4 blade to 3 blade, speed?

undercover500

Seaman Apprentice
Joined
Apr 21, 2022
Messages
40
How much speed do you gain, going from a 4 blade to a 3 blade? Assuming RPM's stay the same, or increase a little. Just curious, obviously it won't be the same across the board, but what would you estimate? 1-2mph? 3-4?

We're getting 45mph out of a 4 blade at ~5,600rpm, so a bit over propped, we're going down to a 15 pitch 4 blade soon, as our engine can rev up to 6,300. It'd be interesting to see how fast a 3 blade would push us at 6,300rpm.
 

dingbat

Supreme Mariner
Joined
Nov 20, 2001
Messages
13,768
How much speed do you gain, going from a 4 blade to a 3 blade?
Assuming RPM's stay the same, or increase a little. Just curious, obviously it won't be the same across the board, but what would you estimate? 1-2mph? 3-4?
Depends on the specific prop. Different designs and manufacturers produce diffrent results.
We're getting 45mph out of a 4 blade at ~5,600rpm, so a bit over propped, we're going down to a 15 pitch 4 blade soon, as our engine can rev up to 6,300. It'd be interesting to see how fast a 3 blade would push us at 6,300rpm.
Decreasing the pitch by 13.3% (17/15) and increasing the RPM by 13.3% (5600 to 6328 rpm) is a net gain of Zero
 

QBhoy

Admiral
Joined
Mar 10, 2016
Messages
6,208
What engine is this ? Only engine I know that can pull those rpms is a 115 pro xs 2.1l. Might be one or two others perhaps.
Unless it’s a built thing.
 

undercover500

Seaman Apprentice
Joined
Apr 21, 2022
Messages
40
6300 RPM is a bit high for a production engine.
Our engine is a 115 Mercury Pro XS, WOT rev range of 5300-6300

Full throttle RPM
  • 5300-6300
 

undercover500

Seaman Apprentice
Joined
Apr 21, 2022
Messages
40
Depends on the specific prop. Different designs and manufacturers produce diffrent results.

Decreasing the pitch by 13.3% (17/15) and increasing the RPM by 13.3% (5600 to 6328 rpm) is a net gain of Zero
I understand that is between two comparable 4 blades, but if we went with a 15 pitch 3 blade, would that increase our speed?
 

QBhoy

Admiral
Joined
Mar 10, 2016
Messages
6,208
Our engine is a 115 Mercury Pro XS, WOT rev range of 5300-6300

Full throttle RPM
  • 5300-6300
Great engine. What boat is it on. I’ve got the 115 too. It’s a CT model. Tried loads of props on it so far. Maybe I can help
 

dingbat

Supreme Mariner
Joined
Nov 20, 2001
Messages
13,768
I understand that is between two comparable 4 blades, but if we went with a 15 pitch 3 blade, would that increase our speed?
The number of blades has no bearing on the theoretical forward speed of a prop. Its the pitch of the prop that determines forward speed.

The net result of dropping a blade and decreasing 2" in pitch is roughly a 300-400 RPM increase in WOT.

17" x 5600 = 95,200
15" x 6000 = 90,000
You would actually lose speed.

Four blade props are notoriously inefficient (slip) at high rpm. To see any real increase in speed you need to reduce slip.

I swapped out a good running 4 blade for a very efficient 3 blade prop when I found my slip approaching 17% at WOT. The 3 blade reduced slip and increased speed by roughly 10% (3.5 kts) in the process.
 

undercover500

Seaman Apprentice
Joined
Apr 21, 2022
Messages
40
Great engine. What boat is it on. I’ve got the 115 too. It’s a CT model. Tried loads of props on it so far. Maybe I can help
It is not the CT model, just the regular gear case. It's on a Tahoe T18. With 4 people we hit around 41mph, with 2 people we hit around 43 and with just one I've hit 45mph. So it's definitely got a lot of power and can carry a load well, I'd mainly like to get our RPM's more towards the max of 6,300rpm so that we're not lugging the engine, if some speed comes from it I suppose that would be nice too.
 

QBhoy

Admiral
Joined
Mar 10, 2016
Messages
6,208
It is not the CT model, just the regular gear case. It's on a Tahoe T18. With 4 people we hit around 41mph, with 2 people we hit around 43 and with just one I've hit 45mph. So it's definitely got a lot of power and can carry a load well, I'd mainly like to get our RPM's more towards the max of 6,300rpm so that we're not lugging the engine, if some speed comes from it I suppose that would be nice too.
So what pitch is on it now. Haven’t seen or read this so far. Sorry if I missed it. Is the tachometer defo right ? You’d think that if you’re getting that performance and able to carry a few onboard too..it would seem to be extremely low rpm you’re seeing now…for an engine that likes to be about or over 6000 rpm. That’s why I’m asking around the current pitch and rpm accuracy
 

undercover500

Seaman Apprentice
Joined
Apr 21, 2022
Messages
40
So what pitch is on it now. Haven’t seen or read this so far. Sorry if I missed it. Is the tachometer defo right ? You’d think that if you’re getting that performance and able to carry a few onboard too..it would seem to be extremely low rpm you’re seeing now…for an engine that likes to be about or over 6000 rpm. That’s why I’m asking around the current pitch and rpm accuracy
17 pitch 4 blade, and the RPM's are being recorded through Vessel View. I don't know how much more accurate you can get with a bluetooth module connected into the engine. We're seeing around 5,300rpm with 4 people on it and around 55-5,600rpms with 2-3 people.
 

QBhoy

Admiral
Joined
Mar 10, 2016
Messages
6,208
17 pitch 4 blade, and the RPM's are being recorded through Vessel View. I don't know how much more accurate you can get with a bluetooth module connected into the engine. We're seeing around 5,300rpm with 4 people on it and around 55-5,600rpms with 2-3 people.
Yeah. That will be accurate for sure. Use the same.
 
Top