Is a 14x8 propeller and a 16x7 propeller equal in air force movement in a airboat?
14x8= 112 and a 16x7 = 112 WHAT DO YOU THINK?
I know a 14x8 moves foreward 8" per revolution and a 16x7 moves foreward 7" per revolution .
BUT a 14x8 should be faster. ________________RIGHT?????????????

True, at least on airplanes. The 14X8 will give more top speed, and the 15X7 will have a lower RPM, but will provide faster acceleration. It kind of works like driving your car in next to top gear. Lower speed, but it pulls harder.

Bill, AMA 4720
WACO Brotherhood #1

Sorry, the product of the diameter and the pitch doesn't mean anything. It's just how we express diameter and pitch in writing (similar to a 2x4 block of wood). Other places of the world write 14-8 or 16-7, and subtracting 8 from 14 or 7 from 16 doesn't give anything.

Yes, 14x8 is faster. An engine that spins a 14x8 prop won't be able to spin a 16x7 prop (too big).

It's called LOAD FACTOR. The load factor is a way to measure the total load on the engine by using some simple arithmatic and multiplication.

The equation goes like this:

(Dx2) + Pitch = LOAD FACTOR

D means DIAMETER.

So, for a 14x8 prop you would figure it out like this:
(14X2) + 8 = Load Factor

Which comes out too:
28 + 8 = 36

Now a 16x7 blade would figure out like this:
(16x2) + 7 = Load Factor

Which comes out too:
32 + 7 = 39

So, your Load Factor on the 14x7 is 36
Load Factor on the 16x7 is 39

The higher Load Factor will load the engine more and you'll loose RPM.

BUT, lower RPM doesn't necessarily mean less thrust. You can actually get more thrust from a larger blade with less pitch. Even though it is turning less RPM--it will produce more thrust. Look at thrust as a hole shot. More thrust--more hole shot. You won't get the same top end speed out of a thrust prop, but it will take off quick.

Thats why 3D pilots use big props with only a little pitch. They don't want a fast airplane. They want it to accelerate out of a hover--RIGHT NOW!! And thats what a big prop will do for you. It's like taking off in first gear with a top speed of 15MPH. It's not fast, but it gets to top speed REAL QUICK.

Now look at a high speed racer:
Little prop with lots of pitch. Very high RPM and extremely fast because the small diameter allows the engine to wind up very high. It takes a bit for the engine to wind up and get to full RPM because the pitch is so great. That adds load too.

The problewm with a high speed prop is it takes a while to get wound up. No more hole shot. It's like taking off in overdrive gear with a top speed of 100MPH. It takes a bit to get there, but it's fast.

Rcpilet
You explained it the best .
Thanks

Ok of these two props a 14x6 prop and 13x8 prop both have a load factor of 34. If I understand this the 13x8 will be faster on top end than the 14x6 CORRECT?
Running a 30 cc Homelite engine on a 22"x36" airboat, about 12-14 lbs total wt..

Hi!
Might be so...but you must try it out in practise ...on a particular subject ...different subjects (airplanes ) will give different results. Different propeller makes alo makes a lot of difference.
To go fast...most of us say go APC. For bigger props (over 17") wooden Menz or carbon Mejsliz are best.

Your load factor is the right idea, but I think it is the wrong formula. I think that the following is a closer approximation:

(D^3) x P = load