pitch vs speed vs efficiency
06-23-2011, 05:53 PM
#1
Registered
Thread Starter
pitch vs speed vs efficiency
To what point does pitch cause a problem?
Example:
36P @ 4K RPM to get on plane is slipping way more than a 30P @ 2.5K RPM to get on plane. And that 36P sure as heck isn't getting me on plane at 1200 RPM. This leads me to believe that higher pitched props work better at higher speeds.
Example:
40P @ 100 MPH vs 34P @ 100 MPH
I'm guessing the 34P will be more efficient at this speed because the water is not going by fast enough for the 40 to bite well so it ends up wasting energy paddling the water. Versus the 34 spinning like crazy at 140 MPH and wasting energy each time a blade hits the water, losing thrust because of slight cavitations.
Want some math?
A 4bl 34P spinning at 3K RPM would have had each blade hit the water (surface piercing) 3K times PM. That's 3K RPM X 4 = 12K hits at say 0 % slip/97 MPH. Way more hits=chances for cavitation?
A 4bl 40P spinning at just over 2.5K RPM is 2.5K X 4 = 10K hits at say 0% slip/97 MPH.
At slower speeds do the larger pitched props spend more energy lifting/lowering the back of the boat than at higher speeds?
The end question is:
Is there an ideal pitch for specific speeds? Is a 30P a good prop from 95-105 and do 34's work best in the 100-110's and 40's in the 160's. Or is a 28P better at 110 MPH?
I'm not talking about horsepower and gear ratio. Just a prop turning at an rpm where it is most efficient.
Go ahead and ad classes like surface, semi-surfacing and fully submerged.
And I'm wondering if my boat would do better with a smaller prop spinning faster to achieve 100MPH.
Should I shut up now?
Example:
36P @ 4K RPM to get on plane is slipping way more than a 30P @ 2.5K RPM to get on plane. And that 36P sure as heck isn't getting me on plane at 1200 RPM. This leads me to believe that higher pitched props work better at higher speeds.
Example:
40P @ 100 MPH vs 34P @ 100 MPH
I'm guessing the 34P will be more efficient at this speed because the water is not going by fast enough for the 40 to bite well so it ends up wasting energy paddling the water. Versus the 34 spinning like crazy at 140 MPH and wasting energy each time a blade hits the water, losing thrust because of slight cavitations.
Want some math?
A 4bl 34P spinning at 3K RPM would have had each blade hit the water (surface piercing) 3K times PM. That's 3K RPM X 4 = 12K hits at say 0 % slip/97 MPH. Way more hits=chances for cavitation?
A 4bl 40P spinning at just over 2.5K RPM is 2.5K X 4 = 10K hits at say 0% slip/97 MPH.
At slower speeds do the larger pitched props spend more energy lifting/lowering the back of the boat than at higher speeds?
The end question is:
Is there an ideal pitch for specific speeds? Is a 30P a good prop from 95-105 and do 34's work best in the 100-110's and 40's in the 160's. Or is a 28P better at 110 MPH?
I'm not talking about horsepower and gear ratio. Just a prop turning at an rpm where it is most efficient.
Go ahead and ad classes like surface, semi-surfacing and fully submerged.
And I'm wondering if my boat would do better with a smaller prop spinning faster to achieve 100MPH.
Should I shut up now?
06-23-2011, 06:59 PM
#3
Offshoreonly Advertiser
Offshoreonly Advertiser
A larger pitch requires more torque to rotate causing the engine to require more rpm. At the point that it hits this level, the torque may now over power the prop as well as the higher pitches seeking the surface more and you get ventilation causing a higher rpm to create thrust enough to get the boat moving and on plane. Also 40 pitch props tend to be on high X dimension high HP boats amplifying the venting problem.
40P at 4500 with 1.5:1g.r is 100hph at 12% slip. Pretty efficient.
34P at 7400 is 100 at 12%. Still efficient.
High speeds max rpm create the most stern lift. Different geometry's create less stern lift or more transom leverage.
The ideal pitch for speed is whatever rpm your engine is designed to run at. A 30P prop will be a poor choice for a 100HP engines top speed. Likewise a 19P won't do much good on a turbine. You need to match the prop to the boats personality. I think what you mean is this.............. I have always found 2 times the diameter equals the pitch, is a good recipe for efficiency.
Your talking about a bunch of variables. You realy need to be specific on the boats set up. If there was a magic formula, we'd all have it or be using it.
Brett
40P at 4500 with 1.5:1g.r is 100hph at 12% slip. Pretty efficient.
34P at 7400 is 100 at 12%. Still efficient.
High speeds max rpm create the most stern lift. Different geometry's create less stern lift or more transom leverage.
The ideal pitch for speed is whatever rpm your engine is designed to run at. A 30P prop will be a poor choice for a 100HP engines top speed. Likewise a 19P won't do much good on a turbine. You need to match the prop to the boats personality. I think what you mean is this.............. I have always found 2 times the diameter equals the pitch, is a good recipe for efficiency.
Your talking about a bunch of variables. You realy need to be specific on the boats set up. If there was a magic formula, we'd all have it or be using it.
Brett
__________________
Brett Anderson / BBLADES Professional Propellers
920-295-4435 http://www.bblades.com/
[email protected]
Brett Anderson / BBLADES Professional Propellers
920-295-4435 http://www.bblades.com/
[email protected]
