What causes this??

[nickens21]

I've ran a few different props in the past and never really had anyone tell me the true cause of this!! When u take off to get on plane I notice some props will spin up in rpms jst as the bow breaks over!! It's like it slips only at that break over point!! Bites hard on the bottom end when taking off so its not slipping through out the entire hole shot!!

Thanks Jesse...:cool

 

[K.Kiser]

The prop pierces the surface and momentarily cavitates as the boat flattens is what I've always thought..

 

[whipper]

This isnt all propellers but most high performance props are designed to spool up rpm before they bite to enhance hole shot and acceleration. The explanation why this occers is very technical. Its has to do with negative pressure exeeding the the waters vapor point cousing pressure bubbles of water vapor being thrust by the blades. The vapor point has no thrust untill theres enough momentum to be turned back to water. Thats why when ya pick up speed it starts to grab. Does that make sence?:big grin

 

[Alli-drenaline Rush]

I'm an airplane pilot for fun, and one of the things that we are taught is how to recognize a phenomenon called P-factor, which is an asymmetric loading of the propeller (prop is pointed in one direction while the motion of the plane is in another direction, setting up more angle of attack on one blade over the other). This is most noticeable in the old (as in World War II vintage) taildragger aircraft on take-off. This phenomenon tends to want to swing the nose around on application of power, so the pilot has to be ready for it.

Now apply this phenomenon to the case of another fluid - water.... You take off, the prop and water are flowing pretty much in the same direction. Then the bow breaks and your boat "mushes" or falls in an arc to the surface. In that moment, the blades are not going exactly in the direction of relative water flow, setting up a higher angle of attack on one or more blade(s) than on the others, promoting cavitation as Whipper writes about. RPM spools up because as Whipper says the vapor bubbles have little mass relative to liquid water, causing the RPM to increase until the flow of water gets re-established along the axis of the prop and the blades have once more something to push against.......

Now what I've just written is either total BS (with a fog factor so high as to be incomprehensible), or it is a Sunday morning flash of hydro-dynamic genius.....

You choose which to believe!!!!

Richard

 

[Alli-drenaline Rush]

In reality it is probably a combination of what Kevin, Whipper and I have written! As Whipper noted, it is a complex phenomenon, involving most likely a combination of cavitation, asymmetric thrust and surfacing prop blades!!!

 

[GFinch]

What is needed is tracton control. Sometimes known as a foot. Or just do what I do, pull a plug wire or two off.

 

[John Richied]

What is needed is tracton control. Sometimes known as a foot. Or just do what I do, pull a plug wire or two off.

You probably already know this, but Gary in a drag race the boats break over point is VERY IMPORTANT.
If the Rrrrrrrrrr’s come up too high when the bow breaks over you are losing valuable time, same thing if the Rpm’s drop too low.

That’s just one more reason I like the Allison manual jackplate… you can move it up or down in small increments to dial in your set-up. A lot of jackplates only move in

 

[nickens21]

All good answers to me!! And I somewhat understand!! But for example: A friend of mine has a 21xrd bullet with a 250xs!! His 24 trophy does not cavitate at all and bites hard! But the 26 he has does it pretty bad!! It seems to me that the smaller wheel would spin the rrrrrr's up more at bow breakover!!

 

[Alli-drenaline Rush]

Hey Jesse, imagine swinging a knife thru water like you're swinging a cleaver on a piece of steak. That's a zero pitch prop. Now turn the blade sideways some and think of the void just behind the blade as the blade goes through the water. That's a 24 pitch prop. Now turn the blade more and repeat. That's a 26 pitch prop. Now put 250 ponies behind that blade and you can imagine how much more of a void the 26 will make over the 24. The 26 is pushing a lot of water and creating a lot of void space. Water takes a small but measuable time to fill that void. Problem is the next blade gets there before the water has had a chance to fill the void, so cavitation starts. The 26 just pushes a lot more water out of the way than the 24, so the 26 cavitates while the 24 hooks up. Hope this rambling makes sense! By the way the rush of water back to fill the void can be so violent that it can eat away the blades over time - most often seen on larger ships. But it can be seen on smaller boats, especially ones with soft metal (aluminum) props....

Richard

 

[Alli-drenaline Rush]

Jesse,

One other point to make is that IMO life is all about compromises... You can't get a plane that lands slow, takes off short, goes fast, flies high, carries a lot of people, sips fuel and costs nothing..... similarly, the 24 will get your friend out of the hole better than the 26, keep the bow up better than the 26 when the wind whips up and he's trying to keep from spearing a 3 ft roller... but the 26 will have better top end than the 24 (unless the 26 is too much prop for the motor at WOT)......

I once hand-loaded some cartridges for a Remington Model 7 youth gun in 7mm-08 caliber. I thought originally that a 140 grain bullet would be best for the gun.... Come to find out, the best group I could get at 100 yds was about 3-4 inches (the barrel is paper thin and only 18.5 inches long), so barrel warp on successive shots and bullet stabilization could be problematic..... By going down to a 120 grain bullet, I was able to get 3-shot 0.5 inch groups at 100 yds and 0.9 inch groups at 200 yds with a 5-10 mph wind.... in this case the gun liked the smaller grain bullet (meaning that the smaller bullet hit the groove for that gun in terms of barrel twist rate and bullet stabilization)... translate this example to your friends' boat..... sounds like he has hit the sweet spot for take-off performance with the 24 given boat loading, power constraints and jackplate height (As mentioned by John Rrrrrrrrr)...... now the only question is whether the 24 gives him enough performance in terms of top-end speed at WOT...... if it's still an issue, then buy a Merc 2.5, do some ECU work and crank that baby up to 9k rip-ums!!!!

 

[TBuck2003]

Great explanations as regards to the airplane prop. Great read... As far as pitch is concerned,less pitch is always going to grab more and HOLD on to the water better, simply because it has less to grab. The 26 trophy is slipping more than the 24 for this reason. Any interruption in the on plane process(bow breaking over) can unload the prop(lose grip) because it is trying to hard to hold on to the water. Not as indepth as some of the answers, but just adding to the great responses!!!!! Remember the angle of the blades is as such that the 26p is rotating 360* to go 26 inches in theory. The 24p is only going 24 inches in theory, to push the boat, and all that is being displaced ,2 more inches is a lot MORE water to be PUSHED upon take off for every one rotation around. Hope this helps,

TBuck

P.S. Ton of factors like cupping Etc... can change results or alter them...

 

[Alli-drenaline Rush]

Good points TBuck about theoretical forward motion and especially the cupping aspect...

 

[nickens21]

Thanks, I purdy much understand what's going on now!! He runs the 24 on there all the time now!! There both trophy props!! And the 26 he actually cracked a blade on it the last time he ran it!!