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Old 05-02-2007, 10:21 PM   #1
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3,4,5 Blade prop question

Hi there!

when i have a 3 blade prop of 20" and if i try a 4 blade and 5 blade prop of same pitch, how will the amount of propblades work to the rpms?

Or is it possible to say " you can see a 20" 5 blader like a, for example, 19" three blader?

all props should be seen with same diameter.

Thanks in advance!
Greetings!
Andi
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Old 05-02-2007, 10:31 PM   #2
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Difficult to say, depends on what the setup needs.

But all other things being equal, a single bladed prop would be the most efficient since a significant amount of power is wasted on the tip (same problem aeroplanes have, which is why many modern aircraft have exotic wing tips)

So a 3 blade should be faster than a 4 blade of identical blade area, pitch, rake, cup etc. (Not even sure if that's possible)
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Old 06-02-2007, 10:59 AM   #3
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more blades - less vibration better holeshot but as Matt said, the extra weight + drag of the 4th blade will mean lower top end.
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Old 06-02-2007, 01:15 PM   #4
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Quote:
more blades - less vibration better holeshot but as Matt said, the extra weight + drag of the 4th blade will mean lower top end.
This is what i already know. I asked this question because there is a propcalculator-software out there in U.S. where you can choose the number of blades ( 3,4 or 5 blades ) before calculating!

I have also written such a program and i want to build this in, if this is important.

The difference is, that my tool is freeware :-)

Greetings!
Andi
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Old 11-02-2007, 01:43 AM   #5
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Quote:
Originally Posted by Fletcher-Andi View Post
This is what i already know. I asked this question because there is a propcalculator-software out there in U.S. where you can choose the number of blades ( 3,4 or 5 blades ) before calculating!

I have also written such a program and i want to build this in, if this is important.

The difference is, that my tool is freeware :-)

Greetings!
Andi



Andi,

you have just entered the twilight zone.......


more blades = theoretically greater blade area total, assuming a given that the blades are equal from one prop to another.

if this were the case, assuming you had enough torque to turn it, then more blade area = more speed with a given that you can achieve the same rpm.

Hence the two blade argument. However the reality is that given the torque profile from the types of engine we use, there will be an optimum of blade number versus pitch to suit the particular usage, and it I guarantee it won't be two blade.

Now complicate this further with the fact that a higher rake will encourage a lift to the bow which should theoretically give less wetted area and therefore a more efficient hull speed, and we have the classic question...............

Do I increase rake to give less wetted area or am I not taking advantage of the available torque hence not achieving use of the total horsepower at maximum rpm.

Then take the fact that I am racing and can afford to do the first kilo off plane because my judgment is top, and all I need is a prop that performs between the torque that arrives between say 3500 and 6500 rpm.

I have considered your question quite deeply and think on balance of arguments that you should plump for a pair of surface driven 5 bladed 30 spoons on 1.35 ratio direct drive with wiffle pattern plate non reversing harbinger drive with exterior cupping of 1 - 2" leaving the internal blade pattern non polished to increase traction.

By the way before we get too detailed what boat are you running this on?
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Old 11-02-2007, 08:45 AM   #6
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Hi!

Thanks for your detailed answer!

I am not a professional race-driver, i only have a Fletcher 199 with a yam 115 on her back.
I never had a race :-) Its just for fun!
If you want, you can take a look at the propcalculator, just click here! to download. Its a smal " Exe " with around 640 kb.
There is also a pic of my brothers boat, its the same fletcher with 225 Yammi.
There are some guys out there who told me that the tool wokrs great....

Best regards!
Andi
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Old 13-02-2007, 01:54 PM   #7
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First of all, put your current numbers into one of the prop calculators in the web, and calculate your prop slip figure. It should be somewhere between 6% and 22%.

You need to know the prop pitch (if it has been labbed or modified, the stamped number may not be the true pitch, however), the drive ratio, the max GPS speed and at what revs you acheive that.

It simply compares the theoretically achievable speed to the real speed.

If you are currently running a high slip figure (over 18%) you may well benefit from an extra blade. At low figures (under 9%) you definitely will not.

When going to an extra blade, expect to drop an inch or two of pitch. A 25" Mirage 3 Blade runs the same revs as 24" Bravo 4 blade at a given speed, for example.

Your slip level drops, and you may gain some speed.
However....
Slip is not inherently bad. You may get the best out of your rig at a high slip level.
Labbed (thinner blades) slip more than standard props, but are most often faster.
The lower a drive is mounted, the less blades you need, and the slip level is lower....but you have more drive in the water, which is resistance.
The higher the drive is mounted (it starts to become a surface drive situation) the more blades you will need to transmit the power, but you have less drive in the water.

Thats about all I know, apart from one last rule.

What works on one boat doesn´t neccessarily work the same on the next one!

Yes, it is the twilight zone.........
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Old 13-02-2007, 03:44 PM   #8
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Hi!

I already know about slip.

Quote:
When going to an extra blade, expect to drop an inch or two of pitch. A 25" Mirage 3 Blade runs the same revs as 24" Bravo 4 blade at a given speed, for example.
That was what i asked for, i wanted to know how the behavior ist between the 3,4,5 blade props!

but finally i think, i will not implement it into my propeller-calculation-software. Because its not for race boats its for boats with " commercial " , unmodified Stainless Steel Props like laser2 for example :-)

Quote:
the stamped number may not be the true pitch, however
Is the progressive pitch the reason for it?

A progressive pitched prop with 20" will have 18"-22".
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Old 13-02-2007, 04:03 PM   #9
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I´m not sure how things are in your power/weight class, but where my boat lies (525 HP at the prop, 2150kg) the four and five blade props can be really nice to drive because the boat feels more connected and responds much more to throttle input, which can be really essential when running in rough water.
It´s not just about top speed.

But the wrong prop can turn a good boat into a f##king handfull.
Been there....

On standard factory props, the pitch varies as much as a half inch in either direction. The moment the prop is labbed (factory or otherwise) its anybody´s guess what the effective pitch is. Labbed props tend to have less cup (the curl up on the trailing edge) and this effects the pitch alot.
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Old 13-02-2007, 05:30 PM   #10
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The Boat-picture in my prop-calculator is the boat i drive, light weight and deep vee, similar to a Phantom 20 but more comfortable, its not a racer, but fast :-)
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Old 14-02-2007, 09:55 PM   #11
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Quote:
Originally Posted by Wild Card 09 View Post
First of all, put your current numbers into one of the prop calculators in the web, and calculate your prop slip figure. It should be somewhere between 6% and 22%.

You need to know the prop pitch (if it has been labbed or modified, the stamped number may not be the true pitch, however), the drive ratio, the max GPS speed and at what revs you acheive that.

It simply compares the theoretically achievable speed to the real speed.

If you are currently running a high slip figure (over 18%) you may well benefit from an extra blade. At low figures (under 9%) you definitely will not.

When going to an extra blade, expect to drop an inch or two of pitch. A 25" Mirage 3 Blade runs the same revs as 24" Bravo 4 blade at a given speed, for example.

Your slip level drops, and you may gain some speed.
However....
Slip is not inherently bad. You may get the best out of your rig at a high slip level.
Labbed (thinner blades) slip more than standard props, but are most often faster.
The lower a drive is mounted, the less blades you need, and the slip level is lower....but you have more drive in the water, which is resistance.
The higher the drive is mounted (it starts to become a surface drive situation) the more blades you will need to transmit the power, but you have less drive in the water.

Thats about all I know, apart from one last rule.

What works on one boat doesn´t neccessarily work the same on the next one!

Yes, it is the twilight zone.........
I,m with you about most things ,but its not that it works the same on all aplications.

For example the higher the drive is mounted the more blades you will need to transmit the power,most fast rigs with extremely high X dimensions ( above the bottem with centerline propshaft)have 3 blade propellors for max speed.

I have labbers on my cat ,15x34 cleavers ,and before I used stock 15x 34 cleavers (both mercury cleavers),my speed increased 3-4 mph ,nothing changed further,rpm,s are about the same.

In the rough stuff usually more blades will help keeping thrust ,and are less vibrating,and softer for the outdrives.

Strange enough The higher I jacked my engines ,the more speed I,ve got,but it did not change a lot in rpm,s ,probably the lowers where having less drag,and therefore the slip was lower.

I had to make 2 external bottem pickups in the sponsons,and route 3/4 ID hoses to both midsections where the water could enter the powerheads,to keep waterpressure up.
centerline propshaft is 2 inches above the bottem now.

I,m turning my props inward ,and the way I mounted my engines ,it gives a lot sternlift ,and allowed the cleavers doing their job perfectly at this high transom setting.

sterling
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Old 18-02-2007, 12:00 PM   #12
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What's a good prop size to run on a Merc Black Max 200 V6 and an 18ft Ring, out of interest? Not interested in massive out and out speeds etc. as I'm not a racer, but a good all-round general purpose choice? Just curious...
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Old 18-02-2007, 02:46 PM   #13
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I think a 25 " Laser 2 could do it for alround things!
If oyu will not reach your RPMs, just take the 23"

Greets!
Andi
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Old 18-02-2007, 10:51 PM   #14
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Quote:
Originally Posted by Fletcher-Andi View Post
I think a 25 " Laser 2 could do it for alround things!
If oyu will not reach your RPMs, just take the 23"

Greets!
Andi
Cheers Andi! I saw one on Fleabay a little while back and I was tempted as it was cheap. Then again, there's nothing really wrong with the standard one that's on there so I couldn't really justify it other than for pose value.
25" would probably help fuel consumption though. Hmm...
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Old 19-02-2007, 03:58 PM   #15
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let me know which prop you drive currently at which max. RPMs, sometimes you will get better performance.

The laser2 gives bowlift. That means a little more speed!
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