Something on the 150 -200 rpm per inch of prop pitch change

[Texasmark]

Props shown. Speed went from 50 @ 5700 with the 24 XP down to 40 with the 21P @ 5800 and 47.5 with the 21P at 6600. So let's extrapolate the 47.5 at 6600 up to 50 which would academically relate to 6600/47.5/50 = 6947 rpm. So I have a 3" pitch delta and a 10 mph speed change at roughly the same rpms. If I up the rpms to 6947 to get the speed near the 50 mph I could extrapolate to get the 6600 on up to the 50 number but the point is I changed pitch 3 inches x the 200 rpms per inch (recommended) numbers and they don't fit.

So assuming I had the 21 to start with and wanted to drop my rpms using the 50 mph speed number I would be working with 50/47.5 x 6600 =6947 rpms calculated. So for a given speed the pitch change was 3" and the rpm change was 6947/5700 = 1247.

So I theoretically have a 1247 rpm change for a 3" pitch change. Using the subscribed number of 150 to 200 rpm per inch (using the big number of 200) for a 3" pitch change would be 600 rpms. But mine changed 1247.

So, in short nothing is cast in concrete and rule of thumb is just a SWAG! You don't know a thing until you run it. Everything else is just that.....SWAGs and buyer beware. Besides you are dealing with A to Z in boats, weight, engines, props, hull efficiency, engine sizes, attitudes of the engine and hull, water and weather conditions, altitude differences, salt vs fresh water, currents vs no currents, state of repair and on and on.

So when you open a thread and say "I need a prop for xyz" here's a sampling of how complicated an answer can be,.

Hope this helps someone.

Mark

 

[Sea Rider]

Re: Something on the 150 -200 rpm per inch of prop pitch change

Hi Texasmark,

Did you played with just pitch or with diam and pitch also ? If with just pitch, you did slight more than 400 rpm per pitch increase. Good to know that (150-200) rpm increase/decrease per pitch, including prop slip constantly mentioned on the prop forums doesn't apply as there are too many tech factors and variasbles to take into account.- Will add optimum leg height into your tech considerations.- That's why I'm not a big fan of cast on stone written theory, prefer the real world parctice.- Thanks for the practical experiment & post.

Happy Boating

 

[drewactual]

Re: Something on the 150 -200 rpm per inch of prop pitch change

it's said that a progressive pitched prop is rated at it's average pitch- NOT the pitch in the center of the blade, not the leading edge, and not the trailing edge...

it would be kinda nice if there was an expression that demonstrated to a consumer how much pitch varies across the blade, and where the rated pitch is located on the blade... It could be the difference between a person thinking a WOT run was using a 21p, when that blades trailing edge is 23p- and then attempting to compare it to another 21p that has a trailing edge less aggressive, or around 22p, and the consumer shaking his head in bewilderment when moving down a pitch to 20p (thinking that is what they need to do) only to discover they are well over their rated RPM's...

the cup as well.. how much and how quickly does it cup in terms of surface area of the blade? I've held props in my hand that had a pronounced lip more akin to a bend than a cup, and I've held props that were advertised as cupped that sure didn't look too cupped to me, if at all... maybe a maker could express cup in terms that matter, something like "at such-and-such angle of attack, the cupping will redirect and give the sensation of an x variable in pitch"... yeah, that's too much to ask, I'm sure, as this variable is linked in an unbreakable way with length, waterline, deadrise, eh- the design of the hull...

vents: are the vents just literal holes tapped into the casing? Or, are the vents slotted with a relief- this is a visual thing, as most of these things I'm pointing out are and can be figured IF you're holding the prop (to a degree), but in today's world of interwebby purchases, it is hard to express... a drilled hole will vent until surface tension on the leading edge creates an area of high pressure within that hole greater than the exhaust, and it will 'seal', as it is easier for the exhaust to escape elsewhere than there... but if there are reliefs in the vents, they may seal quicker (relief on leading edge) or later (relief on trailing edge) than those just tapped into the casing..

crap is confusing...

 

[hwsiii]

Re: Something on the 150 -200 rpm per inch of prop pitch change

Mark, you are definitely right, it ALWAYS goes back to prop blade GEOMETRY, pitch is a starting point ONLY. Pitch can be a useful guideline, but mainly only in the same Make and Model of prop. But blade geometry and motor height are definitely the keys to major increases or decreases in speed and RPM's. That was really brought home again when I did the charts for all of Kenny's prop tests with his boat years ago, as I found as much as 5.7 MPH difference between the same pitches of props (all stainless Steel models), and only a difference of 50 RPM's between the two with a prop slip of 9.3% difference between them, using the Lexor 17" pitch and the Turbo 17" pitch.





In your particular case the differences between the the 21" pitch Ballistic and the 24"pitch Ballistic XL is the XL has another 3/8" of diameter which creates more blade surface area, as well as it has a higher rake angle which produces a higher bow lift and thus less wetted surface area and the Ballistic XL has vent holes for acceleration in it, but I didn't remember the XL having vent holes.That is all I really know about the differences there might be.

I did run an analysis on the two props through my software and it shows the Ballistic XL has a 21.6" True Effective pitch with no slip, and 23.8" Real Effective pitch with a 10% prop slip. The normal Ballistic has a 17" True Effective pitch with no slip, and only 20.2" Real Effective pitch with a 19% prop slip.

There is a correlation between prop slip, True Effective prop pitch and Real Effective prop pitch. I am taking this opportunity to post in this thread to hopefully give people a better understanding of how prop slip directly correlates to the real effective prop pitch that they are getting with any particular prop on their boat and motor combination. True Effective prop pitch is what this particular prop pitch would actually produce, in speed, if there was absolutely no slip involved, although if there was no prop slip then the prop would not be able to push and pull the boat forward. Real Effective prop pitch is what the prop is actually producing in pitch, with the amount of slip that prop produces with that particular boat and motor combination at that particular RPM with its inherent weight and hull design as well as height of the motor on the transom taken into consideration as well.

In your particular case Mark the Ballistic XL is showing an average of 23.8" of pitch at 10% slip and the Ballistic is showing only an 20.2" pitch at 19% slip, therefore producing only 40 MPH with the Ballistic versus the Ballistic XL producing 50 MPH at almost the same RPM. So it is very close to the manufacturers representation on your particular boat and motor combo, but the Ballistic is not producing as close to its manufacturers representation because of the differences in their blade geometry, and your particular boat and motor combo.




Drew, there is a way to calibrate exactly much cup is placed in any particular propeller whether it is on the trailing edge or the blade tip, and the United States Navy has the specs for it, as they can't easily afford to have new props made for warships that have costs running into the hundreds of thousand of dollars. But small propeller manufacturers choose not to use it, and they also reap the rewards of people buying many props for their boats, that is a win win for them. You can't even find a representative at 99% of the propeller manufacturers that have any idea what exact geometry is manufactured into any models of their props so you can make an intelligent decision when buying props, another win win situation for them.


H

 

[Texasmark]

Re: Something on the 150 -200 rpm per inch of prop pitch change

HW. I drilled the ports in the prop as it was not ported and I wanted the XL performance at WOT but wanted more of a hole shot. I originally drilled 1/4" holes and tomorrow I am going to test my enlargement to 5/16". Thanks for the info.

Haven't heard from Walleyehed in a long time. Heard that his dad passed on and it put a dent in his life and his plans. Have no idea as to what he is up to now. He was going to relocate to Florida and participate in off shore racing.

Thanks,
Mark

 

[jimmbo]

Re: Something on the 150 -200 rpm per inch of prop pitch change

I can only comment on that the 200rpm/inch has held up well for me for over 35 yrs. It worked with my dads 1965 90hp, it had gear case with 23:20 ratio; 10' dia prop, 11" pitch did 4400rpm, a 10" dia 9 1/4" pitch did 4800 rpm. My 1984 merc 115hp with 2:1 ratio; a 19 inch did 5400, a 17 did 5800. a 21 did 5000. My 5.7 VP with 1.5:1 ratio; 20" 4900, 22" 4400

 

[dan02gt]

Re: Something on the 150 -200 rpm per inch of prop pitch change

Typically the 150-200rpm/inch is for props in the same family. For example a 21P Laser II vs a 22P Laser II, but you can't use that when comparing a 21P Laser II to a 22P Mirage Plus because of the differences in diameter and blade geometry.

 

[Tail_Gunner]

Re: Something on the 150 -200 rpm per inch of prop pitch change

There are so many different performance category's or type's of performance category's it really hard to define what work's when how and where.For our little runabout's its fairly straightforward and easy to figure out. For myself it's lift vs thrust and finding a balance the hull responds best to. Notice that jpg turbo and stilletto..both overall big lifter's and top speed......ahh then again there is the preference's as to ride quality..midrange or top end..so many prop's and so little time....and in this economy so little money..

Ive been playing with a mini offshore boat regular deep v tab's and so on. But what a pain this thing is trying to balance lift thrust and wetted surface drag into one pitch and prop type. I thought i had the right prop but was fooled but what i think was blade area and drag..Hwsii pointed out the blades were designed to work in a areated water stream...alway's someting.

You should see the blade thickness it a txp ot4 im sure it could cut a log in two.

 

[Frank Acampora]

Re: Something on the 150 -200 rpm per inch of prop pitch change

Hey Mark: I have been following this thread but not commenting.

You are comparing like brands of props with slightly different design. However, my only comment is that you need pitch to go fast, provided the engine can turn it.

Consider a 4 barrel chevy 350 with 4.90 and 2.75 rear end gears. The engine has the power to turn both sets. The 4.90 will give blazing acceleration yet top speed will be low while acceleration with the 2.75 will suffer but ultimate top speed will be greater.

You have relatively the same situation: Your engine is capable of turning a 23 pitch prop. So, at a given RPM, speed will be higher than the 21 pitch. Additionally, as mentioned above, the 23 pitch has MORE blade area, leading to less slip.

When you compare props with different blade area and pitch, all bets are off and the 200 rule of thumb does not apply. I experienced much the same thing when I switched from stock Force 13 X21 stainless to Ballistic 13 X 21 pitch. RPM went way down, more than expected but speed stayed the same. (My engine was not capable of turning the Ballistic faster)

Note that the 200 RPM is a RULE OF THUMB. It is simply a good starting point and surely, as you have demonstrated, not etched in stone.

 

[Texasmark]

Re: Something on the 150 -200 rpm per inch of prop pitch change

Hi Texasmark,

Did you played with just pitch or with diam and pitch also ? If with just pitch, you did slight more than 400 rpm per pitch increase. Good to know that (150-200) rpm increase/decrease per pitch, including prop slip constantly mentioned on the prop forums doesn't apply as there are too many tech factors and variasbles to take into account.- Will add optimum leg height into your tech considerations.- That's why I'm not a big fan of cast on stone written theory, prefer the real world parctice.- Thanks for the practical experiment & post.

Happy Boating

Cheers. I too am a real world guy. You just can't even guess accurately or even close in most cases. So all these folks logging on here for us to provide them with the optimum prop especially when they give us nothing to guess from are just........well I'll leave it at that.

The diameter change I assume was a prop designer's thing. If you look at a series of props and go through various pitch changes you will notice that the diameter moves around quite a bit. As long as it changes fractions of an inch I don't worry about it.

As I mentioned in my post, part of the change was boat dynamics. Having a light boat with a stepped hull, not a pad boat like a bass boat, but an alum boat with 4 distinct flat changes of 1" across the hull at the transom, the faster you go the less hull is in the water and so the speed disproportionally changes.

Mark