Propeller Efficiency Curves
03-01-2002, 11:59 AM
#1
Anyone ever see actual propeller efficiency curves for the props we all play with, Mirage, Bravo, Hydromotive, etc.? It might help us when we are trying to optimize a setup. We all think about power and torque and RPM and slip, but all the boat knows is thrust and drag.
Prop efficiency = Thrust HP/propshaft HP
Thrust HP = Speed x Thrust (ft/sec x lbs)/550
1 HP = 550 ft-lbs/sec
Mercury says that 80% is about the best efficiency you can get and that value varies according to slip(angle of attack). If slip is too high, then the amount of propshaft HP that actually gets converted into forward thrust is reduced. If slip is too low, the amount of forward thrust is also reduced, as you use up too much power trying to force too much blade area through the water.
I have attached a chart from Mercury's propeller book. This chart shows efficiency dropping to 70% in a fairly narrow range of slip(angle of attack). This could make a big difference in speed since it is ultimately prop thrust that overcomes the drag of the hull and drive.
In this example Mercury says the maximum efficiency occured at 3-4 degrees angle of attack which works out to about 10% slip, what you might expect with a Mirage on a vee hull.
I have a feeling that some of the "mysteries" of prop selection could be better understood if we had access to the real curves from the prop manufacturers. Anyone out there with this info?
You guys labbing props have anything to say about this?
Prop efficiency = Thrust HP/propshaft HP
Thrust HP = Speed x Thrust (ft/sec x lbs)/550
1 HP = 550 ft-lbs/sec
Mercury says that 80% is about the best efficiency you can get and that value varies according to slip(angle of attack). If slip is too high, then the amount of propshaft HP that actually gets converted into forward thrust is reduced. If slip is too low, the amount of forward thrust is also reduced, as you use up too much power trying to force too much blade area through the water.
I have attached a chart from Mercury's propeller book. This chart shows efficiency dropping to 70% in a fairly narrow range of slip(angle of attack). This could make a big difference in speed since it is ultimately prop thrust that overcomes the drag of the hull and drive.
In this example Mercury says the maximum efficiency occured at 3-4 degrees angle of attack which works out to about 10% slip, what you might expect with a Mirage on a vee hull.
I have a feeling that some of the "mysteries" of prop selection could be better understood if we had access to the real curves from the prop manufacturers. Anyone out there with this info?
You guys labbing props have anything to say about this?
Last edited by tomcat; 03-04-2002 at 12:41 PM.
03-03-2002, 10:32 AM
#2
Look at the prop efficiency curve and tell me, if the prop you were using hit maximum efficiency at 5% slip and your test data told you that you were at 10% slip, what would you do? Would you try something to reduce slip or change to a prop that had it's best efficiency at 10%? I don't have the answer, I'm just trying to figure this out.
Don't say lab the prop, because the prop would then have a different curve. It makes sense that thinning the blades should increase overall efficiency by reducing the power required to move the blades through the water, a good thing to do in any case, but it doesn't address the original problem. Or does it? I don't know.
Don't say lab the prop, because the prop would then have a different curve. It makes sense that thinning the blades should increase overall efficiency by reducing the power required to move the blades through the water, a good thing to do in any case, but it doesn't address the original problem. Or does it? I don't know.
Last edited by tomcat; 12-18-2002 at 06:49 PM.
03-04-2002, 12:34 PM
#3
For more on this topic check out the Bravo vs. Hydro thread in the Testing and Upgrades Forum.
What do you make of this graph?
Used with permission www.aerodyn.org.
What do you make of this graph?
Used with permission www.aerodyn.org.
03-04-2002, 05:40 PM
#6
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Tomcat,
I don't know if i understand your question, but as you know the way the efficiency of a propeller is calculated is the actual speed as compared to (divided by) the theoretical speed if the prop were "screwed" through a solid (times 100). There are prop efficiency calculators all over the place.
Graphs for compressible fluid flow (air) have absolutely no bearing on props in water, unless you are trying to get familiar with terminology. Water for our purposes is considered incompressible.
Thrust of the prop is effectively impossible to calculate unless you know a lot about the boat's bottom, ie all of the coefficients of drag etc. You can "back" into it as you have shown if you know the propshaft horsepower, however it is not a factor that we need to know directly, of concern obviously, but not necessary in our efficiency numbers.
Angle of attack is a little understood ingredient in our world as while pitch is a simple concept, trim is not as it changes the entering blade one way and the exiting blade another. Additionally, the boat bottom changes angle with waves etc. These kinds of sciences are called empirical because experimentation is often the only reliable way to see what is happening on an system wide basis.
Does this help???!!!
Ted
I don't know if i understand your question, but as you know the way the efficiency of a propeller is calculated is the actual speed as compared to (divided by) the theoretical speed if the prop were "screwed" through a solid (times 100). There are prop efficiency calculators all over the place.
Graphs for compressible fluid flow (air) have absolutely no bearing on props in water, unless you are trying to get familiar with terminology. Water for our purposes is considered incompressible.
Thrust of the prop is effectively impossible to calculate unless you know a lot about the boat's bottom, ie all of the coefficients of drag etc. You can "back" into it as you have shown if you know the propshaft horsepower, however it is not a factor that we need to know directly, of concern obviously, but not necessary in our efficiency numbers.
Angle of attack is a little understood ingredient in our world as while pitch is a simple concept, trim is not as it changes the entering blade one way and the exiting blade another. Additionally, the boat bottom changes angle with waves etc. These kinds of sciences are called empirical because experimentation is often the only reliable way to see what is happening on an system wide basis.
Does this help???!!!
Ted
03-04-2002, 06:03 PM
#7
Kaamacat - I put the two callers in touch with one another. The first caller gave up on the boat; he felt the salesman was playing dumb when it came to questions about the boat's performance problem. Others that have looked at the boat felt that the salesman wasn't that interested in talking to them. There's a funny vibe going on.
My own paranoid opinion is that either the boat weighs an extra 1000 lbs for some reason, or the engines are 7.4L - 310 HP with 502 MPI stickers. That's the only way I can get the numbers to make sense in my power vs. speed program. The drives they are advertising as Speedy #3s look like TRS with the lowercases painted, so the sticker theory has to be checked.
It's too bad, the boat looks great and the price is a steal, maybe just too good to be true.
My own paranoid opinion is that either the boat weighs an extra 1000 lbs for some reason, or the engines are 7.4L - 310 HP with 502 MPI stickers. That's the only way I can get the numbers to make sense in my power vs. speed program. The drives they are advertising as Speedy #3s look like TRS with the lowercases painted, so the sticker theory has to be checked.
It's too bad, the boat looks great and the price is a steal, maybe just too good to be true.
Last edited by tomcat; 03-04-2002 at 07:32 PM.
03-04-2002, 07:30 PM
#8
Hello Ted - Thanks for the response, I will address your points in order, then ask the question again.
What you describe as propeller efficiency is the equation for slip. It's prop slip calculators that are all over the place. Efficiency is power out/power in. It's only related to slip in that maximum efficiency occurs at a certain slip for a given prop.
The terminology for air propellers and water propellers is the same, the difference is the density of the fluid. It's even OK to compare them since air flow is essentially non-compressible in the operating range of air propellers. The small pressure differences that can be generated by a propeller fan can be ignored and the flow treated as non-compressible with very small errors. This isn't true of high speed blowers and screw compressors, but it' s OK for propeller fans and even low speed radial fans. The second and third curves I have shown are for air propellers. I would love to see some for water propellers, but I expect them to be similar.
Thrust...without it nothing happens, so it is important, but hard for us to measure. So we watch our speedo and prop for max speed. With the equations listed above, we could calculate thrust if we knew propshaft HP and efficiency but I agree that the absolute value of thrust is not important. What I want to know is "Have I got the most possible thrust out of the power I put into the prop?" For this I need to be able to plot my test data on a graph of the prop efficiency to see where I am. Check the thread on Bravo vs. Hydromotive for more on this point.
Angle of attack and slip are very closely related. In the first prop efficiency curve attached above, Mercury plots efficiency against angle of attack or slip. They make no distinction, so let's just concentrate on slip. (For a good explanation of the relationship between angle of attack and slip see Mercury's book "Everything You Wanted to Know About Propellers.")
To keep things simple let's leave trim out of it and say that the prop thrust is directed parallel to the surface of the water and the correct angle of the hull for minimum wetted surface is provided by the aerodynamic lift of the tunnel. Sorry vee guys, but this is why cats are faster; they're not wasting thrust to hold the bow up. According to Mercury, in this situation a fast cat would have an ideal slip of 5%. They have the experience and the prop efficiency data so I assume they know what they're talking about.
So here's the question: What do you do if you are measuring 10% slip in your tests?
What you describe as propeller efficiency is the equation for slip. It's prop slip calculators that are all over the place. Efficiency is power out/power in. It's only related to slip in that maximum efficiency occurs at a certain slip for a given prop.
The terminology for air propellers and water propellers is the same, the difference is the density of the fluid. It's even OK to compare them since air flow is essentially non-compressible in the operating range of air propellers. The small pressure differences that can be generated by a propeller fan can be ignored and the flow treated as non-compressible with very small errors. This isn't true of high speed blowers and screw compressors, but it' s OK for propeller fans and even low speed radial fans. The second and third curves I have shown are for air propellers. I would love to see some for water propellers, but I expect them to be similar.
Thrust...without it nothing happens, so it is important, but hard for us to measure. So we watch our speedo and prop for max speed. With the equations listed above, we could calculate thrust if we knew propshaft HP and efficiency but I agree that the absolute value of thrust is not important. What I want to know is "Have I got the most possible thrust out of the power I put into the prop?" For this I need to be able to plot my test data on a graph of the prop efficiency to see where I am. Check the thread on Bravo vs. Hydromotive for more on this point.
Angle of attack and slip are very closely related. In the first prop efficiency curve attached above, Mercury plots efficiency against angle of attack or slip. They make no distinction, so let's just concentrate on slip. (For a good explanation of the relationship between angle of attack and slip see Mercury's book "Everything You Wanted to Know About Propellers.")
To keep things simple let's leave trim out of it and say that the prop thrust is directed parallel to the surface of the water and the correct angle of the hull for minimum wetted surface is provided by the aerodynamic lift of the tunnel. Sorry vee guys, but this is why cats are faster; they're not wasting thrust to hold the bow up. According to Mercury, in this situation a fast cat would have an ideal slip of 5%. They have the experience and the prop efficiency data so I assume they know what they're talking about.
So here's the question: What do you do if you are measuring 10% slip in your tests?
03-04-2002, 11:10 PM
#10
THRUST AND DRAG
The hull and drive hanging in the water produce drag. The hull is planing and the drag results from the friction of water on the wetted surface of the hull bottom and from the displacement of water. The drive is partially submerged and the drag results from friction and from the displacement of water. Just to give you some idea, when Mercury was testing their Blackhawk surface piercing drive they said that dragging a Bravo drive through the water at 60 mph created 350 lbs of drag, while the Blackhawk only created 35 lbs of drag at the same speed. There is also aerodynamic drag from the hull and deck moving through the air, but this is a lot less than the hydrodynamic drag.
So drag can be measured in lbs. If you wanted to you could drag a boat behind a tow boat with a spring scale on the tow rope and measure the lbs of drag at different speeds. This would create a hull curve of drag vs. speed. This curve is a parabola; drag goes up in proportion to the square of speed.
The propeller creates thrust, also measured in lbs. Since you have already measured the drag of the hull and drive at different speeds you know how much thrust the prop is delivering when the boat is moving at those different speeds.
Propellers produce thrust in proportion to their RPM, diameter or blade area, and the density of water. It takes power to do this. As RPM increases, the thrust increases, but because the propeller is being spun by an engine that has a horsepower curve that rises and falls when you pass the peak horsepower, the engine and prop together produce a thrust curve that looks like a hump.
When you plot these two curves on the same graph, top speed, maximum RPM and maximum thrust all occur at the intersection of the two curves. This is shown on the attached graph. The only difference is we don't tow boats around with a spring scale in the tow rope. We put engines of different known HP into the boat and see what different speeds we get. After doing this many times we develop empirical equations that make the hull curve. As a result the attached graph is propshaft HP vs. speed, not thrust/drag vs. speed. This approach works just as well for predicting speed. The exception is when you change drive heights to reduce drive drag, and change props (in particular blade area) to maximize the conversion of propshaft HP to thrust. But that's another story.
The attached curve shows two different engines in the same hull.
So drag can be measured in lbs. If you wanted to you could drag a boat behind a tow boat with a spring scale on the tow rope and measure the lbs of drag at different speeds. This would create a hull curve of drag vs. speed. This curve is a parabola; drag goes up in proportion to the square of speed.
The propeller creates thrust, also measured in lbs. Since you have already measured the drag of the hull and drive at different speeds you know how much thrust the prop is delivering when the boat is moving at those different speeds.
Propellers produce thrust in proportion to their RPM, diameter or blade area, and the density of water. It takes power to do this. As RPM increases, the thrust increases, but because the propeller is being spun by an engine that has a horsepower curve that rises and falls when you pass the peak horsepower, the engine and prop together produce a thrust curve that looks like a hump.
When you plot these two curves on the same graph, top speed, maximum RPM and maximum thrust all occur at the intersection of the two curves. This is shown on the attached graph. The only difference is we don't tow boats around with a spring scale in the tow rope. We put engines of different known HP into the boat and see what different speeds we get. After doing this many times we develop empirical equations that make the hull curve. As a result the attached graph is propshaft HP vs. speed, not thrust/drag vs. speed. This approach works just as well for predicting speed. The exception is when you change drive heights to reduce drive drag, and change props (in particular blade area) to maximize the conversion of propshaft HP to thrust. But that's another story.
The attached curve shows two different engines in the same hull.
