Progressive Pitch Props

[Fastjeff57]

Boy, is there a lot to learn about props!

I've built 3 of them so far, using stainless blades 'removed' from factory props and welded to a custom made hub. In each I tried different shapes and pitches, using a cobbled up pitch block and measuring them with a homemade rake and pitch measuring deal.

The results have been---ah---educational. My latest prop (in the photo) after three tweeks, is now holding the motor down to 5,800 rpms (where I want it), but the boat is slower than with another prop I made that holds the motor down to 5,300 rpms. (My respect for Ron Hill and others who know what they're doing grows daily!)

This latest prop, after tweeking, has a pitch of 10 " initially, going to 12 " at the mid point, to 20 " at the aft end (not counting cupping). Obviously, that is not what I intended, and with so much progressive pitch it's really inefficient (hence the lousy top speed).

Getting to my question (at last), I've measured progressive increasing pitch on several factory props (of about an inch total), and noticed that cleavers seem to have a negative (reducing) pitch. I was wondering what the thinking is on this subject, and the theory behind it.

Can anyone explain the progressive pitch concept?

Thanks,

Jeff

[zul8tr]

Here is my understanding. Progressive pitch prop blades were developed to accomplish better acceleration as well as top speed in the same set of blades. In theory progressive pitch is a duel purpose prop. At a given radius there will be a set of progressive pitches from the lead edge to trail edge, and these will change at each radius being less as the radius increases. A progressive pitched propeller will generally have less cavitation than a constant pitch blade (at a given radius) and will provide increased acceleration and higher top speed if designed right. To do this the leading edge will be a lower pitch than a constant pitch blade so the water is picked up under less pushing resistance force and this aids in acceleration (like a lower gear). The water then flows along the blade surface with increasing pitch to the trailing edge where it has been accerleated into a tighter thrust cone (greater force) to aid in top speed. As an anology look at a jai alai sesta .......

http://www.google.com/#q=jai+alai+sesta

In the return mode of the ball (pelota) it starts at a low pitch in the sesta to allow the players arm to exert great accleleration at a lower force, than the ball quickly accelerates to the end of the sesta (more pitch and greater top speed) and off it goes at close to 190 mph.

The complex geometry of progressive pitch props is one of the reasons that testing is the only way to these props evaluate props (as well as others).

[Yellowjacket]

Getting to my question (at last), I've measured progressive increasing pitch on several factory props (of about an inch total), and noticed that cleavers seem to have a negative (reducing) pitch. I was wondering what the thinking is on this subject, and the theory behind it.

Can anyone explain the progressive pitch concept?

Thanks,

Jeff

The regressive pitch (less pitch at the tip) is intended to improve the efficiency of the prop at high speed. The idea behind it is that with a progressive pitch there is a big difference in pressure between the suction and pressure sides of the blade near the tip. This causes the water on the pressure side to want to roll over to the other side near the tip. If you "unwind" the tip, there is less pressure gradient and less of a tendency for the water to roll over the tip and reduce the efficiency. The same trick is used on turbine blades to reduce tip losses, or the tendency for the air to spill over the tip of the blade. Same kind of thing as why jet planes use "winglets" to reduce wing tip losses. Probably also helps low speed acceleration too.

[zul8tr]

The regressive pitch (less pitch at the tip) is intended to improve the efficiency of the prop at high speed. The idea behind it is that with a progressive pitch there is a big difference in pressure between the suction and pressure sides of the blade near the tip. This causes the water on the pressure side to want to roll over to the other side near the tip. If you "unwind" the tip, there is less pressure gradient and less of a tendency for the water to roll over the tip and reduce the efficiency. The same trick is used on turbine blades to reduce tip losses, or the tendency for the air to spill over the tip of the blade. Same kind of thing as why jet planes use "winglets" to reduce wing tip losses. Probably also helps low speed acceleration too.

Are you referring to the tip at the max diameter or there abouts? I am referring to the progressive pitch of the pushing side of the blade in the direction of the arc from lead to trail edge at any given radius.

[Fastjeff57]

Yellowjacket is (I believe) referring to many of the cleavers I've seen, where the pitch DECREASES from front to rear (ie: regressive pitch).

Good info!

Jeff

[zul8tr]

Here is Ron Hill's take on pregresdsive pitch props

http://www.boatracingfacts.com/forums/showthread.php?41-Progressive-Pitch

[Fastjeff57]

Love your quotes! Here's one I especially like: "If two men think alike, one of them is not needed!" (Henry Drela)

Jeff

[zul8tr]

Love your quotes! Here's one I especially like: "If two men think alike, one of them is not needed!" (Henry Drela)

Jeff

There are many out there but Patton really has hundreds of good ones.