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Old 18-06-2007, 10:48 PM   #1
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Pitch difference

Anyone know if there is a way of working out how much pitch you need to drop if you are changing from a three bladed prop to a four???
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Old 18-06-2007, 11:06 PM   #2
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Ballpark dropping or adding a blade 4 to 3 to 4 will increase/ decrease by 250 RPM approx
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Old 18-06-2007, 11:46 PM   #3
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about an inch. roughtly.
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Old 19-06-2007, 12:57 PM   #4
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Anyone know if there is a way of working out how much pitch you need to drop if you are changing from a three bladed prop to a four???

Mercury say if you run a 21" 3 blade you will need to go up to a 22" 4 blade
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Old 19-06-2007, 06:55 PM   #5
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Quote:
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Mercury say if you run a 21" 3 blade you will need to go up to a 22" 4 blade
woulda though it was the other way round due to more drag from the 4th blade etc
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Old 19-06-2007, 09:24 PM   #6
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woulda though it was the other way round due to more drag from the 4th blade etc
I was thought it was the other way round to?
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Old 20-06-2007, 06:02 PM   #7
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I was thought it was the other way round to?

Perhaps, but I was only quoting what I read on the mercury web site what they wrote about there trophy plus propellors.

So maybe it only applies to them.
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Old 20-06-2007, 06:18 PM   #8
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If you have three blades pushing water around, and you add a fourth blade, you are going to move more water around than before, despite the extra drag due to the additional blade. So to keep the engine about where it was, you are going to have to move slightly LESS water now.

The question is do you understand what pitch is exactly?

To move LESS water does the pitch become bigger or smaller?



For a single revolution of the prop, the 'perfect' prop will move forward the number of inches defined by the pitch number. So a single blade of a 22 inch pitch prop will move forward 22 inches, and a single blade of a 21 inch prop will move forward 21 inches. So it is clear that the higher the pitch number the MORE water moved per revolution.

So as you now want to move LESS water per blade, you would need a lesser pitch number, so that the engine remains balanced in the equasion.


So, a 22 inch three blade should be replaced with approximately a 21 inch four blade to get the same effect.

Approximately, but a good place to start.

Four blades have higher ineficiencies due to drag and interfearance, but better properties regarding balance and effects in the water.

It is a tough call to do the switch. The question you should really ask is WHY you think you need to go from three to four? What do you anticipate to gain from doing so?
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Old 20-06-2007, 06:43 PM   #9
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There's lots of theory with props, but in truth, the best way to go, is to try every prop you can get your hands on that's roughly the right pitch/size and make a judgement from actual results.

As an example, I recently tried of pair of Mercury Mirage plus, 29's they are 3 bladers (std dia which I 'think' is 14 3/4")

the boat was slightly faster, and topped out at slightly lower rpm than my own props. both sets are absolutely std, no mods.

my own props are Mercury Bravo ones, 15 1/4" x 30" (four blade)

To me, this outcome was unexpected, as the higher pitch, larger diameter, and greater total blade area, left me expecting the Mirages to over rev, and give lower speed.

It's a black art, that's for sure.
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Old 20-06-2007, 07:23 PM   #10
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It's a black art, that's for sure.
Can you still that that ! Thought you had to say cloured or minority art
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Old 20-06-2007, 07:41 PM   #11
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yup, I can definitely that that
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Old 20-06-2007, 07:49 PM   #12
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[QUOTE=Jon Fuller;111587]There's lots of theory with props, but in truth, the best way to go, is to try every prop you can get your hands on that's roughly the right pitch/size and make a judgement from actual results.

My thoughts exactly.



Reading a bit more into 4 blades, the trophy plus in particular, was as far as i am aware, was developed for bass fishing boats in the states. it was made to increase your hole shot with as little loss in top speed as possible.

This is only what I have read up on ,not tested.

As for my previous post this is where I read it

http://www.mercuryracing.com/product...trophyplus.php
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Old 20-06-2007, 08:26 PM   #13
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yep thats prob because some of these newer engines (just like the 300 promax) aren't fastest when run on the limiter, so yes if your running on the limiter changing up a pitch will most likely be quicker but when changing prop from 3 to 4 blade you will also loose abit of speed.
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Old 20-06-2007, 08:59 PM   #14
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It's simple logic and fact, that if you're on the limiter, going up on pitch will improve speed, but I don't believe it's 'fact' that 4 bladers are slower than 3 bladers.

Hydromotives, herrings etc, i'm sure there are some that are as quick as any 3 bladers. But that's my point, it aint a science, the variables are infinate, try everything you can borrow.
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Old 20-06-2007, 10:10 PM   #15
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Quote:
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But that's my point, it aint a science, the variables are infinate, try everything you can borrow.
Exactly wot im doin!
By trying different props you can find different propellors for different conditions.
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Old 20-06-2007, 10:38 PM   #16
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I reckon I can I safely say that if you took the blades off a 4 blader and made em into an identical 3 blader, you new 3 blader would either be faster or slower than the 4 blader.
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Old 20-06-2007, 10:50 PM   #17
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I reckon I can I safely say that if you took the blades off a 4 blader and made em into an identical 3 blader, you new 3 blader would either be faster or slower than the 4 blader.
Actually, you can say a bit more than that, but it goes to the question - what problem are you trying to solve? Why do you want to change props? A diameter change or a design change may be more efficient for solving the specific issue you may be having.

If you just want a general theoretical rule of thumb then its 1 inch when you change.
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Old 20-06-2007, 11:12 PM   #18
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In theory from what I've read, the fastest of all props shoud be a one blader.

Probably vibrates a little though.
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Old 21-06-2007, 01:21 AM   #19
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Quote:
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In theory from what I've read, the fastest of all props shoud be a one blader.

Probably vibrates a little though.
In theory, you are correct. Mathematically it is the perfect prop. But the imbalance screws the performance.

So you go for a twin blade, then you hit other issues, so a three blade is a good compromise. IMO the three blade is the best allround prop, but if you want to solve a specific issue, number of blades is just one of about 10 parameters you can play with. Another biggie is diameter, as is sweap.
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Old 21-06-2007, 07:24 AM   #20
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What about sooty & sue?
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