same motor/RPM/batts, and loading is ok the move should be to a smaller prop but with more pitch for more speed. Its sort of a normal assumption. Racers are bound to be better informed as they share much better information between themselves. A props diameter=thrust. Bigger means more. But since any hull can have too big a prop, you have to use a prop that does not have adverse effects. Like hull control issues, prop walk/torque roll/etc. Thats where you can size down but change the pitch. That in effect moves the prop forward a greater amount for every revolution. Some of you already know that going down in dia, but increasing the pitch, increases speed. Now add tip cupping and you can even get more speed. Another method for more speed is simply changing your prop depth if its not set to the optimum depth. Thats called ventilation, not cavitation. 2 different beasts.

Ventilation occurs when the prop captures or directs air (surface piercing props) and it literaly pushes down the air into the water. Some of you guys have probably have seen prop blades with holes drilled in them. Thats an attempt to allow the air to escape off the pressure side of the blade. I tried it but dont believe it. In my mind all that does is reduce effective blade surface but some swear great results.

There are many changes that can be done to a prop to "wake it up". You dont hear too many guys talking about spooning or camber. Thats where you roll the props edge from the leading edge to the trailing edge. That in effect increases a props progressive pitch. You can get some really decent tips off of the rcprops.com website. Paul has good advice to offer anyone willing to read and learn.

John

[FighterCat57]

Good Stuff. Thanks for taking the time to write that out!
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Kawasaki Ar80