Propeller Power Curve - Moderated Discussion Areas

  ContinuousWave
  Whaler
  Moderated Discussion Areas
  ContinuousWave: Whaler Performance
  Propeller Power Curve

search | FAQ | profile | register | author help

Author Topic:   Propeller Power Curve

jimh posted 01-13-2008 11:46 AM ET (US)
Please use this discussion exclusively for questions, comments, or criticism of my article
Propeller Power Curve
http://continuouswave.com/whaler/reference/propellerPowerCurve.html
I will be glad to answer any questions, acknowledge any comments, or defend my method against reasonable criticism with follow-up comments.
tmann45 posted 01-13-2008 01:30 PM ET (US)
It has been a few years since I read Gerr's book. I remember it was focused mostly on displacement hulls, which I was most interested in at that time. Do his calculations apply as well to planning type hulls as well as displacement type?
Tom
jimh posted 01-13-2008 01:54 PM ET (US)
I cannot speak for Mr. Gerr's work. You'd have to ask him if it applies.
My calculations are derived independently of Gerr's (as he does not show any derivation of his formula) , and I have based my analysis on the work of Crouch. The Crouch speed prediction formula is for moderate planing hulls. We have independently confirmed its accuracy many times with test results, so I feel it is entirely applicable to boats such as my Boston Whaler REVENGE. Using that as a basis I have derived a propeller power curve formula which seems to be in general agreement with that proposed by Gerr.
I rely on Gerr's assessment that n=2.7 is a good average value, that's all.
[Also, I should add that I do not agree with the characterization that Gerr's propeller handbook is limited to boats with displacement hulls. That is not an accurate characterization in my opinion, and I would recommend Gerr's book to people with non-displacement hull boats.]
jimh posted 01-13-2008 03:17 PM ET (US)
The concept that there should be some extra power available from the engine at certain speeds is important. You cannot accelerate the engine unless it has some extra power. Therefore, the amount of extra power available at a particular engine speed is a good indication of how fast the boat will accelerate to a higher speed.
As you can see in the graphs, the hypothetical two-cycle engine I diagram based on an E-TEC power curve will be able to accelerate faster than the hypothetical four-cycle engine power curve based on a Honda BF150.
jimh posted 01-13-2008 03:44 PM ET (US)
I also explored the differences in using various values of n , from 2.2 to 3.0, as suggested. I added an addendum which shows a plot of this using the SST 15-inch propeller as a reference.
Peter posted 01-13-2008 06:16 PM ET (US)
Nice article. This should develop into an interesting discussion. There are some parallels to this article by David Pascoe regarding diesels. www.yachtsurvey.com/GasNdiesel.htm

It would be nice if the outboard manufacturers published power torque and fuel consumption curves like the marine diesel manufacturers do. See, for example, 84.241.129.114/uploads/products/pdf/BY/6BY220Z.pdf .

Tohsgib posted 01-14-2008 11:30 AM ET (US)
I agree that torque would be a better medium than HP. Also your engine although it hits 5900rpm it does not make 225hp there, more likely around 5500rpm.
jimh posted 01-14-2008 11:38 AM ET (US)
It is possible to speculate about the power curve on my engine, but that speculation is not the focus of the article. For the purpose of the article, which is to derive a propeller curve from some reasonable data, the assumption was made that the engine produced 225-HP in the range for which the manufacturer specified it would.
If the manufacturer published a power curve for the engine, it would, of course, be useful for the purpose of developing a power curve for the propeller, but, since this data was not available, I just took the manufacturer at his word and used 225-HP. This assumption is explicitly mentioned in the article.
The effect of assuming, for example, that the engine only produced, say, 210-HP, would be just to move the curve slightly, and not to change its overall shape very much.
newt posted 01-14-2008 04:53 PM ET (US)
Jim, forgive me if this comes across wrong, but what is the point? What is purpose does the Propeller Power Curve? Is the curve a tool to use in selecting or comparing an engine?
I'm not trying to be a wise ass, I just don't get it.
cooper1958nc posted 01-14-2008 05:25 PM ET (US)
Jim is to be appreciated for attempting some of these derivations.
Derivations like these are helped when "units" are obeyed. In other words, if a constant is used to translate RPM to HP (or whatever), it must have appropriate units, like HP/RPM. The constants shown are all dimensionless, which is confusing.
More substantively, the propeller power curve graphed, which is just a of graph y= x ^2.7, is not related to the hull at all. It might be fairly correct if the propeller were being run in a test tank, but not if the boat is underweigh.
A planing hull's resistance curve is not a monotonic function of speed, but has an inflection point where a transition occurs from displacement to planing speed. The torque required to turn the propeller follows this curve as well, so it looks nothing like the curve you show.
In proof of this, it often takes near full throttle to get over the planing transition. There is little reserve torque available, because the "torque required" and the "torque available" are almost equal.
The error in the mathematics that led to the curve is I believe, when you multiply RPM * f(RPM)^2 and get RPM ^2.7. f(RPM) is the tipoff that it is a FUNCTION of RPM, which in this case is a function that looks like a steep climb followed by a shallow step, then gradual climb after that. You can't ignore the FUNCTION.

cooper1958nc posted 01-14-2008 05:34 PM ET (US)
Methinks it would be much better to forget about the propeller for now. To predict reserve torque (power) at any RPM,
1. go to the published full-throttle torque vs RPM curves, plot them. that gives power available.
2. To find power actually required, go to fuel consumption data for the hull and engine. Use a s/f/c estimate to estimate power required at different speeds. Plot that. If you use an s/f/c for a given point (full throttle) that will automatically take the prop issues into consideration, at least as a first approximation.
3. The difference is the "reserve power." You will see it looks different from your curves.
towboater posted 01-14-2008 05:49 PM ET (US)
Last night I spent a good hour looking over Mercury engine/prop performance results for 60 HP EFI Big Foot. The tests included a large variety of hulls and prop dia/pitch.
Nice job Merc. I am leaning your way.

[urlhttp://northamerica.mercurymarine.com/look_deeper/index.php?Filter=11[/url]
I read all this because, coincidentally, I am poised to repower and interested to know how Merc would use DATA to put X wheel on Y engine for Z hull. Thanks again for your time and hard work Professor (Jimh)...and patience, I will get there.
I am a little confused with portions of your article. One para refers to obtaining power via PITCH whereas my local Pro prop expert refers to power via DIA and speed via PITCH.
Otherwise, I understand you imply adding pitch as a general term to put more load on the engine in this case. Or, maybe I have it backwards. I am certainly interested in learning more about the relationship of PITCH and DIA.
I also wonder, as long as Merc and the others are testing engines/props on various hulls, why dont they hook a tow rope up to a bollard pull meter? Actually, it wouldnt matter which hull they used.
http://www.steerprop.com/bollardpullcode.pdf
Seems to me, this test eliminates all assumptions regarding slip and hull efficiency in regards to prop power.
mk
cooper1958nc posted 01-14-2008 06:27 PM ET (US)
Static thrust (bollard pull) is not a good indicator of how high speed propellers will function at speed. They are operating at a wierd angle of attack for one. For another reason, the lower pitch and higher diameter will always outpull.
jimh posted 01-14-2008 08:21 PM ET (US)
newt--What's the point? To try to have a better understanding of how propellers and engines interact.
Coop'--Yes, the curve is a graph of y=cx^2.7, so are all propeller power curves drawn with an exponent of n=2.7; this is more or less a given in the definition of the propeller power curve. As the method says, it is "arbitrary" that we choose the propeller power curve to meet the engine power curve at maximum engine RPM.
Being able to see a propeller power curve gives you a chance to compare the power required by the propeller with the power available from the engine.
newt posted 01-14-2008 09:04 PM ET (US)
OK, I get it now. It is interesting to see the relationship between power required and power available.
The first thing that pops into mind is the advantage of a transmission. According to your graph, at 4000 RPM your propeller requires around 60 hp, however the engine can produce 60 hp at 2000 RPM. Think of the fuel savings if you could switch into second gear and cruise at half the RPM!
Jim, wouldn't your engine power curve more resemble that of the E-TEC? For example, my Johnson 150 engines produce maximum HP at 5000 RPM and your engine likely peaks before maximum RPM also.
cooper1958nc posted 01-14-2008 09:05 PM ET (US)
But jimh, a propeller has no "power curve" independent of the hull any more than a tire has a "power curve" independent of the car. If run in a test tank, true, it has a defined power curve. But when it is moving, its RPM to power ratio (defined by the torque required) depends on how easy the boat is to move at that speed.
The "power curve" you show is not applicable to a propeller on a planing boat. That curve looks like a test tank or displacement hull.
The engine produces power, which is torque times RPM. The propeller converts torque to thrust. The Delivered power is force (thrust) times speed. If the hull has high resistance at 16 mph it takes more torque to move it. It might take a little less torque to move it at 20 mph, depending on the hull. The power required would be slightly more or slightly less, but certainly not (16/20)^2.7.
towboater posted 01-14-2008 09:52 PM ET (US)
Jim,
SPEED?
isnt the topic
Propeller POWER curve?
I am conjecting, BP test will reveal how much pulling power or static thrust the combo of any engine/prop will produce independant of the hull factors.
Certainly this assumes the combo will reach OEM safe operating RPMs. If safe operating RPMs are not reached or exceeded
A. not enough/too much HP
B. too much/not enough wheel
BP testing would establish a base line.
I realize the setup that has the highest BP rating may not be the best setup for a Gzillion hull performance factors.
BP is a simple test.
It will reveal whether or not a 150 hp Brand X has more static thrust or torque or whatever you want to call it than a 150 hp Brand Y using the same wheel.
With recent comments about fuel consumpion at cruise the highest % of concern. I think it would be interesting to calculate best performance at WOT vrs a POWER performance setup and see which setup provides best fuel economy at cruise.
Thx Peter for the link to Pasco. He mentions one of the most common mistakes one can make is to underpower a boat. No prob there.
jimh posted 01-14-2008 10:11 PM ET (US)
Coop'--the propeller power curve is derived from the function of the hull speed increasing with horsepower according to Crouch's formula. If Crouch's formula is accurate in describing that relationship (and most agree it is) then the influence of the hull is incorporated into the HULL FACTOR, which is a constant that is given to each hull type. Since the hull influence is given by a constant, it is also appropriate that it is given as a constant in the propeller curve.
If you want to explore the power-versus-speed right as the hull transitions from displacement mode to planing mode, I am sure it will be more complicated. But even Crouch does not try to describe this transition area.
The propeller power curve can be compared for a few different propellers. When I get a chance I will add an additional graph showing a few different propeller power curves drawn. Again, these will be estimates based on my observation of what maximum engine speed these propellers allow.
jimh posted 01-14-2008 10:46 PM ET (US)
towboater--I am glad that you found it interesting to read other information about propellers available elsewhere. There is an enormous number of discussions about propellers, how to select them, where to buy them, and so on, but I have not found too much about propeller power curves. That lack of information was one of the reasons I undertook to write about propeller power curves.
As for the topic of bollard pull, using bollard pull to assess propeller performance, and other elements of propeller design for best bollard pull, I am sure they are of interest to people who plan to do a lot of towing with their boats, however, most recreational small boat owners are more interested in getting good moderate speed performance on plane from their boats. I do not think that bollard pull will be an important factor in selecting a propeller.
If you do not mind, I suggest that you start a new and separate discussion about the use of bollard pull testing as a way to select or evaluate propellers for moderate speed planing hulls. It should be an interesting topic, and I am looking forward to seeing the theory behind your method.
jimh posted 01-14-2008 11:35 PM ET (US)
I added two more graphs to the article to show the propeller power curves of two additional propellers plotted against the engine power curve. In the second graph I expanded the horsepower scale so you can see just how much more power these larger propellers will need to turn them to 6,000-RPM (engine crankshaft speed)--much more than my old 225-HP motor can handle.

towboater posted 01-15-2008 12:24 AM ET (US)
so be it.
itl posted 01-15-2008 03:23 AM ET (US)
Why not just use Crouch's formula to calculate power what propeller takes in certain boat and certain speed/rpm? Then you can subtract that power from your engine power curve and you get the extra power what you can use for accelerating your boat.
jimh posted 01-15-2008 08:57 AM ET (US)
itl--Using Crouch's speed prediction formula to develop a propeller power curve is precisely what I have just demonstrated.
If you have an alternative method for developing a propeller power curve, please feel free to show it and how you derived it.
jimh posted 01-15-2008 09:05 AM ET (US)
coop'--Regarding the propeller power curve being derived "in a test tank" I can only say that the interaction between a hull and water is the same in a test tank as it is in open water, other than the influence of some bottom effects which might occur due to the test tank being slightly shallower than deep ocean water, or from the influences of waves creating dynamic loads. However, I do not see any analysis of boat propulsion and horsepower required which includes factors for dynamic loads like head seas. By not including the influences of sea conditions in my propeller power curve I do not see that I have taken a different course than any others. These formula are too simple to include analysis of wave influences on the interaction of the hull and the water.
The use of test tanks to derive data about the performance of a hull is a very common approach and has been proven to have outstanding faithfulness to the results obtained in the real world. So I am not prone to discard or devalue the information about propeller power curves based on your assessment that it represents a situation as might occur in a test tank.
Peter posted 01-15-2008 10:16 AM ET (US)
Newt -- Jim's engine is rated at 5500 RPM with a top RPM of 6000 RPM. Your 150 is rated at 5000 RPM with a top RPM of 5500 RPM.
Coop' -- The marine diesel industry uses propeller power curves (I think with n=2.7) to illustrate their products power production capability relative to such propeller power curve. These propeller power curves ARE used for planing vessels as evidenced by the provision of propeller power curves on the power and torque charts for stern drive configured marine diesels such as the one in the above link. To the best of my knowledge, stern drive configurations are rarely ever used for displacement applications. The marine diesel manufacturers do not try to simulate the on-plane transition point because it varies from hull to hull and its basically irrelevant if the motor does not have enough reserve power to climb over the bow wave.
itl posted 01-15-2008 04:39 PM ET (US)
Say, we have a boat which weight 2500lbs (with 150hp Honda engine and gears), hull factor is 180. Honda power curve shows that engine is actually producing around 160hp and that is the max. power output. We also know our boat performance and it is known how fast it goes when engine is running 3000-6000rpm.
With Crouch's formula, we can calculate that propeller is taking power as following:
160hp 6000rpm 45.5mph (top end)
130hp 5000rpm 41.0mph
100hp 4000rpm 36.0mph
70hp 3000rpm 30.1mph
Then we compare these power ratings to Honda powercurve provided by engine manufacturer:
160hp 6000rpm
140hp 5000rpm
110hp 4000rpm
80hp 3000rpm
We notice that propeller took 10hp less than engine is producing in 3000-5000rpm range and we can accelerate our boat with that 10hp reserve until we reach the top end where engine and propeller powercurves cross :)
Previous data is taken from hat and my results probably does not meet with real life situation. Honda powercurve is same as is in jimh's propeller power curve article.
jimh posted 01-16-2008 08:07 AM ET (US)
itl's approach is interesting. You drive your boat at a particular speed, deduce the horsepower from Crouch's formula (using a predetermined HULL FACTOR), and note the RPM. Then you compare the horsepower the engine power curve predicts at that RPM. However, there is no way to write it down as an equation. The individual boat determines the function.
When I get a chance I will use some of my own data and graph the results using the approach suggested.

Hop to:

Contact Us | RETURN to ContinuousWave Top Page

Powered by: Ultimate Bulletin Board, Freeware Version 2000
Purchase our Licensed Version- which adds many more features!
© Infopop Corporation (formerly Madrona Park, Inc.), 1998 - 2000.

Author	Topic: Propeller Power Curve
jimh	posted 01-13-2008 11:46 AM ET (US) Please use this discussion exclusively for questions, comments, or criticism of my article Propeller Power Curve http://continuouswave.com/whaler/reference/propellerPowerCurve.html I will be glad to answer any questions, acknowledge any comments, or defend my method against reasonable criticism with follow-up comments.
tmann45	posted 01-13-2008 01:30 PM ET (US) It has been a few years since I read Gerr's book. I remember it was focused mostly on displacement hulls, which I was most interested in at that time. Do his calculations apply as well to planning type hulls as well as displacement type? Tom
jimh	posted 01-13-2008 01:54 PM ET (US) I cannot speak for Mr. Gerr's work. You'd have to ask him if it applies. My calculations are derived independently of Gerr's (as he does not show any derivation of his formula) , and I have based my analysis on the work of Crouch. The Crouch speed prediction formula is for moderate planing hulls. We have independently confirmed its accuracy many times with test results, so I feel it is entirely applicable to boats such as my Boston Whaler REVENGE. Using that as a basis I have derived a propeller power curve formula which seems to be in general agreement with that proposed by Gerr. I rely on Gerr's assessment that n=2.7 is a good average value, that's all. [Also, I should add that I do not agree with the characterization that Gerr's propeller handbook is limited to boats with displacement hulls. That is not an accurate characterization in my opinion, and I would recommend Gerr's book to people with non-displacement hull boats.]
jimh	posted 01-13-2008 03:17 PM ET (US) The concept that there should be some extra power available from the engine at certain speeds is important. You cannot accelerate the engine unless it has some extra power. Therefore, the amount of extra power available at a particular engine speed is a good indication of how fast the boat will accelerate to a higher speed. As you can see in the graphs, the hypothetical two-cycle engine I diagram based on an E-TEC power curve will be able to accelerate faster than the hypothetical four-cycle engine power curve based on a Honda BF150.
jimh	posted 01-13-2008 03:44 PM ET (US) I also explored the differences in using various values of n , from 2.2 to 3.0, as suggested. I added an addendum which shows a plot of this using the SST 15-inch propeller as a reference.
Peter	posted 01-13-2008 06:16 PM ET (US) Nice article. This should develop into an interesting discussion. There are some parallels to this article by David Pascoe regarding diesels. www.yachtsurvey.com/GasNdiesel.htm It would be nice if the outboard manufacturers published power torque and fuel consumption curves like the marine diesel manufacturers do. See, for example, 84.241.129.114/uploads/products/pdf/BY/6BY220Z.pdf .
Tohsgib	posted 01-14-2008 11:30 AM ET (US) I agree that torque would be a better medium than HP. Also your engine although it hits 5900rpm it does not make 225hp there, more likely around 5500rpm.
jimh	posted 01-14-2008 11:38 AM ET (US) It is possible to speculate about the power curve on my engine, but that speculation is not the focus of the article. For the purpose of the article, which is to derive a propeller curve from some reasonable data, the assumption was made that the engine produced 225-HP in the range for which the manufacturer specified it would. If the manufacturer published a power curve for the engine, it would, of course, be useful for the purpose of developing a power curve for the propeller, but, since this data was not available, I just took the manufacturer at his word and used 225-HP. This assumption is explicitly mentioned in the article. The effect of assuming, for example, that the engine only produced, say, 210-HP, would be just to move the curve slightly, and not to change its overall shape very much.
newt	posted 01-14-2008 04:53 PM ET (US) Jim, forgive me if this comes across wrong, but what is the point? What is purpose does the Propeller Power Curve? Is the curve a tool to use in selecting or comparing an engine? I'm not trying to be a wise ass, I just don't get it.
cooper1958nc	posted 01-14-2008 05:25 PM ET (US) Jim is to be appreciated for attempting some of these derivations. Derivations like these are helped when "units" are obeyed. In other words, if a constant is used to translate RPM to HP (or whatever), it must have appropriate units, like HP/RPM. The constants shown are all dimensionless, which is confusing. More substantively, the propeller power curve graphed, which is just a of graph y= x ^2.7, is not related to the hull at all. It might be fairly correct if the propeller were being run in a test tank, but not if the boat is underweigh. A planing hull's resistance curve is not a monotonic function of speed, but has an inflection point where a transition occurs from displacement to planing speed. The torque required to turn the propeller follows this curve as well, so it looks nothing like the curve you show. In proof of this, it often takes near full throttle to get over the planing transition. There is little reserve torque available, because the "torque required" and the "torque available" are almost equal. The error in the mathematics that led to the curve is I believe, when you multiply RPM * f(RPM)^2 and get RPM ^2.7. f(RPM) is the tipoff that it is a FUNCTION of RPM, which in this case is a function that looks like a steep climb followed by a shallow step, then gradual climb after that. You can't ignore the FUNCTION.
cooper1958nc	posted 01-14-2008 05:34 PM ET (US) Methinks it would be much better to forget about the propeller for now. To predict reserve torque (power) at any RPM, 1. go to the published full-throttle torque vs RPM curves, plot them. that gives power available. 2. To find power actually required, go to fuel consumption data for the hull and engine. Use a s/f/c estimate to estimate power required at different speeds. Plot that. If you use an s/f/c for a given point (full throttle) that will automatically take the prop issues into consideration, at least as a first approximation. 3. The difference is the "reserve power." You will see it looks different from your curves.
towboater	posted 01-14-2008 05:49 PM ET (US) Last night I spent a good hour looking over Mercury engine/prop performance results for 60 HP EFI Big Foot. The tests included a large variety of hulls and prop dia/pitch. Nice job Merc. I am leaning your way. [urlhttp://northamerica.mercurymarine.com/look_deeper/index.php?Filter=11[/url] I read all this because, coincidentally, I am poised to repower and interested to know how Merc would use DATA to put X wheel on Y engine for Z hull. Thanks again for your time and hard work Professor (Jimh)...and patience, I will get there. I am a little confused with portions of your article. One para refers to obtaining power via PITCH whereas my local Pro prop expert refers to power via DIA and speed via PITCH. Otherwise, I understand you imply adding pitch as a general term to put more load on the engine in this case. Or, maybe I have it backwards. I am certainly interested in learning more about the relationship of PITCH and DIA. I also wonder, as long as Merc and the others are testing engines/props on various hulls, why dont they hook a tow rope up to a bollard pull meter? Actually, it wouldnt matter which hull they used. http://www.steerprop.com/bollardpullcode.pdf Seems to me, this test eliminates all assumptions regarding slip and hull efficiency in regards to prop power. mk
cooper1958nc	posted 01-14-2008 06:27 PM ET (US) Static thrust (bollard pull) is not a good indicator of how high speed propellers will function at speed. They are operating at a wierd angle of attack for one. For another reason, the lower pitch and higher diameter will always outpull.
jimh	posted 01-14-2008 08:21 PM ET (US) newt--What's the point? To try to have a better understanding of how propellers and engines interact. Coop'--Yes, the curve is a graph of y=cx^2.7, so are all propeller power curves drawn with an exponent of n=2.7; this is more or less a given in the definition of the propeller power curve. As the method says, it is "arbitrary" that we choose the propeller power curve to meet the engine power curve at maximum engine RPM. Being able to see a propeller power curve gives you a chance to compare the power required by the propeller with the power available from the engine.
newt	posted 01-14-2008 09:04 PM ET (US) OK, I get it now. It is interesting to see the relationship between power required and power available. The first thing that pops into mind is the advantage of a transmission. According to your graph, at 4000 RPM your propeller requires around 60 hp, however the engine can produce 60 hp at 2000 RPM. Think of the fuel savings if you could switch into second gear and cruise at half the RPM! Jim, wouldn't your engine power curve more resemble that of the E-TEC? For example, my Johnson 150 engines produce maximum HP at 5000 RPM and your engine likely peaks before maximum RPM also.
cooper1958nc	posted 01-14-2008 09:05 PM ET (US) But jimh, a propeller has no "power curve" independent of the hull any more than a tire has a "power curve" independent of the car. If run in a test tank, true, it has a defined power curve. But when it is moving, its RPM to power ratio (defined by the torque required) depends on how easy the boat is to move at that speed. The "power curve" you show is not applicable to a propeller on a planing boat. That curve looks like a test tank or displacement hull. The engine produces power, which is torque times RPM. The propeller converts torque to thrust. The Delivered power is force (thrust) times speed. If the hull has high resistance at 16 mph it takes more torque to move it. It might take a little less torque to move it at 20 mph, depending on the hull. The power required would be slightly more or slightly less, but certainly not (16/20)^2.7.
towboater	posted 01-14-2008 09:52 PM ET (US) Jim, SPEED? isnt the topic Propeller POWER curve? I am conjecting, BP test will reveal how much pulling power or static thrust the combo of any engine/prop will produce independant of the hull factors. Certainly this assumes the combo will reach OEM safe operating RPMs. If safe operating RPMs are not reached or exceeded A. not enough/too much HP B. too much/not enough wheel BP testing would establish a base line. I realize the setup that has the highest BP rating may not be the best setup for a Gzillion hull performance factors. BP is a simple test. It will reveal whether or not a 150 hp Brand X has more static thrust or torque or whatever you want to call it than a 150 hp Brand Y using the same wheel. With recent comments about fuel consumpion at cruise the highest % of concern. I think it would be interesting to calculate best performance at WOT vrs a POWER performance setup and see which setup provides best fuel economy at cruise. Thx Peter for the link to Pasco. He mentions one of the most common mistakes one can make is to underpower a boat. No prob there.
jimh	posted 01-14-2008 10:11 PM ET (US) Coop'--the propeller power curve is derived from the function of the hull speed increasing with horsepower according to Crouch's formula. If Crouch's formula is accurate in describing that relationship (and most agree it is) then the influence of the hull is incorporated into the HULL FACTOR, which is a constant that is given to each hull type. Since the hull influence is given by a constant, it is also appropriate that it is given as a constant in the propeller curve. If you want to explore the power-versus-speed right as the hull transitions from displacement mode to planing mode, I am sure it will be more complicated. But even Crouch does not try to describe this transition area. The propeller power curve can be compared for a few different propellers. When I get a chance I will add an additional graph showing a few different propeller power curves drawn. Again, these will be estimates based on my observation of what maximum engine speed these propellers allow.
jimh	posted 01-14-2008 10:46 PM ET (US) towboater--I am glad that you found it interesting to read other information about propellers available elsewhere. There is an enormous number of discussions about propellers, how to select them, where to buy them, and so on, but I have not found too much about propeller power curves. That lack of information was one of the reasons I undertook to write about propeller power curves. As for the topic of bollard pull, using bollard pull to assess propeller performance, and other elements of propeller design for best bollard pull, I am sure they are of interest to people who plan to do a lot of towing with their boats, however, most recreational small boat owners are more interested in getting good moderate speed performance on plane from their boats. I do not think that bollard pull will be an important factor in selecting a propeller. If you do not mind, I suggest that you start a new and separate discussion about the use of bollard pull testing as a way to select or evaluate propellers for moderate speed planing hulls. It should be an interesting topic, and I am looking forward to seeing the theory behind your method.
jimh	posted 01-14-2008 11:35 PM ET (US) I added two more graphs to the article to show the propeller power curves of two additional propellers plotted against the engine power curve. In the second graph I expanded the horsepower scale so you can see just how much more power these larger propellers will need to turn them to 6,000-RPM (engine crankshaft speed)--much more than my old 225-HP motor can handle.
towboater	posted 01-15-2008 12:24 AM ET (US) so be it.
itl	posted 01-15-2008 03:23 AM ET (US) Why not just use Crouch's formula to calculate power what propeller takes in certain boat and certain speed/rpm? Then you can subtract that power from your engine power curve and you get the extra power what you can use for accelerating your boat.
jimh	posted 01-15-2008 08:57 AM ET (US) itl--Using Crouch's speed prediction formula to develop a propeller power curve is precisely what I have just demonstrated. If you have an alternative method for developing a propeller power curve, please feel free to show it and how you derived it.
jimh	posted 01-15-2008 09:05 AM ET (US) coop'--Regarding the propeller power curve being derived "in a test tank" I can only say that the interaction between a hull and water is the same in a test tank as it is in open water, other than the influence of some bottom effects which might occur due to the test tank being slightly shallower than deep ocean water, or from the influences of waves creating dynamic loads. However, I do not see any analysis of boat propulsion and horsepower required which includes factors for dynamic loads like head seas. By not including the influences of sea conditions in my propeller power curve I do not see that I have taken a different course than any others. These formula are too simple to include analysis of wave influences on the interaction of the hull and the water. The use of test tanks to derive data about the performance of a hull is a very common approach and has been proven to have outstanding faithfulness to the results obtained in the real world. So I am not prone to discard or devalue the information about propeller power curves based on your assessment that it represents a situation as might occur in a test tank.
Peter	posted 01-15-2008 10:16 AM ET (US) Newt -- Jim's engine is rated at 5500 RPM with a top RPM of 6000 RPM. Your 150 is rated at 5000 RPM with a top RPM of 5500 RPM. Coop' -- The marine diesel industry uses propeller power curves (I think with n=2.7) to illustrate their products power production capability relative to such propeller power curve. These propeller power curves ARE used for planing vessels as evidenced by the provision of propeller power curves on the power and torque charts for stern drive configured marine diesels such as the one in the above link. To the best of my knowledge, stern drive configurations are rarely ever used for displacement applications. The marine diesel manufacturers do not try to simulate the on-plane transition point because it varies from hull to hull and its basically irrelevant if the motor does not have enough reserve power to climb over the bow wave.
itl	posted 01-15-2008 04:39 PM ET (US) Say, we have a boat which weight 2500lbs (with 150hp Honda engine and gears), hull factor is 180. Honda power curve shows that engine is actually producing around 160hp and that is the max. power output. We also know our boat performance and it is known how fast it goes when engine is running 3000-6000rpm. With Crouch's formula, we can calculate that propeller is taking power as following: 160hp 6000rpm 45.5mph (top end) 130hp 5000rpm 41.0mph 100hp 4000rpm 36.0mph 70hp 3000rpm 30.1mph Then we compare these power ratings to Honda powercurve provided by engine manufacturer: 160hp 6000rpm 140hp 5000rpm 110hp 4000rpm 80hp 3000rpm We notice that propeller took 10hp less than engine is producing in 3000-5000rpm range and we can accelerate our boat with that 10hp reserve until we reach the top end where engine and propeller powercurves cross :) Previous data is taken from hat and my results probably does not meet with real life situation. Honda powercurve is same as is in jimh's propeller power curve article.
jimh	posted 01-16-2008 08:07 AM ET (US) itl's approach is interesting. You drive your boat at a particular speed, deduce the horsepower from Crouch's formula (using a predetermined HULL FACTOR), and note the RPM. Then you compare the horsepower the engine power curve predicts at that RPM. However, there is no way to write it down as an equation. The individual boat determines the function. When I get a chance I will use some of my own data and graph the results using the approach suggested.