If a 16X9 3-blade fixed propeller can load an engine to maximum 3050rpm, and is replaced with a 15X10 propeller (same type of propeller but with 1 inch less diameter and 1 inch more pitch) how will this affect the maximum rpm? Will the change increase or reduce maximum rpm and by how much?
A bit of prop talk
- Thread starter Philip_Oceanis 361
- Start date
Don't want to hijack this thread but it seems like a place to start, please, let me know if I should move this to a new thread.
Ok, that seems like a good way to arrive at a power number. I have a Hunter 28 with a Yanmar 2gm20. Current prop is a RH15x13. I can only get about 2800 rpm before it starts to overload and blow smoke and fuel out of the exhaust. My research says I'm way over propped since top RPM is supposed to be about 4000 and cruising RPM is about 3400.
In looking to a smaller prop I understand that a rule of thumb is 1 degree of pitch or 1 inch of diameter should equate to about 1000 rpm. So using that rule something like a 14x8 or a 13x9 would put us closer to the proper size.
Now my question, when looking at a prop where do we get the torque for low end power? My intuition says that engine torque comes from the diameter and drive comes from the pitch, or am I thinking this through wrong. Once moving, the boat will go to hull speed however, when maneuvering at slow speeds, stopping and starting, I want to ensure the boat will respond and don't want to go too low on torque for that initial stopping power.
So a larger prop with less pitch or a smaller prop with more pitch, which way should I go?
Thoughts?
Ok, that seems like a good way to arrive at a power number. I have a Hunter 28 with a Yanmar 2gm20. Current prop is a RH15x13. I can only get about 2800 rpm before it starts to overload and blow smoke and fuel out of the exhaust. My research says I'm way over propped since top RPM is supposed to be about 4000 and cruising RPM is about 3400.
In looking to a smaller prop I understand that a rule of thumb is 1 degree of pitch or 1 inch of diameter should equate to about 1000 rpm. So using that rule something like a 14x8 or a 13x9 would put us closer to the proper size.
Now my question, when looking at a prop where do we get the torque for low end power? My intuition says that engine torque comes from the diameter and drive comes from the pitch, or am I thinking this through wrong. Once moving, the boat will go to hull speed however, when maneuvering at slow speeds, stopping and starting, I want to ensure the boat will respond and don't want to go too low on torque for that initial stopping power.
So a larger prop with less pitch or a smaller prop with more pitch, which way should I go?
Thoughts?
I could be wrong (that's why I am asking anyway) but I think a change of 1inch diameter = change of 2 or 2,5 inches of pitch. So are you sure maximum rpm will not increase?
The exact answer is very complex because there are blade area ratios, blade shapes, .
What you are doing is pushing water and you 15x10 will move a particle of water farther than the 16x9 but it won't move as many particles.
What you are doing is pushing water and you 15x10 will move a particle of water farther than the 16x9 but it won't move as many particles.
Philip,
There is a rudimentary method to answer your question.
This test can be at the dock tied up or, while underway.
It's all about pitch vs. HP vs. RPM.
Simply, based on the manuf's. specs, if throttle wide out, you cannot get to the MAX RPM, you are over pitched. If you exceed the MAX RPM's, you are under pitched. It's that simplistic.
A 1" diameter difference is not that much of a factor, it's the number associated to blade pitch. Each pitch number times a full 360 revolution, will advance your boat by an additional 1 foot.
So, if just curious, use the simple method I mentioned to see what happens.
CR
There is a rudimentary method to answer your question.
This test can be at the dock tied up or, while underway.
It's all about pitch vs. HP vs. RPM.
Simply, based on the manuf's. specs, if throttle wide out, you cannot get to the MAX RPM, you are over pitched. If you exceed the MAX RPM's, you are under pitched. It's that simplistic.
A 1" diameter difference is not that much of a factor, it's the number associated to blade pitch. Each pitch number times a full 360 revolution, will advance your boat by an additional 1 foot.
So, if just curious, use the simple method I mentioned to see what happens.
CR
Philip,
I should also mention that the free space from a prop blade @ 12 o'clock should not be less than 1" from the bottom of your hull. In fact, if you can get to 1 1/2" you will get better performance. Too little space & your blades will cause air bubbles between the top half of the blade & the hull, thus slowing you down. You will feel this because you will experience, "Cavitation" at this point & feel your stern squat.
There is an article from Sail magazine dated Sept. 1994 on this subject titled, "The Proper Prop: A Guide To Options". See if you can find it pal.
CR
I should also mention that the free space from a prop blade @ 12 o'clock should not be less than 1" from the bottom of your hull. In fact, if you can get to 1 1/2" you will get better performance. Too little space & your blades will cause air bubbles between the top half of the blade & the hull, thus slowing you down. You will feel this because you will experience, "Cavitation" at this point & feel your stern squat.
There is an article from Sail magazine dated Sept. 1994 on this subject titled, "The Proper Prop: A Guide To Options". See if you can find it pal.
CR
